= 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.<Enzyme Nomenclature by the IUBMB>[EC 1.18.1.6, Reaction and Comments]{ref}
In the NADPH-consuming direction, the electron balance is represented by
$$
\mathrm{NADPH+2Adx_{ox}\longrightarrow NADP^++H^++2Adx_{red}}.
$$
The 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.<Enzyme Nomenclature by the IUBMB>[EC 1.18.1.6, reaction written in reverse]{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.<Cholesterol side-chain cleavage enzyme in IUBMB Enzyme Nomenclature>[EC 1.14.15.6, overall reaction and electron source]{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.
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