Electron transfer reactions catalyzed by vitamin B 12 and related compounds: The reduction of dyes and of riboflavin by thiols

Abstract

The reduction of electron acceptors, such as methylene blue (MB) or ribofiavin, by thiols and other electron donors is catalyzed by traces of vitamin B 12 derivatives, e.g., vitamin B 12a, cyanocobalamin, and Factor B. The catalytic reaction with thiols as reducing agents is of zero order with respect to MB. Rate determining is the reduction of the cobalamin from the Co(III) to the Co(II) state by the mercaptide ion. Various cobalamin model compounds, e.g., cobalt phthalocyanine or cobalt porphyrins, are also active. Agents which compete with the mercaptide ion for coordination sites on cobalt act as inhibitors. Alkylcobalamins and 5′-deoxyadenosyl(dimethylben-zimidazolyl)cobamide coenzyme show little catalytic activity around pH 4–5 but become active at higher pH due to the cleavage of the Co-C bond by mercaptide ion. Since Co-alkylation occurs in vitamin B 12 solutions containing thiols at pH 6–7, strong inhibiting effects are observed on addition of n-alkyl halides to the thiolcobalamin reaction solutions. The extreme sensitivity of the catalytic effects observed suggests that cobalamin derivatives may catalyze in vivo electron transfer reactions at the low physiological concentration level.