Immunoquantification of epoxide hydrolase and cytochrome P‐450 isozymes in fetal and adult human liver microsomes

Epoxide hydrolase and three cytochrome P‐450 isozymes were immunochemically determined in microsomes from adult and fetal human liver and tentatively correlated with some enzyme activities. The P‐450 isozymes 5, 8 and 9 present in adult liver could not be positively correlated with the total cytochrome P‐450 concentration spectrophotometrically determined. In fetal liver microsomes, isozyme 8 could not be detected by either electrophoretic or immunochemical procedures. Isozyme 5 was the major isozyme present in the fetal liver and its concentration increased in close relation with the total P‐450 level. As shown previouly, arylhydrocarbon hydroxylase activity was related to the concentration of isozyme 8 in adult liver. In fetal preparations, the absence of isozyme 8 was associated with a very low arylhydrocarbon hydroxylase activity. Aldrin epoxidase and benzphetamine‐N‐demethylase activities were correlated with isozyme 5 concentration, but with different slopes for adult and fetal microsomes: adult preparations catalyzed these two reactions more efficiently. Conversely, the dehydroepiandrosterone 16β‐hydroxylase, also associated with isozyme 5 concentration, was more active in fetal than in adult microsomes. Moreover, if acetanilide hydroxylase increased with isozyme 5 concentration in adult samples, no correlation occurred between activity and P‐450 isozyme level in fetal microsomes. Hydroxylations of lauric acid in positions 11 and 12 and of dehydroepiandrosterone in position 16α increased with total P‐450 concentration but not with isozyme concentrations whatever the age considered. Lastly, epoxide hydrolase activity toward benzopyrene 4,5‐oxide was closely associated with its immunochemically determined level. These results clearly suggest that multiple mechanisms are involved in the regulation of different drug‐metabolizing enzymes in the human fetus.