Monoamine oxidase B (MAO B) was previously shown to catalyze the decarboxylation of cis- (1) and trans-5-(aminomethyl)-3-(4-methoxyphenyl)dihydrofuran-2(3H)-one hydrochloride (2) (Silverman, R. B.; Zhou, J. J. P.; Ding, C. Z.; Lu, X. J. Am. Chem. Sec. 1995, 117, 12895-12896). By [C-14]-labeling of the aryl methoxyl group, it is now shown that the decarboxylated product is 4-(4-methoxyphenyl)butanal (7), which is in the same oxidation state as the substrate. Two other products are produced, 4-carboxy-4-(4-methoxyphenyl)butanal (8), and 5-formyl-3-(4-methoxyphenyl)dihydrofuran-2(3H)-one (9). Only 9 is an oxidation product; 7 and 8 are in the same oxidation state as the substrate (1 or 2). No products are detected under strictly anaerobic conditions. All of these products can be rationalized as arising from the formation of an a-carbon radical, generated either by single-electron amine oxidation and loss of a proton or direct hydrogen atom abstraction to 10 (Scheme 5). This intermediate then can undergo second electron oxidation and hydrolysis of the iminium ion to give 9 (the normal oxidation product). However, it also can suffer either homolytic C-O bond cleavage, decarboxylation, and electron return from the active site to give 7 or heterolytic cleavage and electron return from the active site to give 8. 5-(4-Methoxyphenyl)tetrahydrofuran-2-ol (14), an oxidation product from the intermediate that leads to 7, is not detected. These results suggest that MAO B can catalyze reversible redox reactions.