Jm. Grace et al., ATYPICAL METABOLISM OF DEPRENYL AND ITS ENANTIOMER, (S)-(-N,ALPHA-DIMETHYL-N-PROPYNYLPHENETHYLAMINE BY CYTOCHROME-P450 2D6()), Chemical research in toxicology, 7(3), 1994, pp. 286-290
Debrisoquine 4-hydroxylase is a unique cytochrome P450 that effects ox
idation of protonated substrates at sites distal from the basic nitrog
en. A basic tenet of the several models that have been proposed for th
e active site of P450 2D6 is that oxidation occurs at distances of sim
ilar to 5 or similar to 7 A from the protonated site. In this study, t
he metabolism of both stereoisomers of deprenyl, a therapeutically val
uable monoamine oxidase B inhibitor, was shown to produce N-demethylat
ion 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 approximate to 13:1 over N-depro
pargylation which produced methamphetamine. The K-m and K-cat values f
or formation of methamphetamine, the minor metabolite, were 56 +/- 5 m
u M and 0.63 +/- 0.063 nmol of methamphetamine min(-1) (nmol of P450)(
-1), respectively; the K-cat for nordeprenyl formation was approximate
to 8.2 nmol of nordeprenyl min(-1) (nmol of P450)(-1). Although these
pathways would be the anticipated processes for monoamine oxidases an
d most cytochrome P450s, this mode of biotransformation is not predict
ed by current active site models and represents a novel pathway for P4
50 2D6. Statistical analysis indicates that the therapeutically import
ant L-(-)-isomer was preferentially metabolized [k(cat)/K-m (-)/(+) ra
tio 2.66]. Competitive inhibition of deprenyl metabolism by both quini
dine and quinine with an approximate 10(3) differential confirms that
this metabolic pathway is P450 2D6 mediated. We hypothesize that L-(-)
-deprenyl free base, in equilibrium with the ammonium-carboxylate salt
bridge within the active site, is released and subjected to rapid ele
ctron-transfer oxidation by the active oxygen species. The last step o
f this sequence would be facilitated by the low basicity and low redox
potential of tertiary amines. Thus, deprenyl may represent a novel cl
ass of P450 2D6 substrates that undergo an ''atypical'' 2DG-mediated N
-dealkylation process as a consequence of structural features inhibiti
ng the prototypic process and electronic features facilitating N-oxida
tion reactions.