Developmental biology of heme oxygenase.

The regulation of heme oxygenase activity in the developing neonate is essential to the control of bilirubin production as well as intracellular heme and hemoprotein metabolism. The coordinated activity of the microsomal enzymes, heme oxygenase and NADPH-cytochrome c (P450) reductase, and the cytosolic enzyme biliverdin reductase is responsible for the degradation of heme. The complete reaction sequence requires oxygen and NADPH, and produces bilirubin and carbon monoxide in equimolar amounts. Although heme oxygenase expresses a rather broad range of substrate affinities, the oxidative degradation of heme is exclusively alpha-specific. Heme oxygenase is found in several tissues, with significant activity levels in the liver, spleen, and erythropoeitic tissue. Heme oxygenase activity is inducible by heme and other metalloporphyrins, hormones, starvation, stress, toxins, and xenobiotics. Heme oxygenase induction is generally considered to be the result of an increased protein synthesis and gene transcription. This hypothesis is supported by recent studies of the heme oxygenase gene that identified inducer element binding sites responsive to metal administration, heat shock, and nutrient availability. In the developing fetus and neonate, hepatic heme oxygenase activity and mRNA levels are elevated above that of the adult. This suggests that the elevated heme catabolism observed in neonates may be associated with an increased transcription of the heme oxygenase gene. The apparent induction of hepatic heme oxygenase during the neonatal period is probably the result of tissue-specific and time-dependent transcriptional regulating factors including potentially hormones and heme. Several metalloporphyrins, such as the tin and zinc porphyrin complexes, inhibit heme oxygenase activity and thus have therapeutic potential for the treatment of neonatal jaundice. Recent studies suggest that the meso- and bis-glycol derivatives of these metalloporphyrins may be more potent inhibitors of heme oxygenase activity in vitro and in vivo than the protoporphyrin structures. As structural analogues of heme, however, these compounds may also have other less desirable effects on the regulation of heme and hemoprotein metabolism, particularly in the developing neonate.