Expression of heme oxygenase gene in rat and human liver.

Developmental changes of microsomal heme oxygenase were studied. In human fetal liver, the enzyme activity was 8 times higher than that detected in the adult liver. On the other hand, adult livers contained 4 times more cytochrome P450 than fetal livers. Elevated heme oxygenase activity in fetal liver was not due to modulators present in the microsomes. Similar to human, rat fetal liver also contained high enzyme activity which appeared to be regulated at the transcriptional level. Hybridization analysis of rat liver RNAs with cDNA for rat heme oxygenase revealed that the level of mRNA for the enzyme was 3-fold higher in the fetus than in the adult. The low cytochrome P450 content may be due in part to the high heme oxygenase activity in fetal livers.