Cholesterol side-chain cleavage in the rat adrenal cortex: isolation of a cycloheximide-sensitive activator peptide.

A cytosolic peptide activator (Mr approximately equal to 2,200) of cholesterol side-chain cleavage in the adrenal cortex has been isolated from normal corticotropin-treated rats and from rats implanted with the MtT/F4 corticotropin-secreting pituitary tumor. The isolation techniques were those common to peptide hormone purification, including tissue extraction into a highly acidic medium, gel filtration, and reverse-phase HPLC. The amino acid composition has been determined on acid hydrolysates. The activity of this adrenal peptide is acutely increased in hypophysectomized animals treated with corticotropin, and this increase is blocked by cycloheximide. The addition of activator peptide to adrenal mitochondrial preparations results in a rapid stimulation of pregnenolone formation that is dependent on activator concentration and a source of NADPH. In the absence of NADPH, addition of activator peptide to adrenal mitochondria increases the rate of cholesterol association with side-chain cleavage cytochrome P-450. The peptide therefore exhibits properties that are believed to characterize the hypothetical corticotropin-dependent labile activator of adrenal steroidogenesis.