| Abstract |
Omeprazole (OMP) is metabolised by CYP2C19 and CYP3A4 to 5-hydroxyomeprazole and omeprazole sulfone, respectively. It has been demonstrated that polyunsaturated fatty acids (PUFAs) are substrates and potent inhibitors of CYP2C19. We hypothesise that the addition of bovine serum albumin (BSA) to incubations of human liver microsomes (HLM) will sequester PUFAs that are released from the membranes of HLM during the course of incubation, therefore providing true estimates of kinetic parameters. This study investigated the effect of BSA on the kinetics of OMP 5-hydroxylation and sulfoxidation by microsomes from 5 human livers. In the absence of BSA, 5-hydroxyomeprazole and omeprazole sulfone formation followed two enzyme Michaelis-Menten kinetics. The mean apparent (± S.D.) kinetic parameters for OMP 5-hydroxylation were: Km1= 8.63 ± 6.83 mM, Vmax1 = 97.3 ± 46.7 pmol/min.mg and CLint1 = 13.8 ± 8.39 mL/min.mg; Km2 = 156 ± 123 mM, Vmax2 = 239 ± 77.4 pmol/min.mg and CLint2 = 2.34 ± 1.47 mL/min.mg. The mean apparent (± S.D.) kinetic parameters for OMP sulfoxidation were: Km1= 16.5 ± 11.5 mM, Vmax1 = 63.0 ± 60.2 pmol/min.mg and CLint1 = 3.16 ± 1.50 mL/min.mg; Km2 = 195 ± 105 mM, Vmax2 = 240 ± 116 pmol/min.mg and CLint2 = 1.38 ± 0.54 mL/min.mg. In the presence of 2% BSA, 5-hydroxyomeprazole formation also followed two enzyme Michaelis-Menten kinetics whereas omeprazole sulfone formation followed a single enzyme Michaelis-Menten model. In the presence of 2% BSA, the Km1 and Vmax1 for OMP 5-hydroxylation were significantly reduced (2 to 4 fold) consequently CLint1 was increased (2 fold). Hence, the addition of BSA to the reaction incubations had minor effect on the CYP2C19 catalysed high affinity component (Km1 and Vmax1) of OMP 5-hydroxylation, whereas the effect of BSA on OMP sulfoxidation was less clear. Further investigations of the effect of BSA on OMP 5-hydroxylation and sulfoxidation by recombinant CYP2C19 and CYP3A4 are required to clarify the role of BSA on in vitro OMP kinetics. |
Publication
Journal Publication
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