Monoamine oxidase inhibitory constituents of propolis: kinetics and mechanism of inhibition of recombinant human MAO-A and MAO-B

Abstract

Propolis is the resinous material that bees gather from leaf buds, flowers and vegetables. Propolis extracts contain constituents with a broad spectra of pharmacological properties and are important ingredients of popular dietary supplements. Propolis extracts were evaluated in vitro for inhibition of recombinant human monoamine oxidase (MAO)-A and MAO-B. The dichloromethane extract of propolis showed potent inhibition of human MAO-A and MAO-B. Further fractionation identified the most active fractions as rich in flavonoids. Galangin and apigenin were identified as the principal MAO-inhibitory constituents. Inhibition of MAO-A by galangin was about 36 times more selective than MAO-B, while apigenin selectivity for MAO-A vs. MAO-B was about 1.7 fold. Apigenin inhibited MAO-B significantly more potently than galangin. Galangin and apigenin were further evaluated for kinetic characteristics and the mechanism for the enzymes' inhibition. Binding of galangin and apigenin with MAO-A and -B was not time-dependent and was reversible, as suggested by enzyme-inhibitor binding and dissociation-dialysis assay. The inhibition kinetics studies suggested that galangin and apigenin inhibited MAO-A and -B by a competitive mechanism. Presence of prominent MAO inhibitory constituents in propolis products suggests their potential for eliciting pharmacological effects that might be useful in depression or other neurological disorders. The results may also have important implications in drug-dietary supplement interactions.

Figure 1

Structure of galangin (1) and apigenin (2).

Figure 2

Kinetic characteristics of inhibition of recombinant human MAO-A with [A] galangin and [B] apigenin; V = nmoles/min/mg protein and S = substrate kynuramine concentration (μM).

Figure 3

Kinetic characteristics of inhibition of recombinant human MAO-B with [A] galangin and [B] apigenin; V = nmoles/min/mg protein and S = substrate kynuramine concentration (μM).

Figure 4

Time-dependent inhibition of recombinant human MAO-A by galangin (0.25 μM), apigenin (1.2 μM) and clorgyline (7.5 nM). The remaining activity was expressed as % of activity. Each point represents mean ± S.D. of triplicate values.

Figure 5

Time-dependent inhibition of recombinant human MAO-B by galangin (14.0 μM), apigenin (4.0 μM) and l-deprenyl (0.050 μM). The remaining activity was expressed as % of activity. Each point represents mean ± S.D. of triplicate values.

Figure 6

Analysis of nature of binding of galangin and apigenin with recombinant human MAO- A by recovery of catalytic activity of the enzyme after dialysis dissociation. Each bar shows mean ± S.D. of triplicate values.

Figure 7

Analysis of nature of binding of galangin and apigenin with recombinant human MAO- B by recovery of catalytic activity of the enzyme after dialysis dissociation. Each bar shows mean ± S.D. of triplicate values.