Polyphosphoinositides:stimulator of mitochondrial cholesterol side chain cleavage and possible identification as an adrenocorticotropin-induced, cycloheximide-sensitive, cytosolic, steroidogenic factor.

ACTH treatment in vivo enhances the activity of adrenal cytosol for supporting pregnenolone synthesis in adrenal mitochondria. Upon fractionation of adrenal cytosol by gel filtration through Sephadex G200, pregnenolone synthesis-stimulating activity is eluted just after 3H2O(Kd = 1.1--2.1). This late eluting steroidogenic activity is enhanced by ACTH treatment and diminished by cycloheximide treatment, which also inhibits adrenal protein synthesis and ACTH-induced steroidogenesis. Chromatography on Sephadex G-10 yields even further separation of steroidogenic factor(s) from inactive substances; it appears that the active factor(s) adsorbs to Sephadex and that such adsorption is enhanced by a greater degree of cross-linking in the gel. The phospholipids, cardiolipin, diphosphoinositide, and triphosphoinositide, are recovered partly in adrenal cytosol and, by virtue of their polyphosphorylated head group, specifically stimulate mitochondrial pregnenolone synthesis. It seems likely that the ACTH effect on cytosolic steroidogenic activity is due to changes in polyphosphoinositides, since these phospholipids are specifically stimulated by ACTH, cycloheximide blocks the ACTH-induced increase in polyphosphoinositides, and these phospholipids comigrate with steroidogenic activity during gel filtration of adrenal cytosol. These findings along with other findings from our laboratory suggest that polyphosphoinositides may function as a cycloheximide-sensitive mediator in the steroidogenic action of ACTH.