Relationship of the single-electron reduction potential of quinones to their reduction by flavoproteins

Abstract

The aerobic and anaerobic metabolism of a series of quinones of known single-electron reduction potential has been studied using flavoenzymes catalyzing single-electron reduction. Metabolism was more closely related to single-electron reduction potential than to structural features or lipid solubility of the quinones studied. The pattern of quinone reduction with purified NADPH-cytochrome P-450 reductase was similar to that seen with NADH: ubiquinone oxidoreductase with NADPH as the cofactor; the lower limit for reduction was a quinone single-electron reduction potential of −240 mV. The lower limit for quinone reduction with purified NADH-cytochrome b 5, reductase and NADH: ubiquinone oxidoreductase with NADH as the cofactor was a single-electron reduction potential of −170 mV. With all three enzymes there was a decreased quinone metabolism at higher single-electron reduction potentials. The same pattern of quinone metabolism was seen using purified or microsomal NADPH-cytochrome P-450 reductase and purified or microsomal NADH-cytochrome b 5, reductase respectively. Microsomal quinone metabolism under aerobic conditions showed an increased V max and an unchanged K m , compared to metabolism under anaerobic conditions.