3.1. Antioxidant Activity
The biochemical reactions that continuously occur inside every single living cell are the driving force that sustains life. Yet many of these reactions generate free radicals, i.e., atoms, molecules or ions with unpaired electrons that are highly unstable and reactive [
70]. Several internal (e.g., mitochondrial activity, peroxisomes, etc.) and external factors (e.g., tobacco smoke, radiation, solvents, etc.) stimulate the generation of free radicals [
71]. The balance between the generation of free radicals and their neutralization by endogenous antioxidants is quite delicate. If the scale tips in favor of an excessive number of free radicals, cells will start to suffer the effects of oxidative stress, which can include severe cellular damage (primarily to DNA, proteins and lipids) and even death [
72].
Damage caused by free radicals has been associated with several serious illnesses, such as cancer, cardiovascular diseases (e.g., hypertension, atherosclerosis, cerebrovascular accidents, vasculitis), neurological disorders (e.g., Parkinson’s, Alzheimer’s, Huntington’s, autism), renal disorders (e.g., glomerulonephritis), liver disorders, rheumatoid arthritis, adult respiratory-distress syndrome, auto-immune diseases (e.g., systemic lupus erythematosus), inflammation, cataracts, gastric ulcers, hemochromatosis, among others [
72]. Several studies have also shown a link between oxidative stress and the aging process [
73,
74,
75]. One of the most substantial causes of aging is the accumulation of functional damage on cells, particularly damage affecting DNA [
71]. Oxidative stress, especially the stress caused by exposure to UV radiation, is also largely responsible for skin aging [
73]. Given all the undesirable effects of free radicals, it follows that reducing those effects could help in the prevention of several serious illnesses and delay the aging process. Enter antioxidants.
As photosynthetic organisms, marine algae are exposed to a combination of intense light and high concentrations of oxygen that stimulates the generation of free radicals. The fact that marine algae show remarkable resistance to oxidative damage suggests that their cells possess highly effective antioxidant defenses [
76]. Some of the antioxidant substances most frequently found in algae include polyphenolic compounds, vitamins and photosynthetic pigments [
2].
Several studies conducted with
Asparagopsis spp. have revealed that these algae produce compounds with antioxidant activity. The most significant results can be seen in
Table 1.
Zubia et al. [
77] tested the antioxidant activity of dichloromethane–methanol (1:1) extracts of several Rhodophyta from the coast of France. Among the tested species,
A. armata extracts were among those that exhibited the highest free-radical-scavenging activity in the DPPH (2,2-diphenyl-1-picrylhydrazyl) assay. However, the obtained results (an EC
50 of 6.25 mg/mL) indicate a weak antioxidant activity. A large concentration of extract, 104 to 446 times larger than the concentration of the positive controls used, was required in order to produce the same antioxidant effect. Using the same assay, Rhimou et al. [
78] obtained better results for the methanol extract of
A. armata from the Mediterranean Moroccan coast, but a concentration 78 to 107 times higher than that of the positive controls was still required in order to produce the same effect. Both Zubia et al. [
77] and Rhimou et al. [
78] also tested the antioxidant activity of
A. armata extracts using the β-carotene/linoleic acid assay. Zubia et al. [
77] verified that the extract of
A. armata had inferior results when compared to the extracts of the other studied species, with a percentage of oxidation inhibition varying between 8.02% and 4.92% for concentrations varying between 50 and 500 μg/mL, respectively. Surprisingly, the percentage of inhibition in this assay seems to be inversely proportional to the concentration of the extract. This did not occur for any of the other tested species or the positive controls and seems to suggest that the
A. armata extract could have pro-oxidant rather than antioxidant activity, which seems to go against the results of the DPPH assay. However, other factors could influence these results. The DPPH and β-carotene/linoleic-acid-assay test for different mechanisms of antioxidant action and the results of these two assays will not always correlate. Additionally, the β-carotene/linoleic-acid assay seems to be affected by many factors that limit its reproducibility. In a paper by Dawidowicz and Olszowy [
83], the authors found that both the type of solvent and the volume of solvent used in the measuring system significantly influence the results of the assay. As an example, the researchers found that the volume of ethanol used was inversely proportional to the antioxidant activity of BHT. Rhimou et al. [
78] also had very poor results for the methanol extracts of
A. armata using the β-carotene/linoleic-acid assay. Looking at the results of these two studies, it seems that the methanol extract of
A. armata contains a larger amount of antioxidant compounds than the dichloromethane–ethanol extract. However, there are significant differences for similar controls between the two studies, making it difficult to compare results. These differences could be the result of methodological differences between the two studies.
Neethu et al. [
81] used several methods to test the antioxidant activity of chloroform, methanol, petroleum ether, and ethyl acetate extracts of
A. taxiformis. In the hydrogen-peroxide-scavenging assay, all the extracts showed scavenging activity in a concentration-dependent manner. However, it was significantly lower than the activity observed for the positive control (ascorbic acid), which was surprisingly low itself, at 17.59 % inhibition at 500 µg/mL. Other examples found in literature [
84,
85] show that ascorbic acid is a far better scavenger of hydrogen peroxide. In the assay that tests the capacity for superoxide-radical scavenging, it was observed that the percentage of scavenging increases with the concentration of the extract. Among the tested extracts, methanol (85%) and chloroform (79%) exhibited highest scavenging activity at 500 μg/mL. The positive control (ascorbic acid) showed a similar scavenging effect (87%) to that of the methanol extract. The FRAP (ferric-reducing-antioxidant power) assay revealed maximum antioxidant activity for the chloroform extract (67%) at 100 μg/mL. The positive control (ascorbic acid) showed a similar capacity to reduce Fe
3+ (73%) when compared to the extracts.
Nunes et al. [
82] studied the antioxidant activity of
A. taxiformis extracts as well as their ferrous-ion-chelating activity, which measures the ability of secondary antioxidants to inhibit oxidation through an indirect approach. The results can be seen in
Table 1. These researchers used two different extraction methods (M1, by sonication and stirring, and M2, using Soxhlet extraction) to obtain each extract and found that there were significant discrepancies in the results for extracts that used the same solvent but different extraction methods. The ethanol extract obtained using M1 had the best results in both antioxidant-activity assays and was the extract with the largest amount of total phenolic compounds and chlorophyll a, which are compounds known to have antioxidant activity.
These studies suggest that algae of the genus Asparagopsis produce metabolites that exhibit antioxidant activity. Further research is required to isolate the active compounds and to determine their mode of action.
3.2. Cytotoxic Activity
Historically, cancer has always been, and still is, a major cause of human fatalities. Prospects are not reassuring either, since the number of cancer cases is expected to increase as populations grow larger, older and continue to adopt lifestyles that increase the risk of cancer [
86]. It is estimated that, in 2020 alone, the number of new cancer cases worldwide was around 19.3 million and that 10 million people died of the disease [
87].
In the last three decades, almost 80% of all cancer-fighting drugs approved by the FDA (Food and Drug Administration, USA) were natural products or synthetic products based on natural products [
88] and 60% of all commercially available anticancer drugs were of natural origin [
89].
There are not many published papers about the anticancer activity of
Asparagopsis spp., but there is enough available information to suggest that these algae have some potential as producers of cytotoxic compounds. Zubia et al. [
77] tested the cytotoxic activity of the dichloromethane–methanol (1:1) extracts of several Rhodophyta from the French coast against Daudi cells (in vitro model of Burkitt’s lymphoma), Jurkat cells (in vitro model of T-cell leukemia) and K562 cells (in vitro model of myeloid leukemia). The results for the
A. armata extracts can be seen in
Table 2. No positive control was used and the IC
50 values were not calculated, making it difficult to draw any conclusions. However, a large concentration of extract seems to be required in order to produce a significant effect.
Alves et al. [
89] tested the cytotoxic and antiproliferative activity of methanol and dichloromethane extracts of several algae from the Portuguese coast against an in vitro model of human hepatocellular carcinoma (HepG-2 cells). At a concentration of 1 mg/mL for 24 h, the methanol extract of
A. armata was capable of reducing the percentage of viable cells to 11.22 ± 2.98 % of the control population. The dichloromethane extract further reduced that population to 1.51 ± 0.38%. The IC
50 values can be seen in
Table 2. While this is a very significant reduction in the percentage of viable cells, it is important to note that 1 mg/mL is a very high concentration, as are the IC
50 values. The same is true in the antiproliferative assay, where a very large concentration of extract, 30 to 50 times larger than that of the positive control tamoxifen, was required in order to produce that same effect. Alves et al. [
90] also found that methanol and dichloromethane extracts of
A. armata have cytotoxic and antiproliferative activity against Caco-2 cells, an in vitro model of human colorectal cancer, and Nunes et al. [
91] found that a chloroform–methanol (2:1) extract of
A. taxiformis had some cytotoxic activity against A549 cells, an in vitro model of lung cancer cells.
These studies suggest that
Asparagopsis spp. produce metabolites with some anticancer activity. However, a lot of research remains to be done in order to isolate, identify and purify the molecules that have anticancer potential and to understand the intracellular signaling processes that are associated with the cytotoxicity and/or cell cycle regulatory mechanisms [
89].
3.3. Antimicrobial Activity
The appearance of bacterial strains that are resistant to multiple antibiotics, i.e., the multiresistance issue, has prompted researchers to look for new products with antimicrobial activity [
92]. Since many marine organisms, including algae, produce bioactive metabolites as a response to ecological pressure, the exploration of this chemical diversity has led to the discovery of several compounds with antimicrobial activity [
93]. Marine pathogenic bacteria can have a serious impact on algae [
94,
95,
96]. In addition, epiphytic bacteria can negatively affect algae through the increase in hydrodynamic drag and the inhibition of photosynthesis due to fouling (i.e., the accumulation of microorganisms, plants or animals on the surface of submerged organisms or structures) [
97]. Notwithstanding the constant threat of bacterial epibiosis, algae can keep themselves relatively free of bacterial diseases and excessive fouling [
96,
98]. These observations have spurred research on algae as a source of antimicrobial compounds [
99].
Paul et al. [
100] observed that dichloromethane and methanol extracts of
A. armata exhibited antibacterial activity against
Vibrio spp.,
Escherichia coli,
Pseudomonas aeruginosa and
Staphylococcus spp. and used gas chromatography and mass spectrometry to determine the composition of the algal extracts. Bromoform and dibromoacetic acid were the dominant compounds in the extracts of
A. armata, and both these halogenated compounds were found to be capable of inhibiting the growth of all the tested bacterial strains. In this study, the researchers also found that antibacterial metabolites were produced in specialized cells and that
A. armata possesses systems that allow the release of these metabolites into the surrounding seawater. By creating an artificial seawater medium deprived of bromine, the researchers found that the algae that were unable to produce halogenated compounds were colonized by a significantly larger number of epiphytic bacteria than those that did produce halogenated compounds. These results suggest that the ecological function of these metabolites is associated with defense against colonization by epiphytic bacteria.
Bansemir et al. [
101] tested the activity of dichloromethane, methanol and water extracts of several algae against several fish pathogenic bacteria (
Aeromonas salmonicida ssp.
salmonicida,
Aeromonas hydrophila ssp.
hydrophila,
Pseudomonas anguilliseptica,
Vibrio anguillarum,
Yersinia ruckeri). The results of the disk-diffusion assay can be seen in
Table 3. Only the dichloromethane extracts exhibited significant antibacterial activity and
A. armata was the alga that showed the strongest activity out of the tested species. As an example, the MIC (minimal inhibitory concentration, i.e., the lowest concentration of antibiotic that prevents bacterial growth) for the dichloromethane extracts of
A. armata against
V. anguillarum was <100 μg/mL. This value might be significantly larger than the MIC of the positive control oxytetracycline (0.5 μg/mL) but that is to be expected. While oxytetracycline was used in a purified state and at a known concentration, extracts are complex mixtures of several compounds, which means that whatever compounds are responsible for the antibacterial activity could be highly diluted. Bansemir et al. [
101] highlighted the potential of
A. armata’s bioactive compounds as prophylactics or therapeutic agents in the treatment of fish bacterial infections that could be used in aquaculture and fishkeeping. The inclusion of
A. armata in fish diets could be an alternative to the application of extracts, fractions or purified compounds, according to the researchers. However, further research on the possible toxicity of the algae against fish and on the stability and metabolization of algal bioactive compounds is required in order to confirm whether these ideas can be employed in the aquaculture industry.
Mata et al. [
109] investigated whether
A. taxiformis has the potential to be integrated into fish aquaculture systems and to act as a therapeutic agent against fish pathogenic bacteria. Like
A. armata,
A. taxiformis is also capable of producing and releasing halogenated antibacterial metabolites into the surrounding seawater [
109]. The authors quantified the release, accumulation, and residence of such metabolites in the cultivation medium and used this medium to test its in vitro activity against
Streptococcus iniae, a common fish pathogen that is often a problem in aquaculture systems. The results suggest that the medium delayed but could not inhibit the growth of
S. iniae, even though a treatment with concentrations of bromoform and dibromoacetic acid three orders of magnitude higher (70 mg/L and 32 mg/L, respectively) than that of the water that was directly sourced from an operational cultivation tank did inhibit the growth of
S. iniae by more than 80%. However, increasing the concentration of metabolites becomes harmful to fish even at non-antibacterial concentrations. These results suggest that
A. taxiformis’ metabolites have limited potential in the treatment of barramundi (
Lates calcarifer) infected with
S. iniae.
Salvador et al. [
102] tested the activity of solid algal mass (i.e., obtained without solvents) and of methanol extracts of 82 Iberian macroalgae against three Gram-positive bacteria, two Gram-negative bacteria and one yeast. The results showed that the algae with the strongest antimicrobial activity belonged to the order Bonnemaisoniales, particularly
A. armata, which was one of the algae that exhibited the strongest activity and the broadest spectrum (
Table 3). Based on the observed activity, the researchers suggested the potential of algal metabolites as natural preservatives in cosmetics.
Pinteus et al. [
93] tested the activity of methanol,
n-hexane and dichloromethane extracts of twelve algae from the Portuguese coast against
Escherichia coli (Gram-negative bacteria),
Bacillus subtilis (Gram-positive bacteria) and
Saccharomyces cerevisiae (yeast). Even though there were no positive results against
E. coli, the extracts of
A. armata exhibited the strongest activity against
B. subtilis. The activity was significantly weaker than that of the positive control chloramphenicol. Nonetheless, that is to be expected since, as previously mentioned, extracts are complex mixtures in which the bioactive compounds can be present at very small concentrations. The
n-hexane and dichloromethane extracts of
A. armata were also capable of inhibiting the growth of
S. cerevisiae, as can be seen in
Table 3. The positive control amphotericin had a significantly better performance, but again the difference could be attributed to the diluted state of the bioactive metabolites present in the extracts.
Greff et al. [
69] tested the antibacterial activity of two newly discovered brominated cyclopentones: mahorone and 5-bromomahorone. Both cyclopentones exhibited moderate antibacterial activity against the human pathogen
Acinetobacter baumannii, as can be seen in
Table 3, as well as the results against other bacteria and fungi. There was also some relevant activity against Methicillin-resistant
Staphylococcus aureus (MRSA). The choice to use MIC
80 as a metric for antibacterial activity, instead of the more common MIC or MIC
50, is an odd one, and makes it hard to compare these results to those of other studies.
Besides the antibacterial activity per se, a study by Jha et al. [
110] suggests that
Asparagopsis spp. produce compounds that can interrupt quorum sensing. Quorum sensing is a population density-dependent gene-regulation system mediated by bacterial extracellular-signaling molecules that is used by bacteria to regulate the formation of biofilms (i.e., organized microbial communities that sheath themselves in extracellular polymeric substances that can confer antibiotic resistance to the bacteria) [
111] and their pathogenicity and production of virulence factors [
112]. Therefore, quorum sensing could be the ideal target for antipathogenic drugs that could be an alternative to antibiotics, which have led to the emergence of antibiotic resistance in bacteria [
113]. It has been shown that quorum-sensing inhibitors can increase the susceptibility of biofilms to antibiotics both in vitro and in vivo [
114]. Biofilms can have adverse effects on several important structures such as water-supplying pipes, air ducts and industrial fermenters. They cause erosion, clogging and the formation of slippery coatings on surfaces, as well as the accumulation of potentially harmful bacteria [
115,
116]. Marine biofilms (fouling) are one of the main causes of economic loss in maritime industries [
117,
118]. The most common strategy employed to fight fouling involves the application of paints and coatings that contain anti-fouling chemicals that prevent the accumulation of organisms on structures. These paints and coatings have been mostly based on heavy metals such as copper, chromium and tin. However, many studies have demonstrated that heavy metals have negative impacts on several marine microorganisms and mollusks [
119,
120]. Since marine algae keep themselves relatively free of fouling, it was suggested that, in addition to microbial compounds, marine algae could also produce quorum-sensing inhibitors. Indeed, the first quorum-sensing inhibitor was isolated from the red alga
Delisea pulchra [
121]. Jha et al. [
110] observed that, in addition to inhibiting the growth of
Chromobacterium violaceum, methanol extracts of
A. taxiformis and some of their fractions also inhibited quorum sensing in
C. violaceum and
Serratia marcescens.
Pinteus et al. [
106] investigated the ability of
A. armata to inhibit the growth of several marine and freshwater bacteria and marine microalgae, in the context of finding greener alternatives to common antifouling solutions. The results can be seen in
Table 3. The researchers also fractioned the crude methanol–dichloromethane (1:1) extract using vacuum liquid chromatography. The fractions were, generally, more effective than the crude extracts at inhibiting bacterial growth. This points out the importance of isolating the compounds that are responsible for this activity, as they are present in low concentrations in the crude extracts. The results also suggest that
A. armata produces several products with broad-spectrum biocidal activity. The researchers also investigated the inhibition of bacterial biofilm formation, which is an essential step in the fouling process. All but two fractions and crude extracts were effective against biofilm formation by
Vibrio parahemolyticus, with percentages of inhibition ranging from 13% to 30%, in concentrations ranging from 125 to 500 µg/mL. In the case of biofilm production by
B. subtilis, the crude extract was the most effective, with the highest inhibitory activity ranging between 49% and 59%, with extract concentrations of 125 and 500 µg/mL, respectively. The extracts and fractions also showed inhibitory activity against the enzyme acetylcholinesterase, which is essential to the settling of invertebrates in a specific substrate, meaning that compounds produced by
A. armata are capable of inhibiting fouling by acting on several key stages of the process, such as bacterial and algal growth, biofilm production and chemical signaling.
Activity against
Leptospira spp., the bacteria responsible for the disease leptospirosis, has also been observed. Vedhagiri et al. [
122] tested the antibacterial activity of methanol extracts of
A. taxiformis against several strains and isolates of
Leptospira javanica and calculated the MIC, MBC (minimum bactericidal concentration) and EC
50 values, which ranged from 100–400 μg/mL, 200–1600 μg/mL and 34.48–322.9 μg/mL, respectively. The results for the extracts were often comparable to those of the positive controls penicillin and doxycycline.
Antiprotozoal activity against
Leishmania donovani, one of the protozoans responsible for causing the disease leishmaniosis, has also been reported. Genovese et al. [
107] found that extracts from both
A. armata and
A. taxiformis had a significant inhibitory effect
on L. donovani. Results are in
Table 3 and suggest that the active compounds are non-polar or moderately polar. LC-MS analysis indicates that brominated compounds are responsible for this activity.
Although several studies have demonstrated some degree of antimicrobial activity of extracts from
Asparagopsis spp., most of these studies use the disk-diffusion method, or variations of it. This method is based on the inhibition of plated bacteria by antibiotics present in paper disks. The antibiotics diffuse through the agar medium and are present at progressively lower concentrations as the distance to the disk increases. If the antibiotic is effective, no bacteria will grow around the disk. The area around the disk where no bacterial growth is observed is referred to as the inhibition zone or inhibition halo. The larger the zone, the more sensitive the tested bacteria are to the tested antibiotics. However, even though the MIC of an antibiotic influences the size of the inhibition zone, this test cannot be used to determine the MIC value, mostly because the diameter of the inhibition zone is also a function of the initial antibiotic concentration and of its solubility and rate of diffusion in the agar. This means that the disk-diffusion method cannot be used to directly compare the effectiveness of two different antibiotics [
123] and makes it hard to compare the results of studies that use this method. The method is, however, good enough to show that antimicrobial activity does take place. It is also important to standardize the units used to quantify activity, so that different studies can be compared.
Most research so far has been done on the activity of extracts and not pure compounds, which means that there is further work to be done to isolate active compounds and determine their mode of action.
3.4. Antiviral Activity
The last two years have been a harsh reminder of the dangers that viruses pose to mankind. At the time of writing, COVID-19, caused by the virus SARS-CoV-2, has claimed more than six million lives worldwide. Viruses are all around us. They are the most abundant “lifeforms” in the ocean [
124], with an estimated 10
30 viral particles existing in the oceans alone [
125]. They exist in the atmosphere, with billions of viral particles depositing on a single square meter above the atmospheric boundary layer [
126]. They lurk in the smallest bits of soil [
127]. If the estimated 10
31 individual virus existing on Earth were laid end to end, they would stretch for 100 million light years. It is also estimated that 10
23 viral infections happen each second in the ocean [
128]. Viruses are ubiquitous, have a remarkable ability to mutate [
129], and have zoonotic capabilities. Indeed, most new infectious diseases have a zoonotic origin [
130], in which the virus is capable of crossing the species divide and infecting humans. It is estimated that, in mammal and bird hosts alone, there are approximately 1.67 million undiscovered viral species in key zoonotic families, and experts claim that we can reasonably expect that between 631,000 and 827,000 of these unknown viruses have zoonotic potential [
130]. With this in mind, we can safely assume that it is only a matter of time before the next viral outbreak strikes. Additionally, as COVID-19 revealed, we, as a species, are wildly unprepared to deal with a pandemic. Therefore, it is important to keep researching natural products to find compounds with antiviral activity.
At least three studies have demonstrated that extracts and compounds isolated from
Asparagopsis spp. have antiviral potential. Shalaby and Shanab [
131] tested the antiviral activity of
A. taxiformis extracts and pure compounds against H5N1, the virus that causes avian influenza. They found that the extracts, particularly petroleum ether and water extracts, were capable of up to 99.9 % antiviral activity, mainly through the inhibition of virus adsorption into the host cell. More results can be seen in
Table 4.
Haslin et al. [
133] investigated whether sulfated-cell-wall polysaccharides from the gametic, carposporic and tetrasporic stages of
A. armata had any in vitro antiviral activity against HIV. They found that, while the carposporic polysaccharides were ineffective, syncytia (i.e., large, multinucleate and, in the case of HIV infection, nonfunctional cells) formation was completely suppressed by the gametic and tetrasporic polysaccharides at 10 μg/mL after the 7th day of infection. Their results suggest that these compounds block the replication of HIV and the formation of syncytia between infected and uninfected cells after the infection. During infection, they inhibit an early step of viral replication, seemingly due to the interaction between the polysaccharides and viral enzymes. The polysaccharides were also found to be non-toxic to host cells. Rhimou et al. [
132] found that extracts of
A. armata had an inhibitory effect on the in vivo replication of the
Herpes simplex type 1 virus (HSV-1) in Vero cells (African-green-monkey-kidney cell line). The most effective was the water extract, as can be seen in
Table 4. Since the antiviral activity was directly proportional to the extract polarity, the results suggest that that the compounds with antiviral activity are mostly polar. The polar extracts were also the least cytotoxic.
These results suggest that both A. armata and A. taxiformis produce compounds with antiviral activity. Further studies are required to isolate and purify these compounds, as well as to investigate whether these extracts and compounds are effective against other viruses.
3.5. Enzyme Inhibition Activity
Enzyme inhibitors are important therapeutic agents. Some famous examples include simvastatin, which is a hydroxy-methylglutaryl-coenzyme-A (HMG-CoA)-reductase inhibitor that decreases low-density-lipoprotein cholesterol (LDL-C), triglycerides and apolipoprotein B and increases high-density-lipoprotein cholesterol [
134], sildenafil, which is a cGMP-specific phosphodiesterase-type-5 inhibitor that is commercially sold under the brand name Viagra and used in the treatment of erectile dysfunction [
135], and antibiotics such as penicillin and vancomycin that inhibit enzymes involved in bacterial peptidoglycan synthesis [
136].
Acetylcholinesterase inhibition is the basis of most approved therapies for Alzheimer’s disease [
137], which is the most prevalent neurocognitive disorder in the world and is estimated to be responsible for about two thirds of the dementia cases worldwide [
138]. Since current therapeutics have limited potential and serious side effects, researchers have been looking for new therapeutic agents in the natural world, including marine algae. Bettencourt [
139] observed that the dichloromethane extracts of
A. taxiformis inhibited the activity of acetylcholinesterase with an IC
50 of 116.50 μg/mL, as can be seen in
Table 5. Nunes et al. [
91] also tested the activity of extracts of
A. taxiformis against acetylcholinesterase and butyrylcholinesterase. The IC
50 values obtained are in
Table 5. While the positive controls donepezil and galantamine were far more effective at inhibiting acetylcholinesterase, both extracts were more-effective inhibitors of butyrylcholinesterase than donepezil. Custódio et al. [
79] reported the inhibitory activity of the
A. armata methanol extract against acetylcholinesterase as being 58.4% ± 1.0 at a concentration of 10 mg/mL, while the positive control galantamine inhibited the enzyme by 90.3% ± 0.6 at a concentration ten times lower, while the result for butyrylcholinesterase at the same extract concentration was 66.8% ± 1.3, which is much lower than the 80.3% ± 0.1 inhibition by galantamine at 1 mg/mL. It should be noted that 10 mg/mL is an extremely large concentration, too large to be of interest, even in an extract, and also that presenting results as IC
50 values is more useful than percentages of inhibition when comparing inhibition power.
Using colorimetric assays, Oumaskour et al. [
105] found that dichloromethane–methanol (1:1) extracts of
A. armata could inhibit both elastase and phospholipase A2 (PLA2) (see
Table 5). The researchers suggest that, due to this inhibitory activity,
A. armata metabolites can have an anti-inflammatory effect.
An enzyme whose inhibition is of interest to the cosmetics industry is tyrosinase, which is the main enzyme involved in the synthesis of melanin, a skin-darkening pigment. Therefore, tyrosinase inhibitors are actively pursued for the formulation of skin-bleaching cosmetic products [
140]. Only one study was found that investigated the tyrosinase-inhibitory activity of
Asparagopsis spp. Custódio et al. [
79] observed that, at a concentration of 10 mg/mL, the methanol extract of
A. armata inhibited the activity of tyrosinase by 81.4%. The positive control arbutin was able to inhibit tyrosinase activity by 78.0% at a concentration of 1 mg/mL. Nonetheless, it is worth noting once again that that whatever inhibitor(s) may be present in the extract are probably present at low concentrations.