Resolution of the rat brain heme oxygenase activity: absence of a detectable amount of the inducible form (HO-1).

In the present study we report on the detection of a distinct pattern of heme oxygenase isoform composition in the rat brain. In this organ only the noninducible form of heme oxygenase, HO-2, could be clearly detected. This pattern of composition distinguishes the brain from other organs tested to date, namely the liver, testis, and spleen. The rat brain microsomal fraction displayed a rather impressive rate of heme oxygenase activity. This fraction also exhibited a rate of NADPH-cytochrome P-450 reductase activity that was sufficient to fully support the oxygenase activity. The brain microsomal fraction was solubilized and subjected to ion-exchange chromatography on DEAE-Sephacel. The chromatographic elution pattern of heme oxygenase activity was compared with those of the liver and testis. In the brain only one peak of heme oxygenase activity was detected. The peak exhibited an elution profile similar to that of HO-2 of the liver and the testis. The presence of an activity peak was not detected in the elution profile at the region where the inducible isoform of heme oxygenase, HO-1, was expected. Cross-reactivity was observed between the solubilized brain microsomal fraction and antiserum to the testis HO-2 when subjected to Ouchterlony double diffusion immunoanalysis. A reaction was not observed when antiserum to liver HO-1 was employed. The presence of HO-2 in the brain microsomal preparation was also established by Western immunoblotting analysis. A protein having a mobility that was identical to the purified testicular HO-2 (Mr 36,000) was present in the brain microsomal preparation when probed with antiserum to HO-2. However, our attempts to demonstrate the presence of HO-1 in the brain microsomal preparation by a similar technique, but using antiserum to HO-1, were not successful. It is proposed that HO-2 is responsible for the bulk, if not all, of the brain microsomal heme oxygenase activity. It is further proposed that tissue-specific regulatory mechanisms are responsible for both the refractory response of the brain heme oxygenase to known metallic inducers and the absence of a detectable amount of the HO-1 isoform.