title: Kinetic evidence for the involvement of multiple forms of human liver cytochrome P-450 in the metabolism of acetylaminofluorene.
abstract: The kinetics of 2-acetylaminofluorene (AAF) metabolism have been studied in liver microsomes from four human subjects over a substrate range of 0.02-200 microM. The N-hydroxylation of AAF was best described by a single enzyme system with a mean Km of 1.63 microM and a mean Vmax of 61 pmol mg-1 protein min-1. Biphasic kinetics for the 7-hydroxylation of AAF in all four subjects were observed and a two enzyme system best described the data. The mean Km and Vmax for the high affinity, low-capacity enzyme were 0.69 microM and 30 pmol mg-1 protein min-1, respectively, while for the low affinity, high capacity enzyme the mean Km was 75 microM and the mean Vmax was 286 pmol mg-1 protein min-1. In one of the four subjects studied the 5-hydroxylation of AAF was similarly resolved into a two enzyme system. The 1-hydroxylation of AAF in human microsomes was a major reaction and was best described by a single enzyme system. The 3- and 5-hydroxylations of AAF were minor metabolic pathways. Non-classical Michaelis-Menten kinetics were observed for the 9-hydroxylation of AAF and at high substrate concentrations this was the major metabolite formed. These data demonstrate the involvement of at least two forms of human cytochrome P450 in the metabolism of AAF.
mesh_terms: D015073:2-Acetylaminofluorene / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D006900:Hydroxylation; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology*
chemical_list: D007527:Isoenzymes; D003577:Cytochrome P-450 Enzyme System; D015073:2-Acetylaminofluorene