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The oxidative metabolism of disopyramide (DIS) enantiomers to mono-N-dealkyldisopyramide in mouse hepatic microsomes was studied in vitro. When (R-)-DIS was used as a substrate Lineweaver-Bulk plot was a single line with the apparent Km value of 0.23 mM and Vmax value of 1.05 nmol/mg protein/min. When S(+)-DIS was used as a substrate the plot was biphasic, and two apparent Km values (Km1 = 0.04 and Km2 = 0.77, mM) and two Vmax values (Vmax1 = 0.58 and Vmax2 = 1.53, nmol/mg protein/min) were obtained. When racemic DIS was used as a substrate the plot was also biphasic, while the plot observed at the lower concentration region was shifted to the upper region in comparison with that estimated from the sum of the individual N-dealkylase activities of each enantiomer. These results suggest that the N-dealkylation of R(-)-DIS is carried out by very limited cytochrome P-450 isozyme(s), but that of S(+)-DIS is carried out by multiple cytochrome P-450 isozymes; that is, N-dealkylations of R(-)- and S(+)-DIS are involved in the different cytochrome P-450 isozymes each other, particularly at relatively higher concentration of the substrates.

Stereoselectivity in N-Dealkylation of Disopyramide Enantiomers in Mouse Hepatic Microsomes