Debrisoquine 4-hydroxylase is a unique cytochrome P450 that effects oxidation of protonated substrates at sites distal from the basic nitrogen. A basic tenet of the several models that have been proposed for the active site of P450 2D6 is that oxidation occurs at distances of approximately 5 or approximately 7 A from the protonated site. In this study, the metabolism of both stereoisomers of deprenyl, a therapeutically valuable monoamine oxidase B inhibitor, was shown to produce N-demethylation and N-depropargylation of the sole basic nitrogen in the molecule by recombinant cytochrome P450 2D6. N-Demethylation of L-(-)-deprenyl leading to nordeprenyl was favored by approximately 13:1 over N-depropargylation which produced methamphetamine. The Km and kcat values for formation of methamphetamine, the minor metabolite, were 56 +/- 5 microM and 0.63 +/- 0.063 nmol of methamphetamine min-1 (nmol of P450)-1, respectively; the kcat for nordeprenyl formation was approximately 8.2 nmol of nordeprenyl min-1 (nmol of P450)-1. Although these pathways would be the anticipated processes for monoamine oxidases and most cytochrome P450s, this mode of biotransformation is not predicted by current active site models and represents a novel pathway for P450 2D6. Statistical analysis indicates that the therapeutically important L-(-)-isomer was preferentially metabolized [kcat/Km (-)/(+) ratio = 2.66]. Competitive inhibition of deprenyl metabolism by both quinidine and quinine with an approximate 10(3) differential confirms that this metabolic pathway is P450 2D6 mediated.(ABSTRACT TRUNCATED AT 250 WORDS)