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 present time, endogenous substrates for such an enzyme remain to be identified. A possible linkage between the CYP2D6 enzyme and serotonergic transmission has been recently reported by our group. In the same manner, structurally related enzymes such as CYP3A could also be related to endogenous compounds.


METHODS
CYP3A activity was measured using the enzyme-specific substrate midazolam in human liver microsomes. Several neurotransmitters, precursors, and their metabolites, corresponding to three different metabolic routes, were 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 1-hydroxylase activity measured in microsomes from five human liver samples indicated Km values (mean +/- SD) of 5.8 +/- 4.9 microM, and Vmax values of 1.7 +/- 1.4 nmol min(-1) per mg microsomal protein in all the samples used in the study. Of the 14 substances analyzed, adrenaline, serotonin, and 5-hydroxytriptofol were full inhibitors of CYP3A enzyme activity (Ki values of 42.3, 26.4, and 43 microM, respectively). The remaining substances were weak inhibitors or had no inhibitory effect.


CONCLUSIONS
Brain CYP3A activity could be modulated by some neurotransmitters and precursors.
Martínez
C
C
Department of Pharmacology, Medical School, University of Extremadura, Badajoz, Spain.
Gervasini
G
G
Agúndez
J A
JA
Carrillo
J A
JA
Ramos
S I
SI
García-Gamito
F J
FJ
Gallardo
L
L
Benítez
J
J
D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004837:Epinephrine / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D006917:Hydroxytryptophol / Q000378:metabolism / Q000494:pharmacology; D066298:In Vitro Techniques; D007654:Ketoconazole / Q000494:pharmacology; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D008874:Midazolam / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism* / Q000494:pharmacology; D010089:Oxidoreductases, N-Demethylating / Q000378:metabolism*; D020033:Protein Isoforms / Q000378:metabolism; D012701:Serotonin / Q000378:metabolism* / Q000494:pharmacology; D005680:gamma-Aminobutyric Acid / Q000378:metabolism* / Q000494:pharmacology
D004791:Enzyme Inhibitors; D018377:Neurotransmitter Agents; D020033:Protein Isoforms; D006917:Hydroxytryptophol; D012701:Serotonin; D005680:gamma-Aminobutyric Acid; D003577:Cytochrome P-450 Enzyme System; D001189:Aryl Hydrocarbon Hydroxylases; D051544:Cytochrome P-450 CYP3A; D010089:Oxidoreductases, N-Demethylating; D008874:Midazolam; D007654:Ketoconazole; D004837:Epinephrine