Influence of sex and inducer treatment on the high- and low-affinity forms of hepatic microsomal erythromycin N-demethylase in rats.

To study the regulation of the multiple forms of erythromycin N-demethylase, we determined the influence of sex and inducer treatment on this mixed-function oxidase activity in adult Wistar rats injected intraperitoneally once daily for 4 days with dexamethasone, pregnenolone-16 alpha-carbonitrile, phenobarbital, or 2% Tween 80 (control). Based on the results from a computer curve-fitting procedure (ENZFITTER) as well as Eadie-Hofstee and Lineweaver-Burk plots, at least two forms of erythromycin N-demethylase were present in control, dexamethasone, pregnenolone-16 alpha-carbonitrile, and phenobarbital-treated male rats and in control and dexamethasone-treated female rats. Only a high-affinity form was apparent in pregnenolone-16 alpha-carbonitrile and phenobarbital-treated female rats. Therefore, more than one form of erythromycin N-demethylase exists in hepatic microsomes from adult rats, depending on sex and inducer treatment. As well, at a substrate concentration commonly used in the erythromycin N-demethylase assay, the relative contribution of the high- and low-affinity forms to the enzyme activity varies with sex and inducer treatment.