In vitro evidence against the oxidation of quinidine by the sparteine/debrisoquine monooxygenase of human liver.

Competitive inhibition studies using human liver microsomes have shown that quinidine (QD) has an exceptionally high affinity (60 nM) for the genetically variable cytochrome P-450 that catalyzes the formation of 4-hydroxydebrisoquine and dehydrosparteines from debrisoquine and sparteine. The present study examined the effect of sparteine and debrisoquine on the oxidation of QD by microsomes prepared from two human livers. QD and its major metabolite 3-hydroxy-QD were measured by quantitative TLC. QD 3-hydroxylation followed saturable single-site kinetics over a 1-250 microM range of QD concentrations. The Km and Vmax of the reaction in the two liver specimens were 47.5 +/- 3.5 microM and 58.7 +/- 5.9 microM, and 0.36 +/- 0.08 and 0.29 +/- 0.02 nmol of 3-hydroxy-QD/mg of protein/min. Sparteine and debrisoquine (250 microM) had no effect on this QD 3-hydroxylase activity. Furthermore, near-saturation of the sparteine/debrisoquine isozyme by 250 microM sparteine had no effect on the oxidation of QD by all routes (measured by QD disappearance from an initial level of 70 nM during an 8-hr incubation period). These observations indicate that none of the major oxidative reactions of QD are catalyzed by the sparteine/debrisoquine isozyme; QD may simply bind to this cytochrome P-450, without being oxidized by it.