Deuterium isotope effect studies on the MAO-B catalyzed oxidation of 4-benzyl-1-cyclopropyl-1,2,3,6-tetrahydropyridine

Previous studies have established that 1-cyclopropyl-4-phenyl-1,2,3,6-tetrahydropyridine is an efficient time- and concentration-dependent inhibitor of the flavin-containing enzyme monoamine oxidase B (MAO-B). This behavior is consistent with a proposed mechanism-based inactivation pathway which proceeds via an initial single electron transfer step to generate an unstable cyclopropylaminyl radical cation intermediate that alkylates an active site functionality via the ring opened primary carbon centered radical. More recently we have found that, in addition to being an inhibitor, the corresponding 1-cyclopropyl-4-benzyl-1,2,3,6-tetrahydropyridine species is an excellent MAO-B substrate, behavior which may not be consistent with the obligatory formation of a cyclopropylaminyl radical cation intermediate. In an attempt to gain further insight into the mechanism associated with the MAO catalyzed oxidation of 1,4-disubstituted tetrahydropyridines, we have undertaken deuterium isotope effect studies on the substrate and inhibitor properties of this 4-benzyl-1-cyclopropyltetrahydropyridine derivative. A normal isotope effect was observed on k(cat)/K-M. Although the good substrate properties of this compound prevented an accurate estimate of k(inact) and K-I, we did observe a very modest inverse isotope effect on the rate of inactivation of 0.1 mu M MAO-B by 500 mu M inactivator. The results are discussed in terms of possible mechanisms for the MAO-B catalyzed oxidation of 1,4-disubstituted 1,2,3,6-tetrahyrdopyridines.