Monoacetyldapsone inhibition of dapsone N-hydroxylation by human and rat liver microsomes

Dapsone (DDS) is metabolized by N-hydroxylation and N-acetylation to DDS hydroxylamine (DDS-NOH) and monoacetyldapsone (MAD), respectively. The activities of these two alternative and independent reactions vary widely between individuals and show an inverse relationship during chronic DDS therapy. Toxicity observed during DDS therapy has been attributed to DDS-NOH. The observation of reduced toxicity in rapid acetylators, who are also poor hydroxylators, therefore, raised the possibility that MAD may be inhibiting DDS-NOH formation. This hypothesis was tested in human and rat liver microsomes. Human liver microsomes hydroxylated DDS with a lower affinity (KM 2-fold greater) and lower maximal catalytic activity (Vmax 12-fold lower) than that of the rat. The relative catalytic activity (Vmax/KM) was 22-fold higher in rat compared with human liver microsomes. Furthermore, MAD was a potent inhibitor of DDS N-hydroxylation by rat liver microsomes (52% inhibition at 0.01 mM MAD) compared with human liver microsomes (23% inhibition at 0.4 mM MAD). Human, but not rat, liver microsomes deacetylated MAD to DDS by an NADPH independent mechanism. These results show that substantial differences exist in DDS N-hydroxylase between rats and humans, with respect to substrate affinity, enzyme activity, and susceptibility to inhibition, such that information obtained from the rat should not be extrapolated to humans. We conclude that MAD is a potent inhibitor of DDS-NOH formation in rat liver microsomes. The degree of inhibition in human microsomes, however, suggest that MAD is unlikely to be a significant modulator of enzyme activity in vivo.