CPR-Centered Redox Interactions Between Cytochrome P450 Oxidoreductase and Cytochrome c Oxidase: Potential...

The intracellular pool of nicotinamide adenine dinucleotide phosphate (NADPH) is a limited resource for which multiple enzyme systems compete. Among these systems, microsomal NADPH-cytochrome P450 oxidoreductase (CPR, POR) plays a central role in electron transfer to cytochrome P450 enzymes and can also reduce other redox-active electron acceptors in vitro. This review analyzes the molecular basis of competition between CPR-mediated cytochrome P450 reduction and cytochrome c reduction in vitro and discusses whether this can be extrapolated to an indirect interplay between CPR and cytochrome c oxidase (COX) in intact cells. In vitro, reduced cytochrome c (Fe²⁺) acts as a competitive inhibitor of NADPH-dependent cytochrome P450 reduction, apparently by binding to an overlapping site on CPR. However, CPR and COX are physically separated by subcellular compartmentalization, and cytosolic and mitochondrial NADPH pools are largely separate; therefore, CPR and COX should not be considered direct competitors for a common NADPH pool under physiological conditions. Potential pharmacological implications include modulation of drug metabolism, cellular energy homeostasis, and toxicity when both systems are challenged, but these remain hypothetical and require direct flux measurements in intact cells.