An evaluation of potential mechanism-based inactivation of human drug metabolizing cytochromes P450 by monoamine oxidase inhibitors, including isoniazid

To characterize potential mechanism-based inactivation (MBI) of major human drug-metabolizing cytochromes P450 (CYP) by monoamine oxidase (MAO) inhibitors, including the antitubercular drug isoniazid. Human liver microsomal CYP1A2, CYP2C9, CYP2C19, CYP2D6 and CYP3A activities were investigated following co- and preincubation with MAO inhibitors. Inactivation kinetic constants (K-I and k(inact)) were determined where a significant preincubation effect was observed. Spectral studies were conducted to elucidate the mechanisms of inactivation. Hydrazine MAO inhibitors generally exhibited greater inhibition of CYP following preincubation, whereas this was less frequent for the propargylamines, and tranylcypromine and moclobemide. Phenelzine and isoniazid inactivated all CYP but were most potent toward CYP3A and CYP2C19. Respective inactivation kinetic constants (K-I and k(inact)) for isoniazid were 48.6 mu M and 0.042 min(-1) and 79.3 mu M and 0.039 min(-1). Clorgyline was a selective inactivator of CYP1A2 (6.8 mu M and 0.15 min(-1)). Inactivation of CYP was irreversible, consistent with metabolite-intermediate complexation for isoniazid and clorgyline, and haeme destruction for phenelzine. With the exception of phenelzine-mediated CYP3A inactivation, glutathione and superoxide dismutase failed to protect CYP from inactivation by isoniazid and phenelzine. Glutathione partially slowed (17%) the inactivation of CYP1A2 by clorgyline. Alternate substrates or inhibitors generally protected against CYP inactivation. These data are consistent with mechanism-based inactivation of human drug-metabolizing CYP enzymes and suggest that impaired metabolic clearance may contribute to clinical drug-drug interactions with some MAO inhibitors.

Genetic screening for susceptibility to depression: can we and should we?

Objective: To summarize current knowledge about genetic susceptibility to mood disorders and examine ethical and policy issues that will need to be addressed if robustly replicated susceptibility alleles lead to proposals to screen and intervene with persons at increased genetic risk of developing mood disorders. Method: Empirical studies and reviews of the genetics of unipolar and bipolar depression were collected via MEDLINE and psycINFO database searches. Results: A number of candidate genes for depression have been identified, each of which increases the risk of mood disorders two- or threefold. None of the associations between these alleles and mood disorders have been consistently reported to date. Conclusions: Screening the population for genetic susceptibility to mood disorders is unlikely to be a practically useful policy (given plausible assumptions). Until there are effective treatments for persons at increased risk, screening is arguably unethical. Screening within affected families to advise on risks of developing depression would entail screening children and adolescents, raising potentially serious ethical issues of consent and stigmatization. Genetic research on depression should continue under appropriate ethical guidelines that protect the interests of research participants.

Direct electrochemically driven catalysis of bovine milk xanthine oxidase

The complex molybdoenzyme xanthine oxidase (XO) catalyses the oxidation of xanthine to uric acid. Here we report the first direct (unmediated) catalytic electrochemistry of the enzyme in the presence of xanthine. The only non-turnover response (without substrate present) is a sharp two-electron wave from the FAD cofactor at -242 mV vs. NHE (pH 8.0). Upon addition of xanthine to the electrochemical cell a pronounced electrocatalytic anodic current appears at ca. +300 mV vs. NHE, but the FAD peak remains. This is unusual as the onset of catalysis should occur at the potential of the FAD cofactor (the site at which oxygen or NAD+ binds to the enzyme in solution). The observed electrochemical catalysis is prevented by the addition of known XO inhibitors allopurinol or cyanide. (c) 2005 Elsevier B.V. All rights reserved.