MICROSOMAL HEMEPROTEINS P-450LM-2 AND P-450LM-4:COMPARATIVE STRUCTURAL STUDIES*

ABSTRACT Although the two major hemeproteins of rabbit liver microsomes, termed P-450 LM-2 and P-450 LM-4 , are readily distinguishable by their differences in size, substrate specificity, and immunochemical properties, there are nevertheless indications of extensive structural similarity between them. Except for higher values of cysteine and tryptophan in P-450 LM-4 the amino acid compositions correlate quite well. The BrCN digest of P-450 LM-4 shows much higher cross reactivity with anti P-450 CAM antibodies than does the parent protein; similar small hemepeptides were isolated from the BrCN digests of P-450 LM-2 and P-450 LM-4 . Photoaffinity labeling of both hemeproteins with 3′-azidophenobarbital at their substrate binding sites and subsequent digestion with trypsin yielded labeled peptides of similar composition; these labeled peptides were shown to be part of the respective hemepeptides. Comparative peptide mapping of the tryptic digests of the proteins revealed substantial similarity of overall peptide patterns.