title: Biochemical characterization of hepatic microsomal leukotriene B4 hydroxylases.
abstract: omega-Hydroxylation of leukotriene B4 (LTB4) has been reported in human and rodent polymorphonuclear leukocytes; preliminary information indicates that this metabolism is cytochrome P-450 dependent. Therefore, these studies were initiated to characterize the cytochrome P-450-dependent metabolism of LTB4 in other tissues. LTB4 was metabolized by rat hepatic microsomes to two products, 20-hydroxy(omega)-LTB4 and 19-hydroxy(omega-1)-LTB4. The formation of these metabolites was both oxygen and NADPH dependent indicating that a monooxygenase(s) was responsible for these reactions. The apparent Km and Vmax for LTB4 omega-hydroxylase were 40.28 microM and 1202 pmol/min/mg of protein, respectively. In contrast, the apparent Km and Vmax for LTB4 (omega-1)-hydroxylase were 61.52 microM and 73.50 pmol/min/mg of protein, respectively. Both LTB4 omega- and (omega-1)-hydroxylases were inhibited by metyrapone in a concentration-dependent fashion. However, SK&F 525A inhibited LTB4 (omega-1)- but not omega-hydroxylase. In contrast, alpha-naphthoflavone decreased LTB4 omega- but not (omega-1)-hydroxylase activities. The differences in the Km apparent for substrate as well as the differential inhibition by inhibitors of cytochrome P-450 suggest that the omega- and (omega-1)-hydroxylations of LTB4 in hepatic microsomes are mediated by different isozymes of P-450. Furthermore, several additional characteristics of LTB4 hydroxylases indicate that these isozymes of P-450 may be different from those which catalyze similar reactions on medium-chain fatty acids, such as laurate and prostaglandins.
mesh_terms: D000818:Animals; D001571:Benzoflavones / Q000494:pharmacology; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D000072470:Cytochrome P450 Family 4; D007700:Kinetics; D007975:Leukotriene B4 / Q000378:metabolism*; D008297:Male; D013058:Mass Spectrometry; D008748:Methylcholanthrene / Q000494:pharmacology; D008797:Metyrapone / Q000494:pharmacology; D008862:Microsomes, Liver / Q000201:enzymology*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D010634:Phenobarbital / Q000494:pharmacology; D011335:Proadifen / Q000494:pharmacology; D011743:Pyrimidines / Q000494:pharmacology; D051381:Rats; D011916:Rats, Inbred F344; D013696:Temperature
chemical_list: D001571:Benzoflavones; D011743:Pyrimidines; D007975:Leukotriene B4; D008748:Methylcholanthrene; C011512:alpha-naphthoflavone; C006253:pirinixic acid; D003577:Cytochrome P-450 Enzyme System; D011335:Proadifen; D006899:Mixed Function Oxygenases; D000072470:Cytochrome P450 Family 4; C040780:leukotriene B4 20-hydroxylase; D010634:Phenobarbital; D008797:Metyrapone