Structure-activity relationships in the oxidation of benzylamine analogues by bovine liver mitochondrial monoamine oxidase B.

The influence of para and meta substitution of benzylamine on its interaction with bovine liver mitochondrial monoamine oxidase B (MAO B) has been investigated by steady-state and reductive half-reaction anaerobic stopped-flow kinetic approaches. Steady-state kinetic properties of each benzylamine analogue suggest that para or meta substitution does not alter the mechanistic pathway of catalysis [Husain, M., et al. (1982) Biochemistry 21, 595-600]. All analogues tested exhibited Dkcat values ranging from 5.5 to 8.9 and D[kcat/Km(amine)] values ranging from 3.3 to 8.1 D[kcat/Km(O2)] values of approximately 1 are observed for all substrate analogues. Values for Kd were calculated from steady-state isotope effect data [Klinman, J.P., & Matthews, R.G. (1985) J. Am. Chem. Soc. 107, 1058-1060] and are in good agreement with Ks values determined from analysis of the rate of MAO B reduction as a function of benzylamine analogue concentration in reductive half-reaction experiments. A linear correlation of benzylamine analogue Kd values with the hydrophobicity parameter (phi) is observed for the para-substituted analogues where the binding affinity increases with increasing hydrophobicity of the substituent. Statistical treatment of the correlation shows a small negative contribution to binding by the van der Waals volume (VW) of the para substituent. meta-Substituted benzylamine analogues show a decreased binding affinity with the VW of the substituent and no correlation with the hydrophobicity value of the substituents tested. No spectral evidence was found for any flavin radical intermediates during the time course of MAO B flavin reduction in anaerobic reductive half-reduction stopped-flow experiments with any of the alpha,alpha-diprotio- or alpha,alpha-dideuteriobenzylamine analogues tested. The limiting rates of enzyme reduction exhibit large Dk values (6.5-14.1) for all of the analogues tested. para-Substituted benzylamine analogues reduce MAO B with limiting rates that correlate with the steric influence (Es value) of the substituent. Statistical analysis shows the rate of MAO B reduction by para-substituted analogues to be retarded by increased values of Es and, with a smaller contribution, by the hydrophobicity value of the substituent. The rate of MAO B reduction by meta-substituted benzylamine analogues is essentially independent of the nature of the substituent. No evidence was found for any electronic contribution to the rate of MAO B flavin reduction by any of the analogues tested. These data demonstrate the steric orientation of the substrate to be important in the rate of amine oxidation by MAO B and that ring meta substituents favor this orientation.(ABSTRACT TRUNCATED AT 400 WORDS)