The ability of synthetic metalloporphyrins to suppress heme oxygenase activity and bilirubin formation has recently become of considerable clinical and experimental interest for suppression of jaundice in humans, including neonatal hyperbilirubinemia. The present investigation compares the biochemical effects of Sn- and Znprotoporphyrins on the predominant heme oxygenase iso-zyme present in the brain (HO-2) at activity, protein, and transcript levels and describes the ability of Sn-protopor-phyrin to adversely affect this isozyme. Specifically, 6 h after a modest dose (50 μmol/kg, i.v.) of Sn-protoporphyrin, heme oxygenase activity in rat brain was nearly undetectable. In addition, as revealed by Western blot analysis, HO-2 protein level was decreased by 20% and the electrophoretic behavior of the protein in the microsomal membranes was altered. Moreover, the activity of NADPH-cytochrome P-450 reductase, which is required for the oxidation of heme molecule, was markedly decreased (60% of control). Western immunoblot analysis revealed also a pronounced decrease in the reductase protein level. The inducible form of heme oxygenase, HO-1, was not detectable by immunoblotting in brain microsomes of either control or Sn-protoporphyrin-treated animals. Northern blot analyses did not reveal decreases in the levels of the single HO-1 mRNA (1.8 kb) or the two HO-2 transcripts (1.3 and 1.9 kb), suggesting that Sn-protoporphyrin mediates its effects on heme oxygenase isozymes at the protein level. Zn-protoporphyrin, on the other hand, had no deleterious effect on brain parameters presently investigated. The data suggest that Sn-protoporphyrin exerts its inhibitory effects on heme oxygenase activity in the brain through multiple mechanisms involving direct effect on HO-2 protein and its cellular level, inhibiting NADPH-cytochrome P-450 reductase activity, decreasing the cellular content of the reductase, as well as inhibiting heme oxygenase activity by serving as a competitive inhibitor. We suggest that Zn-protoporphyrin may be the safer alternative if metalloporphyrins are to be used to control hyperbilirubinemia. (Pediatr Res 32: 324–329, 1992)