The present investigation provides evidence of the ability of Sn-protoporphyrin to cause striking alterations in adrenal and testicular cytochrome P-450-dependent steroidogenesis and defines the potential of this metalloporphyrin to serve as a cellular toxin. Sn-protoporphyrin is currently used on an experimental basis for treatment of hyperbilirubinemias in humans, including newborn infants. Specifically, in the adrenals of rats treated with a moderate regimen of Sn-protoporphyrin (two doses of 50 mumol/kg, s.c.), marked decreases of 60 to 70% in the microsomal 21 alpha-hydroxylase and the mitochondrial 11 beta-hydroxylase activities were observed after 7 days. Concomitant with these decreases was a significant depression in the adrenal mitochondrial cytochrome P-450 content and a notable reduction (approximately 30%) in serum corticosterone levels. Similarly, in the testes, significant decreases in the microsomal and mitochondrial cytochrome P-450 contents and the microsomal 17 alpha-hydroxylase activity were observed. Serum testosterone level, however, was not decreased, reflecting an absence of decrease in side chain cleavage activity. Metalloporphyrin caused a striking decrease of 65 to 80% in the microsomal heme oxygenase activity in the testes and the adrenals, as well as significant reductions in NADPH-cytochrome P-450 reductase activity of the organs. The decrease in heme oxygenase activity, however, as suggested by Western immunoblotting, apparently resulted, to a large extent, from the loss of enzyme protein integrity rather than a competitive inhibition of activity. At the transcript level, Northern blot analysis using a full length rat testis cDNA probe for heme oxygenase-2 mRNA indicated that Sn-protoporphyrin treatment did not decrease the amount of message for either of the heme oxygenase-2 transcripts (1.3 and 1.9 Kb).(ABSTRACT TRUNCATED AT 250 WORDS)