Chemical suppression of steroidogenesis.

A large number of chemicals are known to interfere with steroidogenesis in the adrenal cortex and other tissues. Many xenobiotics inhibit steroid hormone production as a result of interactions with cytochrome P-450-containing hydroxylases in adrenal mitochondria or microsomes. For example, metyrapone, a compound used clinically in the evaluation of pituitary-adrenocortical function, binds to various cytochromes P-450 in the adrenal, preventing the interactions of steroid substrates with the enzymes and inhibiting steroidogenesis. The mineralocorticoid antagonist, spironolactone, and its major circulating metabolite, canrenone, also competitively interact with adrenal steroid hydroxylases. In addition, spironolactone is converted by adrenal microsomes to an unknown metabolite which promotes the destruction of cytochromes P-450, decreasing the activities of steroid hydroxylases. Carbon tetrachloride is similarly "activated" by adrenal microsomal mixed function oxidases resulting in a decline in steroidogenic enzyme activity. Carbon tetrachloride (in the presence of NADPH) initiates lipid peroxidation in adrenal microsomes but its toxic effects on steroid hydroxylases are fully demonstrable when lipid peroxidation is inhibited by EDTA. A number of heavy metals, including cadmium, also inhibit adrenal steroid hydroxylases. When incubated with adrenal microsomes, cadmium does not affect cytochrome P-450 levels but decreases basal and substrate stimulated NADPH-cytochrome P-450 reductase activity. Although inhibitory effects of many chemicals on steroidogenesis have been described, the toxicological significance as well as definitive mechanisms of action have in most cases yet to be determined.