The role of non-catalytic proteins in the adrenocortical steroidogenic process is vague and speculative. Non-catalytic proteins have been implicated in a variety of ways such as regulation of cholesterol side-chain cleavage activity (the rate-limiting step in steroidogenesis), cholesterol and pregnenolone transport mechanisms, secretory processes, etc. However, to date, no non-catalytic protein has been completely isolated and characterized for the adrenal cortex. The elusive steroidogenic regulatory protein remains to be identified. The adrenal cortex of the guinea pig contains specific steroid-binding proteins which have been only partially purified and characterized: their function is as yet undetermined. For instance, there are proteins which specifically bind cholesterol, cholesterly sulfate, pregnenolone, and pregnenolone sulfate. These proteins are of great interest because the rate-limiting reaction in steroidogenesis is the conversion of cholesterol to pregnenolone or cholesteryl sulfate to pregnenolone sulfate. The pregnenolone-binding protein, which is isolated from the high speed soluble fraction, has been purified to the greatest extent, but the best current preparations still contain numerous contaminants. To further purify the pregnenolone-binding protein, two approaches are currently being used: 1) development of an affinity probe, 2) generation of antibodies to proteins electroeluted from sodium dodscyl sulfate gel slices. Several antisera have now been generated which are presently being examined by Western blot analysis as well as for interaction with the pregnenolone-binding protein using the technique of sucrose density gradient analysis. It is planned in the near future to obtain the capability to utilize high performance liquid chromatography for purifying isolated gel protein bands. In addition to purifying steroid-binding proteins, it is becoming increasingly apparent that selective phosphoproteins will need to be isolated and characterized. The latter task will be difficult and time-consuming, but will be necessary if a specific functional significance is to be ascertained for a particular phosphorylated (or dephosphorylated) protein.
