Laurate omega-hydroxylase activity of human liver microsomes was strongly inhibited by an antibody against rabbit fatty acid omega-hydroxylase P450 4A5, and Western blot analysis with this antibody showed the presence of two immunochemically related proteins with apparent molecular weights of approximately 50 and 52 kDa in all of 14 human liver specimens examined. A fatty acid omega-hydroxylase (designated P450HL omega) was purified to a specific content of 15 nmol of P450/mg of protein from microsomes of a single human liver on the basis of its laurate omega-hydroxylase activity and its reactivity with the P450 4A5 antibody. This P450HL omega showed an apparent molecular weight of 52 kDa on SDS-PAGE. Furthermore, a cDNA clone (designated HL24) has been isolated from a human liver cDNA library by using the cDNA for P450 4A5 as a probe. The sequence of residues 5 through 25 deduced from cDNA HL24 was identical to the NH2-terminal amino acid sequence of P450HL omega except for one undetermined residue. This cDNA encoded a protein of 519 amino acids with a molecular weight of 59,347. The amino acid sequence predicted from the cDNA showed 82% identity with that of P450 4A5. Northern blot analysis showed that the mRNA hybridized to the cDNA is expressed in the human liver and kidney. (ABSTRACT TRUNCATED AT 250 WORDS)