Escherichia coli flavodoxin sepharose as an affinity resin for cytochromes P450 and use to identify a putative cytochrome P450c17/3β- hydroxysteroid dehydrogenase interaction

Flavodoxin Sepharose (Fld Sepharose), a reagent originally developed to demonstrate an interaction between native Escherichia coli Fld and cytochrome P450c17, has been synthesized, using highly expressed (7 μmol Fld/liter E. coli culture) recombinant E. coli Fld, for use as an affinity resin for microsomal cytochromes P450. As a test of the specificity of Fld Sepharose, we have examined the utility of this resin for purification of P450c17 and P450c21 from a relatively crude mixture of solubilized adrenocortical microsomal proteins. Chromatography of this mixture on Fld Sepharose resulted in a three-fold enrichment of cytochrome P450 specific content without spectrally detectable P450 denaturation. Electrophoretic and immunoblot analyses of fractions eluted from the Fld Sepharose column revealed the presence of P450c17 and P450c21, both of which were sufficiently pure, after SDS-PAGE, for identification by N-terminal sequence analysis. Intriguingly, a major protein copurifying with P450c17 and P450c21 was identified as 3β- hydroxysteroid dehydrogenase (3β-HSD) which was subsequently found not to directly bind Fld Sepharose. Purified bovine 3β-HSD covalently linked to Sepharose can bind recombinant bovine P450c17, an interaction which is partially disrupted upon mild heat denaturation of P450c17 or by the nonionic detergent Emulgen. This interaction, however, does not appear to affect P450c17 hydroxylase and lyase activities as measured in vitro. From these results, we propose that 3β-HSD and P450c17 may associate, perhaps as part of a steroidogenic complex, in the endoplasmic reticulum.