Evidence for the Cluster Model of Mitochondrial Steroid Hydroxylase System Derived from Dissociation Constants of the Complex between Adrenodoxin Reductase and Adrenodoxin

Abstract

Using biotinylated adrenodoxin and avidin–Sepharose 4B, dissociation constants for the complex between adrenodoxin reductase and adrenodoxin in the oxidized and reduced states were determined as 50 ± 11 and 296 ± 44 nM, respectively. Concentrations of adrenodoxin reductase/adrenodoxin in the matrix fraction from bovine adrenal cortex, liver, and kidney mitochondria were determined to be 20.2 ± 10.6 μM/120 ± 23 μM, 0.17 ± 0.06 μM/1.79 ± 0.24 μM, and 0.40 ± 0.23 μM/1.33 ± 0.26 μM, respectively. The calculation of the percentage of adrenodoxin reductase in the complex form in the reduced state showed that it is higher than 99% in adrenal cortex mitochondria, providing clear evidence for the cluster model for the mitochondrial steroid hydroxylase system.