CLASSIFICATION OF FUMARATE REDUCTASES AND SUCCINATE DEHYDROGENASES BASED UPON THEIR CONTRASTING BEHAVIOR IN THE REDUCED BENZYLVIOLOGEN FUMARATE ASSAY

Reduction of fumarate by soluble beef heart succinate dehydrogenase ha s been shown previously by voltammetry to become increasingly retarded as the potential is lowered below a threshold potential of -80 mV at pH 7.5. The behaviour resembles that of a tunnel diode, an electronic device exhibiting the property of negative resistance. The enzyme thus acts to oppose fumarate reduction under conditions of high thermodyna mic driving force. We now provide independent evidence for this phenom enon from spectrophotometric kinetic assays. With reduced benzylviolog en as electron donor, we have studied the reduction of fumarate cataly sed by various enzymes classified either as succinate dehydrogenases o r fumarate reductases. For succinate dehydrogenases, the rate increase s as the concentration of reduced dye (driving force) decreases during the reaction. In contrast, authentic fumarate reductases of anaerobic cells (and 'succinate dehydrogenase' from Bacillus subtilis) neither exhibit the electrochemical effect nor deviate from simple kinetic beh aviour in the cuvette assay. The 'tunnel-diode' effect may thus repres ent an evolutionary adaptation to aerobic metabolism.