We investigated the enzyme kinetic basis for the stereoselective disposition of R-and S-omeprazole (OME) and racemic OME in human liver microsomes. OME is primarily metabolized by the hepatic cytochrome P450 enzyme system (CYP2C19 and 3A4). The metabolism of each enantiomer and pseudoracemic OME was studied using unlabeled and 13 C 7 -labeled enantiomers. The enantiomers inhibited each other's metabolism competitively in human liver microsomes and in recombinant CYP2C19 and 3A4. The results obtained with the individual enantiomers allowed successful prediction of the enzyme kinetics for the pseudoracemate. The intrinsic clearance of each enantiomer in a pseudoracemic mixture remained the same as those of the individually incubated enantiomers, although K m and V max decreased. In the pseudoracemate, the relative contribution of CYP2C19 and 3A4 to 5-hydroxylation and 5Ј-O-demethylation of R-OME was comparable to that obtained for the incubation of R-OME alone. For S-OME, however, the presence of its antipode greatly increased the contribution of CYP3A4, with increasing concentrations, compared with that obtained when incubating S-OME alone. The results of our in vitro study clearly show metabolic interactions between the OME enantiomers, which may also occur in vivo. Because the enantiomers of OME produce similar pharmacological effects, the enantiomer interactions should not significantly affect the pharmacodynamics. On the other hand, the use of the S-enantiomer results in less complex enzyme kinetics than those of the racemate; thus, the outcome of its clinical use is more predictable.Omeprazole (OME), a proton pump inhibitor (PPI), has been widely used for many years as an acid inhibitory agent for the treatment of gastric acid hypersecretion disorders. OME is a chiral compound and the sulfinyl group is the chiral center (Fig. 1). It is administered as a racemic (50/50) mixture of the S-and R-enantiomers. Recently, its optical Sisomer has been developed as a new drug (esomeprazole). OME as well as its enantiomers are prodrugs with a common mechanism of action involving chemical rearrangement to a pharmacologically active achiral sulfenamide in the acidic compartment of parietal cells (Lindberg et al., 1986). The formed sulfenamide reacts with sulfhydryl groups of the enzyme H ϩ ,K ϩ -ATPase (the proton pump), which is located in the canaliculi of the parietal cells, thus inhibiting its ability to participate in gastric acid formation. OME and its enantiomers are thus equally potent H ϩ ,K ϩ -ATPase inhibitors (Andersson et al., 2001). However, R-and S-OME show stereoselective disposition because of the enzyme-catalyzed stereoselective metabolism that leads to the higher metabolic stability of esomeprazole compared with its R-isomer and the racemate, which has been demonstrated in vitro using human liver microsomes (Äbelö et al., 2000). The resulting increase in drug exposure in the majority of the population, as indicated by the higher area under the plasma concentration-time curve after esomeprazole ...

The kinetic behaviour of human thrombin has been studied with 26 tripeptidyl-p-nitroanilide substrates protected at the N terminus and with 9 unprotected ones. By the regression analysis of experimentally determined I/&,, k,,, and /cCat/Km values the individual contribution of each side chain of the various substrates to the kinetic parameters was calculated.The contributions to the kinetic parameters of the best substrates provide information about the structure of the binding site. The interaction of subsites S1 and PI, which determines primary specificity, proved to be marginal on the basis of contribution values, though it depends upon this contact whether the substrate is hydrolyzed at all. At subsite Sz proline appeared to be favourable. Subsite S3 plays an important role in efficiency. The best parameters were obtained here with the D configurations of bulky amino acid residues. The aromatic protecting groups applied did not improve the properties of substrates. BzDPhe-Pro-Arg-Nan was predicted by calculation to be better than the protected substrates assayed. The compound was synthesized and tested. Its experimentally determined l/K,,,, 55.1 mM-', was in good agreement with 50.9 mM-' found by calculation.Several serine proteases, e.g. thrombin, trypsin, plasmin and factor Xa, hydrolyze peptide bonds at the carboxylic group of basic amino acids. Their sensitivities to substrate specificity are, however, different. Among them thrombin is the most specific which, apart from a few exceptions, splits next to arginine [1,2] [9] and growth hormones [lo]. According to Muszbek [ I l l in actin it also hydrolyzes next to lysine, whereas hydrolysis does not occur next to ornithine [12].The detailed structure of the substrate-binding sites of thrombin is not yet known. The investigations on the specificity and substrate-binding sites have so far been carried out, in addition to fibrinogen, with peptide derivatives analogous to fibrinogen sequences [13,14] [2,16] and similar hydrophobic side chains, whereas results concerning the S3 substrate-binding subsite are contradictory. Scheraga holds that S3 is rather narrow; it is conceivable, though, that more than one binding domain is operative against long peptides [12].In an attempt to study the substrate-binding sites of thrombin we applied a mathematical method [19], by the aid of which, through the regression analysis of the kinetic constants measured for various tripeptidyl-p-nitroanilide substrates, the interaction between substrate side chains and the

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The chromogenic substrate Bz-Ile-Glu-Gly-Arg-pNA is based on the primary structure preceding the bonds split by factor Xa in bovine prothrombin. Substitution of amino acids in this natural sequence by closely related amino acids has only given inferior substrates. Thus, the natural sequence seems very important. However, we have found that the free γ-carboxyl group of Glu is not indispensable. Substrates with the above structure but having been derivatized on the γ-carboxyl group of Glu, in the form of simple esters or amides, show improved properties. Especially Km of the new substrates compare favourably with our first substrate. Some of the amides also show increased Vmax.These improved properties have made it possible to increase sensitivity, shorten incubation times and lower substrate consumption in several methods where this type of substrate is utilized.