The results presented in this paper demonstrate inhibition of MAO-A and MAO-B as an important pharmacological action of the propolis extracts. A broad spectrum of pharmacological properties, such as anticancer/antitumor, antimicrobial, immunomodulatory, antioxidant and neuroprotective activities, has been exhibited by propolis extracts and preparations [
3,
4,
5]. The broad spectrum of pharmacological and biological properties in propolis preparations may be attributed to the more than 300 natural identified constituents [
6,
24,
25]. The prominent groups of natural products identified in propolis are polyphenols, phenolic aldehydes, coumarins, amino acids and steroids [
1,
2,
25]. The predominant MAO-A inhibitory property of propolis extracts and fractions indicates their possible effect in elevating the levels of biogenic monoamines. A recent study has also reported inhibition of crude MAO activity in rat liver mitochondrial preparations [
26]. These results suggest that use of propolis preparations along with the diet rich in dietary monoamines (most prominently tyramine) may conceivably increase levels of the dietary monoamines in circulation. The protein containing foods have naturally low levels of tyramine, but aging and storage of these foods, for the purpose of developing good taste, results in an increase in levels of tyramine in these foods such as aged cheeses, cured meats, fermented cabbage, soya sauce, fish sauce, yeast extract spreads and broad bean pods. Consumption of these foods is contraindicated with use of irreversible MAO inhibitors (MOAI) particularly for the treatment of depressive disorders [
27,
28].
The original propolis extract showed 10 fold more selectivity for inhibition of MAO-A than MAO-B Further, bioassay guided fractionation identified a fraction rich in flavonoids. Among these flavonoids identified in the active propolis fraction 2, galangin and apigenin were found to be the prominent MAO inhibitory constituents of propolis. Identity of apigenin and galangin in active propolis fraction (fraction #2) was further confirmed with HRESIMS. Previous studies have also reported apigenin and galangin in propolis [
29,
30,
31]. Both galangin and apigenin were significantly less potent against MAO-B as compared to MAO-A. The 30-fold selectivity of galangin for MAO-A and its presence at 13% in propolis extracts indicates that galangin may contribute to selective inhibition of MAO-A by propolis extracts. MAO binding of these constituents with MAO-A and B was competitive with the non-specific substrate (kynuramine), time-independent and reversible. Galangin and apigenin are naturally occurring flavones; commonly present in several plants and fruits. Earlier studies have also demonstrated MAO inhibitory action of flavonoids [
32]. However, this is the first report on inhibition of MAO-A and -B with galangin. Galangin is commonly used as a food additive and has been shown to have broad spectrum of pharmacological activities such as induction of apoptosis and autophagy in HepG2 cells [
33], protection against ischemic injury and potential neuroprotective activity [
34], anti-inflammatory properties via negative regulation of NF-κB [
35], alleviation of liver damage through inhibition of liver fibrosis [
36], attenuation of mast cell mediated allergic inflammation [
37], anti-obesity effect through inhibition of pancreatic lipases [
38], prevention of osteoclastic bone destruction and osteoclastogenesis [
39] and antiparasitic/antibacterial activities. Similarly, potential biological effects reported earlier with apigenin include antioxidative, anti-inflammatory and anticancer and cancer prevention activities [
40]. The significant MAO inhibitory properties of galangin and apigenin as reported herein suggest additional potential utility of these two flavones for alleviation of neurological disorders associated with depletion of monoamines, the most important of which are depression and Parkinson’s disease. The reversible and selective inhibition of MAO-A by propolis extracts and galangin may be important for their potential use in treatment resistant depression (TRD), implicated in 33%–57% cases of depression [
41]. TRD is currently treated by electroconvulsive therapy (ECT) or augmentation of serotonin selective reuptake inhibitors (SSRIs) with antipsychotics. Use of ECT and antipsychotics are associated with safety and tolerability concerns. Depression is hypothesized to result from the deregulation and depletion of monoamine neurotransmitters possibly due to hyperactive MAO-A. Reversible monoamine oxidase-A inhibitors (RIMAs) have shown efficacy in treatment of depression, with better safety and tolerability compared to SSRIs [
41,
42]. With regard to the MAO inhibitory property of galangin and apigenin reported in this study, previous studies have also reported MAO inhibitory activities of a few other flavonoids [
43,
44,
45]. However, the results reported earlier are highly variable due to different enzyme sources and preparations employed in the different studies. Most of these studies were conducted with crude mitochondrial preparations from rat brain, which may not be suitable for determination of selective effect of the test compound on specific MAO isoforms. Apigenin has been shown to inhibit total MAO (A & B) in mouse brain crude mitochondrial preparation with IC
50 of 1 μM. Further, inhibition of MAO-A by apigenin was about 7.5-fold more potent than of MAO-B in these preparations as determined by use of iso-enzyme specific enzyme substrates. The results presented here clearly indicate selectivity of active propolis fractions and the compounds identified from the active fractions.
Potent MAO-A and MAO-B inhibitory property of galangin and apigenin may be important for application of propolis products/extracts for management of neurological disorders, which are associated with depletion of biogenic monoamines. However, issues related to bioavailability of flavonoids, specially limited bioavailability to the brain, have to addressed for their use in treatment or improvements of neurological disorders. Apigenin has also been shown to have a unique monoamine transporter activator activity [
46] and stimulate adult neurogenesis
in vivo and
in vitro, by promoting neuronal differentiation [
47]. This study has also shown that apigenin promoted learning and memory performance in the Morris water task. Based on the previous reports, together with the results reported herein supports the potential application of propolis dietary supplements, consisting of two potent MOAI active flavones galangin and apigenin, in the management of neurological disorders. The results reported in this study suggest the development of propolis preparation standardized for their apigenin and galangin constituents as potential treatments for depression and other neurological disorders.