We report on the detection and characterization of two forms of heme oxygenase in rabbit tissues and provide data suggesting that heme oxygenases in rat and rabbit are not identical and constitute a group of heterogenous proteins. Certain molecular properties, however, are shared by the isozymes in rat and rabbit; the predominant form of the enzyme in control liver and testis is HO-2, in the liver HO-1 is the inducible form, and in the brain HO-1 is not detectable.

HO-1 was purified from liver of rabbits treated with bromobenzene to near homogeneity with a specific activity of 8,270 nmol of bilirubin/mg/h and compared with a homogenous preparation of rat HO-1 with a specific activity of 6,220, also obtained from bromobenzene-treated animals. Rat and rabbit HO-1, on sodium dodecyl sulfate-polyacrylamide gel, had molecular weights of 30,000 and 30,700, respectively. Rabbit HO-2 was partially purified from testis to a specific activity of 386 nmol of bilirubin/mg/h and compared with a purified preparation of rat testis HO-2 with a specific activity of 5,700. Using Western immunoblotting, rabbit HO-2 displayed intense cross-reactivity with antibody raised in rabbit to sodium dodecyl sulfate-denatured rat HO-2, and had a substantially larger molecular weight than the rat HO-2 (42,000 versus 36,000). Rabbit HO-1 did not cross-react with antibody to rat HO-1 which was also raised in rabbit. Unlike the rat enzymes, rabbit HO-1 and HO-2 did not differ in thermolability. It is speculated that HO-1 in rat and rabbit, and possibly HO-2, have evolved from divergent evolution of a common ancestral gene(s).