Modulation of midazolam 1-hydroxylation activity in vitro by neurotransmitters and precursors

Objective: The aim of this study was to find whether endogenous substances could modulate CYP3A activity. There is evidence that CYP3A, a major phase- I xenobiotic metabolizing enzyme, is present in human brain but, at the pre sent time, endogenous substrates for such an enzyme remain to be identified . A possible linkage between the CYP2D6 enzyme and serotonergic transmissio n has been recently reported by our group. In the same manner, structurally related enzymes such as CYP3A could also be related to endogenous compound s. Methods: CYP3A activity was measured using the enzyme-specific substrate mi dazolam in human liver microsomes. Several neurotransmitters, precursors, a nd their metabolites, corresponding to three different metabolic routes, we re assayed as putative modulators of CYP3A enzyme activity. These comprised serotonergic, catecolaminergic, and GABAergic transmitters and precursors. The inhibitory capacity of ketoconazole, a competitive inhibitor of CYP3A, was also analyzed for comparison. Results: The kinetic analysis of the midazolam l-hydroxylase activity measu red in microsomes from five human liver samples indicated K-m values (mean +/- SD) of 5.8 +/- 4.9 mu M, and V-max values of 1.7 +/- 1.4 nmol min(-1) p er mg microsomal protein in all the samples used in the study. Of the 14 su bstances analyzed, adrenaline, serotonin, and 5-hydroxytriptofol were full inhibitors of CYP3A enzyme activity (K-i values of 42.3, 26.4, and 43 mu M, respectively). The remaining substances were weak inhibitors or had no inh ibitory effect. Conclusion: Brain CYP3A activity could be modulated by some neurotransmitte rs and precursors.