Fig. 2.

Electron transport chains and protein components of the vitamin D hydroxylases. (A) In mitochondria, NADPH is oxidized by the flavoprotein ferredoxin reductase, which transfers single electrons through a pool of ferredoxin iron-sulfur proteins to the mitochondrial P450s on the inner membrane. (B) In the endoplasmic reticulum, electron equivalents from NADPH are captured by the NADPH P450 reductase (also known as P450 oxidoreductase, POR). The two electrons from NADPH are transferred sequentially to the microsomal P450 (e.g., CYP2R1). (Used from Ref. 14 with permission.)