The evaluation of 1,4-benzoquinones as inhibitors of human monoamine oxidase

The monoamine oxidase (MAO) enzymes are of considerable pharmacological interest and inhibitors are used in the clinic for the treatment of major depressive disorder and Parkinson's disease. A limited number of studies have shown that the quinone class of compounds possesses MAO inhibition properties. Most notable among these is a report that 2,3,6-trimethyl-1,4-naphthoquinone (TMN), present in extracts of cured tobacco leafs, is a non-selective inhibitor of both MAO isoforms. An older study reports that 1,4-benzoquinone inhibits MAO-A and MAO-B from human synaptosomes. Both 1,4-naphthoquinones and 1,4-benzoquinone are reported to inhibit the MAOs with a reversible mode of action. Since the MAO inhibition properties of additional members of the 1,4-benzoquinone class of compounds have not yet been explored, the present study investigates a small series of four 1,4-benzoquinones which incorporate phenyl, benzyl, benzyloxy and cyclopentyl monosubstitution on C2. The 1,4-benzoquinones were found to be moderately potent MAO inhibitors with IC₅₀ values of 5.03-13.2 μM (MAO-A) and 3.69-23.2 μM (MAO-B). These values are comparable to those recorded for 1,4-benzoquinone of 4.82 μM (MAO-A) and 10.2 μM (MAO-B). Of interest however, is the finding that the 1,4-benzoquinones are irreversible inhibitors of MAO-A since prolonged incubation results in near complete inhibition, and enzyme activity is not recovered by dialysis. MAO-B is much less sensitive to inactivation by the 1,4-benzoquinones. These findings are discussed with reference to a possible mechanism by which irreversible inhibition occurs. It may be concluded that irreversible 1,4-benzoquinone-derived inhibitors may act as probes for investigating quinone reactive sites in the MAOs.