Adrenodoxin reductase

Adrenodoxin reductase is a flavoprotein enzyme that transfers electrons from NADPH to adrenodoxin. Its flavin adenine dinucleotide cofactor links a two-electron donor to a carrier that transfers one electron per reduction. The accepted IUBMB name is adrenodoxin-NADP${}^{+}$ reductase, EC 1.18.1.6.^[ref]

In the NADPH-consuming direction, the electron balance is represented byThe subscripts specify oxidized and reduced adrenodoxin. Two reductions need two electron equivalents, supplied by one NADPH molecule; counting carrier molecules without counting electrons would miss this relation.^[ref]

The reduced carrier can then deliver electrons to a mitochondrial Cytochrome P450, including the cholesterol side-chain cleavage enzyme. That enzyme's net reaction requires six reduced-adrenodoxin equivalents per pregnenolone formed, corresponding ideally to three NADPH equivalents supplied upstream.^[ref]

This separation suggests a useful measurement distinction. Observing NADPH oxidation can establish electron-donor consumption, while detecting pregnenolone establishes a downstream chemical product. The former alone does not demonstrate complete coupling through every stage. Adrenodoxin is recycled as a carrier rather than permanently consumed as six new protein molecules per product.