Physiological and Clinical Aspects of Bioactive Peptides from Marine Animals

Biological molecules in nutraceuticals and functional foods have proven physiological properties to treat human chronic diseases. These molecules contribute to applications in the food and pharmaceutical industries by preventing food spoilage and cellular injury. Technological advancement in the screening and characterization of bioactive peptides has enabled scientists to understand the associated molecules. Consistent collaboration among nutritionists, pharmacists, food scientists, and bioengineers to find new bioactive compounds with higher therapeutic potential against nutrition-related diseases highlights the potential of the bioactive peptides for food and pharmaceutic industries. Among the popular dietary supplements, marine animals have always been considered imperative due to their rich nutritional values and byproduct use in the food and pharmaceutical industries. The bioactive peptides isolated from marine animals are well-known for their higher bioactivities against human diseases. The physiological properties of fish-based hydrolyzed proteins and peptides have been claimed through in vitro, in vivo, and clinical trials. However, systematic study on the physiological and clinical significance of these bioactive peptides is scarce. In this review, we not only discuss the physiological and clinical significance of antioxidant and anticancer peptides derived from marine animals, but we also compare their biological activities through existing in vitro and in vivo studies.


Introduction
Proteins are important nutrients for sustaining life and supporting normal body functions. Proteins and peptides from marine sources provide a rich source of essential amino acids required for human daily needs [1,2]. Inactive peptides within native proteins require proteolytic release (in vivo digestion) or hydrolysis to reveal biological activity [3,4]. Protein hydrolysis results in the conversion of intact proteins into peptides, usually containing no more than 20 amino acids [4]. Current methods for hydrolysis of marine animal muscle and processing waste byproducts include chemical (acid, alkali, or catalytic) hydrolysis [5], enzymatic hydrolysis [6], gamma irradiation hydrolysis [7], subcritical water hydrolysis [8], thermal hydrolysis [9], autolysis [10], and bacterial fermentation [11] (Figure 1). Among those, enzymatic hydrolysis is the most common method of producing hydrolyzed proteins and peptides. Enzymatic hydrolysis improves solubility, water-binding capacity, and heat stability of myofibrillar proteins, modifies emulsifying and foaming properties of hydrolyzed proteins, as well as improves the nutritional quality of foods [12,13]. Desirable physicochemical properties of a hydrolyzed protein could be achieved by controlling hydrolysis parameters, such as time, pH, temperature, and enzyme concentration [14]. The degree of hydrolysis, on the other hand, affects the size and amino acid profile of Bioactive compounds are either produced naturally from plants, animals, fungi, and microorganisms, such as carotenoids, phenolic compounds, polyphenols, and cordymin [16][17][18][19][20], or generated synthetically, such as propyl gallate, t-butylhydroquinone, butylated hydroxytoluene, and butylated hydroxy anisole [21,22]. Bioactive substances from natural sources have gained much interest in recent years due to consumer's preferences and health concerns associated with the use of synthetic food additives [23]. The natural protein hydrolysates and peptides isolated from farm animals, such as cows, chickens, and pigs, are relatively cheap and are easily available sources of nutrients [24,25]. However, outbreaks of certain diseases, including avian influenza, mad cow disease, and foot-and-mouth disease, as well as limited use of extracted proteins from pig skin and bones due to religious reasons, have made it obligatory to find alternative sources [26,27]. Recently, the marine-derived protein hydrolysates and bioactive peptides have gained much attention due to their excellent nutritional composition, numerous health benefits and no reported side effects [6,[28][29][30][31].
An enormous range of marine-based protein hydrolysates and peptides using different hydrolysis methods have been reported in the literature with significant antioxidant, anticancer, antihypertensive, antimicrobial, and immunomodulatory activities [32][33][34]. Antihypertensive peptides or angiotensin-converting enzyme (ACE) inhibiting peptides have been extensively studied [35]. Due to the side effects, such as taste disturbance and skin rashes, associated with the use of synthetic drugs [36], the interest in finding safer and eco-friendly ACE inhibitory compounds from natural sources has increased [37,38]. The ACE inhibitory properties of the peptides isolated from marine sources, including pacific cod skin [39], sardinella [40], flounder fish [41], and Takifugu bimaculatus skin [42], have been reported.
Immunomodulatory peptides prevent or treat immune-suppressing diseases by improving cytochrome or antibody synthesis and supplementing mucosal immunity in the gastrointestinal tract [51]. The protein hydrolysates and peptides from marine sources have shown significant immunomodulatory properties [52]. For example, a low-molecularweight peptide isolated from marine Nibea japonica skin dramatically improved IgG and IgM levels in blood [53]. Furthermore, due to rising cancer cases, the interest in finding a suitable cancer therapy is consistently gaining attention. Traditional cancer treatments, including surgery, chemotherapy, and radiotherapy, could affect healthy cells [54] and suppress the human immune system [55]. The commercially available drugs from natural sources containing bioactive peptides account for 60% of the available anticancer drugs [56]. It could be due to the ability of the anticancer peptides to penetrate and directly bind to the cancer cells via electrostatic interaction [57]. Recent research has claimed that anticancer compounds from marine sources, especially marine natural bioactive products [58] and nanomaterials, as a safer alternative, allowing targeted treatment to cancer cells and not inducing damage to the healthy cells [59,60].
Peptides isolated from marine sources contain essential amino acids [61] and are used in animal and human nutrition. Due to their rich nutritional profile and bioactive properties, marine peptides have several commercial applications in the food, cosmetics, and pharmaceutical industries [62,63]. The in vitro bioactivities of protein hydrolysates have been discussed in the literature; however, a systematic study on the physiological and clinical significance of marine-based purified bioactive peptides is scarce. Hence, there is a need to critically review the physiological and clinical properties of purified bioactive peptides from marine animals. In this review, we discuss the physiological and clinical significance of antioxidant and anticancer properties of peptides purified from marine animals through selected in vitro, in vivo, and clinical studies.

Methods
The literature search was conducted using the PubMed database from 2000 to present. Articles were mostly written in English (in vitro, in vivo, and clinical studies). The key search using a comprehensive list of MeSH (Medical Subject Headings) terms followed: Antioxidants AND peptides, Antineoplastic Agents AND peptides. Additional search words included "marine animal", "marine animal in vitro", "marine animal in vivo", and "marine animal clinical". The search strategy and results are presented in a schematic diagram ( Figure 2). Articles were selected based on their relevance to the topic of this review. For antioxidant studies, 626 articles were analyzed, 576 articles were excluded, and 50 articles were considered in this review. For anticancer studies, 829 articles were analyzed, 809 articles were excluded, and 20 articles were considered. In addition, references in each article were further reviewed to find any relevant study using ScienceDirect, Wiley Online Library, SpringerLink, and Google Scholar databases. Articles were extracted and sorted using EndNote to prevent duplicate citations. gastrointestinal tract [51]. The protein hydrolysates and peptides from marine sources have shown significant immunomodulatory properties [52]. For example, a low-molecular-weight peptide isolated from marine Nibea japonica skin dramatically improved IgG and IgM levels in blood [53]. Furthermore, due to rising cancer cases, the interest in finding a suitable cancer therapy is consistently gaining attention. Traditional cancer treatments, including surgery, chemotherapy, and radiotherapy, could affect healthy cells [54] and suppress the human immune system [55]. The commercially available drugs from natural sources containing bioactive peptides account for 60% of the available anticancer drugs [56]. It could be due to the ability of the anticancer peptides to penetrate and directly bind to the cancer cells via electrostatic interaction [57]. Recent research has claimed that anticancer compounds from marine sources, especially marine natural bioactive products [58] and nanomaterials, as a safer alternative, allowing targeted treatment to cancer cells and not inducing damage to the healthy cells [59,60]. Peptides isolated from marine sources contain essential amino acids [61] and are used in animal and human nutrition. Due to their rich nutritional profile and bioactive properties, marine peptides have several commercial applications in the food, cosmetics, and pharmaceutical industries [62,63]. The in vitro bioactivities of protein hydrolysates have been discussed in the literature; however, a systematic study on the physiological and clinical significance of marine-based purified bioactive peptides is scarce. Hence, there is a need to critically review the physiological and clinical properties of purified bioactive peptides from marine animals. In this review, we discuss the physiological and clinical significance of antioxidant and anticancer properties of peptides purified from marine animals through selected in vitro, in vivo, and clinical studies.

Methods
The literature search was conducted using the PubMed database from 2000 to present. Articles were mostly written in English (in vitro, in vivo, and clinical studies). The key search using a comprehensive list of MeSH (Medical Subject Headings) terms followed: Antioxidants AND peptides, Antineoplastic Agents AND peptides. Additional search words included "marine animal", "marine animal in vitro", "marine animal in vivo", and "marine animal clinical". The search strategy and results are presented in a schematic diagram ( Figure 2). Articles were selected based on their relevance to the topic of this review. For antioxidant studies, 626 articles were analyzed, 576 articles were excluded, and 50 articles were considered in this review. For anticancer studies, 829 articles were analyzed, 809 articles were excluded, and 20 articles were considered. In addition, references in each article were further reviewed to find any relevant study using Sci-enceDirect, Wiley Online Library, SpringerLink, and Google Scholar databases. Articles were extracted and sorted using EndNote to prevent duplicate citations.

Antioxidant Properties
An unbalanced electron in the outermost orbital of reactive oxygen species (ROS) makes these molecules unstable and enables them to react with biological macromolecules, such as proteins, lipids, and DNA [64]. The ROS, including superoxide anion radical (O 2 •− ), hydrogen peroxide radical (H 2 O 2 ), hydroxyl radical ( • OH), peroxyl radical (ROO • ), and hydroperoxyl radical (HO 2 • ), are byproducts produced by mitochondrial aerobic metabolism [65]. An unbalance in the oxidation reaction chain and ROS ratio causes oxidative stress. This leads to the development of chronic diseases, such as diabetes, cardiovascular diseases, Alzheimer's, neurodegenerative diseases, and particularly cancer [66], which is one of the leading causes of mortality worldwide [67]. The oxidative stress generated by ROS is suppressed by antioxidant defense mechanisms, including endogenous and exogenous defense systems [68]. Endogenous antioxidants, such as catalase (CAT), superoxide dismutase (SOD), glutathione (GSH), and glutathione peroxidase (GPx), and exogenous antioxidants, such as carotenoids, vitamin E, vitamin C, and polyphenols, act synergistically to inhibit oxidation reaction, maintain redox homeostasis and reduce cell deterioration via suppressing cell damage [68][69][70]. Therefore, maintaining a balance in antioxidants/ROS ratio is vital to avoid oxidative stress [71].
The antioxidant effects of peptides isolated from fish and other marine animals have been studied extensively in vitro and in vivo (including animal-based studies), while human clinical trials have rarely been reported. The antioxidant properties of peptides isolated from marine animals are listed in Tables 1-3 and are described below.
The structural features of a peptide, such as size, molecular weight, sequence, and amino acid composition, could directly affect the antioxidant activity of a peptide. The low molecular weight of a peptide (< 1.5 kDa) has been linked with higher ROS radical scavenging activities [4,74,87,95,98]. A low-molecular-weight peptide, TCGGQGR (678 Da) from mackerel byproducts, showed DPPH • scavenging activity of 96% and ABTS • scavenging activity of 100% [95]. The tuna backbone peptide, VKAGFAWTANQQLS (1.5 kDa), showed up to 90% • OH scavenging activity at a concentration of 0.05 mg/mL, and no toxicity to human fetal lung fibroblast cells was reported [73]. Exceptionally, some high molecular weight peptides, such as yellowfin sole peptide, RPNFDLEPPY (13 kDa), have also shown high antioxidant activities [72].
The presence of a polar amino acid such as tyrosine (Y) at the C-terminus of a peptide sequence has also been linked with higher antioxidant activities [76,120,121] [78]. The Alaska pollock (Theragra chalcogramma) peptide, LPHSGY (672 Da), showed high • OH scavenging activity of 35% at a concentration of 53.6 µM and also contained tyrosine [88].
The marine animal-based antioxidant peptides were explored from different marine fish species, where mackerel and tilapia contributed the most. The antioxidant peptides prepared using enzymatic hydrolysis at temperatures of < 50 • C and pH < 9 resulted in higher antioxidant activities. Irrespective of the source of isolation, smaller molecular weight peptides showed higher radical scavenging potential. Interestingly, the presence of a hydrophobic amino acid and/or a polar amino acid, such as tyrosine, in the peptide sequence showed higher scavenging activities. Despite the promising in vitro activities, most of the above-mentioned peptides have not been evaluated under in vivo or clinical conditions.

Antioxidant Effects of Bioactive Peptides from Marine Animals-In Vivo Studies
Nazeer et al. purified the peptide KTFCGRH with a molecular weight of 861 Da from a croaker and studied the antioxidant effects in Wistar rats. A total of 100 µg/kg of purified peptide and 20% ethanol were orally administered to experimental rats for 15 days. The rats fed with purified peptides showed improvement in cellular antioxidant enzyme systems and displayed serum CAT, SOD, and glutathione s-transferase (GST) levels of 283.6, 28.42, and 4.3 U/mg protein, respectively. In contrast, the ethanol-administered rates showed CAT, SOD, and GST levels of 196.4, 15.1, and 1.3 U/mg protein, respectively [110]. The purified peptide from horse mackerel viscera, ACFL with a molecular weight of 518 Da, was evaluated in Wistar rats for 15 days. The results showed that 100 µg/kg of peptide and 20% of ethanol-administered rats showed serum CAT, SOD, and GST levels of 290.8, 29.45, and 3.93 U/mg protein, respectively. In contrast, the ethanol-administered rats showed CAT, SOD, and GST levels of 196.4, 15.1, and 1.3 U/mg protein, respectively [111]. Antioxidant properties of the peptide WHKNCFRCAKCGKSL purified from snakehead murrel (Channa striatus) were investigated in wild-type zebrafish. This peptide showed no cytotoxicity to zebrafish larvae at a concentration of 50 µM, and a decrease in malondialdehyde (MDA) levels to 14 µmol/min/mg protein was observed. In contrast, SOD and CAT expressions increased to 22 and 17 U/mg protein, respectively [106].
Hu et al. studied the protective effects of tilapia scales-purified collagen oligopeptides in specific pathogen-free (SPF) Sprague Dawley rats for 30 days [113]. The oral administration of collagen peptides significantly reduced gastric and duodenal ulcer index and MDA content. In comparison, increases in gastric juice pH and expressions of CAT, SOD, and glutathione peroxidase (GPx) were observed [113]. Furthermore, the oligopeptides purified from frigate tuna (Auxis thazard) showed hypouricemic properties. Seven days of treatment of SPF hyperuricemic Kunming mice with tuna oligopeptides resulted in increased SOD and CAT levels of 125 and 0.75 U/mg protein, respectively [112].
The purified marine antioxidant peptides were evaluated using different in vivo models, including albino Wistar rats, SPF-SD mice, and wild zebrafish. The study parameters included changes in body weights, body weight gains, levels of intracellular ROS, levels of lipid peroxidation, and expressions of antioxidant enzyme systems, such as SOD, CAT, GST, and GPx. The reported antioxidant peptides showed enhanced expressions of antioxidant enzymes, reduced MDA levels, and helped to recover oxidative stress.

Antioxidant Effects of Bioactive Peptides from Marine Animals-Clinical Trials
Human clinical studies of purified antioxidant peptides from marine animals have rarely been reported. Kim et al. reported skin improving properties of low-molecularweight collagen peptide (LMWCP) from Sutchi catfish skin (Pangasius hypophthalmus) in 40-60-year-old healthy women [114]. Skin hydration was significantly improved (6fold of placebo), and crow's feet decreased to 2.81 AU when compared to placebo after 6 weeks of treatment. Average roughness of skin wrinkling-0.17 AU (lower than placebo group) and average roughness of skin elasticity-0.71 AU (higher than placebo group) was noted after 12 weeks. In another study, skin improving properties of CELERGEN ® were investigated in 41 adults, including five females. CELERGEN ® is a commercial oral supplement that includes marine collagen peptide from the skin of pollock (Pollachius virens), Atlantic halibut (Hippoglossus hippoglossus), European plaice (Pleuronectes platessa), and plant-derived antioxidants [116]. After 4 months of treatment, the thickness and acoustic density of the dermis layer improved to 4133 µm and 6.3, respectively. However, GPx/GST activity and thickness and acoustic density of the epidermis layer were not changed. Another commercial product, GOLD COLLAGEN ® ACTIVE, produced from fish collagen, showed skin improving properties when evaluated in 122 subjects [115]. This supplement consists of hydrolyzed fish collagen, vitamins, and other active ingredients. It improved skin elasticity by 40% in the younger age group and reduced joint pain by 43% in the elder age group when compared to the placebo group.
The reported clinical studies on purified antioxidant peptides from marine animals have focused on skin properties, such as skin elasticity, skin hydration, and wrinkling. Two of the above-mentioned products have already been approved and available commercially. Nevertheless, some peptides from marine animals have shown prominent in vitro and in vivo antioxidant activities, and clinical trials are lacking, which need to be performed to better understand their bioactivities.

Anticancer Properties
Anticancer peptides have been isolated from various marine animals, such as fish, oysters, mussels, and snails. Anticancer peptides are selective to cancer cells and kill cancerous cells by inhibiting angiogenesis, disrupting tubulin-microtubule balance, and apoptosis [122]. The anticancer properties of several peptides from marine animals have been investigated in vitro and in vivo, including in animal-based studies, and some of these peptides have even been used in human clinical trials. The anticancer properties of peptides isolated from marine animals are listed in Tables 4-6 and are described below.

Anticancer Effects of Bioactive Peptides from Marine Animals-In Vitro Studies
Anticancer peptides from marine animals, especially the phylum Mollusca, have been extensively reported. The peptide, LKEENRRRRD, isolated from sepia ink (Sepia esculenta), inhibited the proliferation of prostate cancer (PC-3) cells and showed a 3.3-fold increase in early apoptosis as compared to the control [127]. In another study, the peptide, AFNIHNRNLL, from mussel (Mytilus coruscus) inhibited various cancer cell lines, including prostate cancer (PC-3), lung cancer (A-549), and breast cancer (MDA-MB-231) cell lines at LC 50 of 0.94, 1.41, and 1.22 mg/mL, respectively, and showed no toxicity to the normal cells [125]. The peptide, LANAK (515 Da), isolated from oyster (Saccostrea cucullata), not only showed strong antioxidant activity [87], but it also showed significant anticancer activity against human colon cancer (HT-29) cells at IC 50 of 90 µg/mL and showed no cytotoxicity to normal kidney epithelial cells of the African Green Monkey [87]. Kahalalide F (KF), derived from marine mollusk (Elysia rufescens), showed anticancer activity by reducing DNA synthesis in prostate cancer cell lines (DU145, PC3, and LNCaP at IC 50 [131]. Besides mollusks, the anticancer activity of fish has also been claimed. Finless sole peptide, GFFALIPKIISSPLFKTLLSAVGSALSSSGGQE, known as pardaxin, inhibited colony formation in MN-11 cells (<90% inhibition) [126]. The Japanese flounder (Paralichthys olivaceus) peptides, RKQCIRKCIRRREPHGKMMIRIRRK and KKYRSQRKIRRMRR-KRKYPSFMQ, induced dose-dependent necrosis in HT-29 cells at a concentration of 500 µM [128]. Spanish mackerel peptides showed a more than 80% survival rate in mouse melanoma cancer (B16F10) cells [129]. The tilapia peptide, GIKCRFCCGCCTPGICGVCCRF-NH 2 , showed a concentration-dependent (50 and 100 µg/mL) inhibition of HeLa and HT1080 cells after 24-72 h treatment. The tilapia peptide inhibited colony formation in HeLa and HT1080 cells by 60% and in HepG2 cells by 50% [130]. Low-molecular-weight antioxidant peptide, FIMGPY (726.9 Da), from skate (Raja porosa) also showed significant anticancer activity against HeLa cells at IC 50 of 4.81 mg/mL and no toxicity to normal NIH3T3 cells [120].
Recent studies have claimed significant anticancer properties of marine-derived peptides, even at low concentrations, such as anticancer peptides isolated from tuna (Thunnus tonggol) [123]. The tuna peptides, LPHVLTPEAGAT and PTAEGGVYMVT, were active against breast cancer (MCF-7) cells at IC 50 of 8.1-0.7 [123]. The isolated peptides had a molecular weight of 390 Da to 1.4 kDa. In another study, a low-molecular-weight peptide (440 Da) isolated from anchovy sauce inhibited human lymphoma (U937) cells by 50% at 31 µg/mL [124]. Interestingly, these peptides also contained hydrophobic amino acids.
The peptides purified from marine animals, including marine mollusks, tilapia, mackerel, and oysters, showed significant in vitro anticancer activities against breast, leukemia, colon, fibrosarcoma, prostate, and cervical cancers. Interestingly, low-molecular-weight peptides containing hydrophobic amino acids showed high anticancer activities. A similar observation was noted in the case of antioxidant peptides. Irrespective of significant in vitro anticancer activities, in vivo activities of these peptides have hardly been reported.

Anticancer Effects of Bioactive Peptides from Marine Animals-In Vivo Studies
Marine animals from different phyla, especially Mollusca and Porifera, have revealed significant anticancer activities, which have been tested through in vitro and in vivo studies. Venoms of marine cone snails, genus Conus, containing more than 100,000 small bioactive peptides, have been used to develop potential therapeutic agents for cardiovascular and nervous systems [133]. The anticancer activity of marine vermivorous cone snail (Conus vexillum) venom was investigated by Abdel-Rahman and colleagues using Swiss albino mice injected with Ehrlich's ascites carcinoma (EAC) cells [133]. Conus venom significantly increased various oxidative stress biomarkers (protein carbonyl content, lipid production, and reactive nitrogen intermediates) of EAC cells after 3-12 h of venom injection. In addition, it significantly reduced CAT and SOD expression levels of EAC cells.
Pardaxin peptide was discovered as a natural agent to inhibit oral squamous cancer [132]. It is derived from finless sole (Pardachirus marmoratus) and have been tested on hamster using an oral squamous cell carcinoma model [132]. Pardaxin at a dose of 75 mg/kg reduced tumor volume to more than half. Furthermore, a 500% decrease in serum prostaglandin E2 (a cancer inducer) levels was observed after taking pardaxin with 5-fluorouracil (an antimetabolite used in clinical therapy). In another study, 14 days of treatment with a high dose of synthetic pardaxin (13 µg/mL) showed a 3-fold reduction in tumor volume of MN-11 cells in C57BL/6 mice, while no side effects were observed in the normal cells [126].

Anticancer Effects of Bioactive Peptides from Marine Animals-Clinical Trials
The anticancer activity of marine animal-derived individual peptides has not been claimed through clinical trials; however, two decades ago, some anticancer compounds containing marine animal-derived peptides were reported, which are currently known as natural chemotherapeutic agents such as Kahalalide F (KF) [140]. KF is a depsipeptide with a composition ranging from tripeptide to tridecapeptide [141]. KF was first isolated from marine mollusk (Elysia rufescens), and it is currently in clinical phase II. KF has shown significant antitumor activities in prostate cancer patients at a recommended dose of 560 µg/m 2 [135]. Another clinical study on 38 cancer patients (colorectal cancer, melanoma cancer, breast cancer, and others) recommended a KF dose of 650 µg/m 2 [134]. However, dose-limiting toxicities were observed after 1 h intravenous infusion [134]. Martín-Algarra and co-workers confirmed 650 µg/m 2 as the safe recommended dose of KF and suggested KF peptide as a safe chemotherapeutic agent [136]. The KF-treated cancer patients showed an average overall survival of 10.8 months, non-cumulative toxicity, and no side effects such as leukopenia or thrombocytopenia when used with other compounds. However, the use of KF alone for cancer treatment has not yet been documented for advanced malignant patients.
Another marine mollusk (Dolabella auricularia) peptide, DOLA-10, currently in clinical trial phase II [138], showed no changes in tumor volume in clinical phase I trials on 22 cancer patients at a maximum dose of 300 µg/m 2 [137]. Pitot et al. recommended a dose of 400 µg/m 2 for patients requiring less than two chemotherapies and 325 µg/m 2 for patients requiring more than two chemotherapies [142]. Therefore, DOLA-10 at a dose of 400 µg/m 2 was used in clinical phase II [138]. Krug et al. studied 400 µg/m 2 of DOLA-10 in 10 NSCLC patients [138], where only 2 patients had a stable disease, 3 patients had grade-4 neutropenia, and 2 patients had grade-3 hyperglycemia. Later, Margolin and colleagues again studied DOLA-10 at 400 µg/m 2 [139] and observed the pharmacokinetic profile at the total body clearance and volume of distribution in the body at a steady state of 2.61 ± 1.9 L/h/m 2 and 28.4 ± 13 L/m 2 . The above-mentioned studies highlight the potential anticancer properties of marine animal-derived peptides; however, further clinical studies are needed to better understand their mechanism of action.

Conclusions and Future Perspectives
The isolation, purification, and characterization of bioactive peptides from marine sources have significantly increased in the last few years. The marine processing byproducts and the underutilized marine organisms, especially fish, have been highlighted as potential sources of bioactive peptides. Different methods, such as proteolytic hydrolysis, chemical hydrolysis, subcritical water hydrolysis, and autolysis, have been used to isolate peptides from marine animals, with enzymatic hydrolysis being the most used. However, the emerging techniques have not been well used. Hence, there is a need to explore the emerging innovative technologies, such as pulsed electric field, ohmic heating, and high hydrostatic pressure processing, to produce peptides with significant bioactivities against human diseases. Even though several studies have reported the bioactivities of marine-based purified peptides, more research is needed to explore their full potential for food, pharmaceutical, and cosmetic industries. While interesting studies on the use of marine animal-based antioxidant and anticancer peptides have been reported in vitro, their bioavailability, functionality, shelf life, and long-term stability need to be addressed through in vivo studies for further use and large-scale production. Furthermore, to use these peptides for human consumption as nutraceuticals, bio-functional foods, or natural additives in food and healthcare products and in biopharmaceuticals, their mechanism of action should be determined through clinical studies to better understand their physiological properties.