Developmental expression of heme oxygenase isozymes in rat brain. Two HO-2 mRNAs are detected

Blot hybridization of RNA isolated from rat brain revealed the presence of two HO-2 homologous transcripts (1.3 and 1.9 kilobases (kb] at all stages of development ranging from 1 day before birth to adulthood. The level of both HO-2 messages appeared to be developmentally regulated and a gradual increase was observed from prenatal day 1 to adulthood. The two transcripts were highly homologous as assayed through hybridization studies using probes derived from the 5' end, middle, and 3' end of a cloned rat testis HO-2 cDNA. The 1.3-kb mRNA was essentially identical in size to the testis HO-2 cDNA. The message was efficiently translated in the brain, and is believed to encode the HO-2 protein. It seems unlikely that the 1.9-kb species represents a precursor of the 1.3-kb mRNA, as it was also translated in vivo, although less efficiently than the smaller mRNA species. Neither of the two HO-2 mRNA species were induced by bacterial endotoxin. Unlike HO-2, only one HO-1 transcript of approximately 1.8 kb could be detected. This transcript was of very low abundance and was not developmentally regulated, but could be increased by bacterial endotoxin. The product of this induced message, however, was not detectable by Western immunoblot analysis using antibody raised against liver HO-1. An immunoprecipitate could be detected in brain microsomes by radioimmunoassay using the same antibody. This protein, however, exhibited antigenic properties different from that of the purified liver HO-1 or that of spleen microsomal HO-1. Brain heme oxygenase activity correlated well with the amount of immunoreactive HO-2 protein and both reflect the abundance of the 1.3-kb mRNA message over the course of development.