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Adrenodoxin is an iron-sulfur cluster-containing electron-transfer protein involved in mitochondrial steroid synthesis. It transfers reducing equivalents to mitochondrial Cytochrome P450 enzymes, including the cholesterol side-chain cleavage enzyme. Its cluster is of the $[2\mathrm{Fe}-2\mathrm{S}]$ type; the structural presence of that cofactor and its interaction with a partner can be investigated separately from catalytic turnover.^[ref]

The electron-transfer chain distinguishes the donor from the enzyme doing the substrate conversion. NADPH supplies electrons upstream through adrenodoxin reductase; reduced adrenodoxin then supplies one-electron equivalents to the cholesterol side-chain cleavage enzyme.^[ref]

The fully coupled overall reaction uses six reduced adrenodoxin equivalents per pregnenolone. Thus an original ideal bookkeeping example producing $2\mu \mathrm{mol}$ of pregnenolone needs $12\mu \mathrm{mol}$ of one-electron donor equivalents. This does not mean twelve micromoles of fresh protein must be consumed: a carrier can be reduced again and reused.

A 2011 study also changed adrenodoxin conditions while measuring carbon monoxide access to reduced haem spectroscopically. This probes an effect of the redox partner on the catalytic protein, beyond merely locating two structures next to one another.^[ref]

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.