David J. Keeling

Studies on the mechanism of action of omeprazole

The effects of omeprazole on preparations of pig gastric (H+ + K+)-ATPase have been studied. Omeprazole was found to inhibit the (H+ + K+)-ATPase activity in a time-dependent manner. Inhibition was more pronounced at pH 6.1 compared with pH 7.4 and decreased as the concentration of (H+ + K+)-ATPase preparation increased. The potency of omeprazole was therefore highly dependent upon the conditions used. When pre- incubated with (H+ + K+)-ATPase preparation (30 micrograms protein/ml) for 30 min at 37 degrees and pH 6.1, omeprazole inhibited the (H+ + K+)-ATPase activity with an IC50 of 3.9 microM. This inhibition was shown to be irreversible in nature. Whilst omeprazole itself was not very potent as an inhibitor of the (H+ + K+)-ATPase activity at pH 7.4 (IC50 = 36 microM), transient acidification of omeprazole resulted in the formation of a compound(s) which produced marked inhibition at this pH (IC50 = 5.2 microM). The effects of omeprazole in the absence of acidification may have resulted from the rate-limiting formation of this compound. Radiolabelled omeprazole was shown to incorporate into the (H+ + K+)-ATPase preparation in a time-dependent and pH-dependent manner. Omeprazole, radiolabelled in three separate positions (the sulphur atom and the two adjacent carbon atoms), incorporated with equivalent time courses suggesting that the incorporation did not involve a fragmentation of the omeprazole molecule. Under conditions shown to produce a 50% inhibition of (H+ + K+)-ATPase activity, [14C] omeprazole had incorporated to a level of 4-5 nmoles/mg protein. Incorporation continued beyond the point required to produce 100% inhibition of (H+ + K+)-ATPase activity and reached 30 nmoles/mg protein after 5 hr. Prior acidification of the omeprazole resulted in a more rapid initial rate of incorporation although the final level of incorporation was lower than for omeprazole. Omeprazole was also shown to interact with the (Na+ + K+)-ATPase from dog kidney. Omeprazole inhibited the (Na+ + K+)-ATPase activity (IC50 = 186 microM). Acid-degraded omeprazole inhibited the (Na+ + K+)-ATPase activity with greater potency (IC50 = 19 microM) and was also shown to incorporate into this enzyme preparation.

The specificity of omeprazole as an (H++K+)-ATPase inhibitor depends upon the means of its activation

Omeprazole (OME) is a novel acid secretion inhibitor, believed to act directly on the gastric proton pump, the (H+ + K+)-ATPase. Inhibition of ATPase activity is associated with an incorporation of [14C]OME into gastric vesicles containing the (H+ + K+)-ATPase, and both processes are greatly enhanced if the OME is exposed to acidic pH. This, and other evidence, suggests that the acidic environment of the (H+ + K+)-ATPase generates from OME a reactive intermediate which covalently inhibits the pump. We have compared the means by which the OME was acid-activated with the specificity of inhibition (amount of incorporation of omeprazole required to produce 100% inhibition of K+-stimulated ATPase activity). The stoichiometry of incorporation has been related to the number of detectable catalytic phosphorylation sites in each preparation (an index of the number of functional pumps). In lyophilised gastric vesicles, where the membrane barriers separating the cytoplasmic and luminal faces of the enzyme are substantially destroyed, incubation with OME at pH 6.1 produced a progressive inhibition and incorporation over 120 min. Complete inhibition of K+-ATPase required 13 +/- 3 (SEM; N = 4) moles of OME incorporation per phosphorylation site. In intact gastric vesicles, under conditions shown independently to result in proton pumping and the acidification of the vesicle interior (150 mM KCl, 9 microM valinomycin, 2 mM Mg-ATP pH 7.0), inhibition and incorporation occurred more rapidly (15 min). Complete inhibition of K+-ATPase required only 1.8 +/- 0.15 (SEM; N = 3) moles of OME per phosphorylation site.(ABSTRACT TRUNCATED AT 250 WORDS)

BY 1023/SK&F 96022 : biochemistry of a novel (H+ + K+)-ATPase inhibitor

The mechanism by which the substituted benzimidazole sulphoxide BY 1023/SK&F 96022 inhibited the (H+ + K+)-ATPase, the enzyme responsible for hydrogen ion secretion in the stomach, was studied in a variety of in vitro preparations. In gastric preparations that were capable of active hydrogen ion transport with consequent lumenal acidification, BY 1023/SK&F 96022 inhibited with high potency and in a time-dependent manner consistent with the acid-induced conversion of the parent benzimidazole sulphoxide to a covalent inhibitor (cyclic sulphenamide). The following IC50 values were obtained for the inhibition of aminopyrine accumulation: intact gastric glands stimulated with 1 mM dibutyryl cAMP, 1.0 microM; permeabilized gastric glands stimulated with 5 mM ATP, 0.42 microM; intact gastric vesicles stimulated with 150 mM KCl, 9 microM valinomycin and 2 mM MgATP, 3.5 microM. In a preparation that could not generate pH gradients, lyophilized gastric vesicles at pH 7.4, BY 1023/SK&F 96022 inhibited K(+)-stimulated ATPase activity with relatively low potency, 70 microM, indicating its good chemical stability at neutral pH. As assessed by ATPase inhibition, this stability was three times greater than that of omeprazole. Inhibition by BY 1023/SK&F 96022 was not reversed by dilution in either permeabilized gastric glands or intact gastric vesicles. Inhibition could, however, be completely reversed by subsequent incubation with 20 mM beta-mercaptoethanol (intact gastric glands) or 100 mM dithiothreitol (intact gastric vesicles) suggesting a disulphide link between inhibitor and enzyme. The concentration of glutathione needed to protect against inhibition by BY 1023/SK&F 96022 was 10,000 times higher in intact, compared with lyophilized, gastric vesicles indicating an interaction with the lumenal (extra-cellular) face of the (H+ + K+)-ATPase. BY 1023/SK&F 96022 and omeprazole were also found to inhibit acidification in purified kidney lysosomes with IC50 values of 194 and 75 microM, respectively. Protection by 10 microM glutathione suggested that this did not result from intralysosomal activation of these inhibitors. Thus, BY 1023/SK&F 96022 has the combined properties of good chemical stability at neutral pH and effective conversion to the cyclic sulphenamide at acidic pH. In this way the activation to the cyclic sulphenamide may be optimally restricted to the parietal cell canaliculus.

Fourier transform infrared spectroscopic studies on gastric H+/K+-ATPase

Suspensions of membrane-bound H+/K+-ATPase in both H2O and 2H2O were investigated using Fourier transform infrared (FT-IR) spectroscopy. Second-derivative techniques were used to reveal the overlapping bands in the 1800-1500 cm-1 region. Analysis of the amide I band shows that the protein component contains substantial amounts of both alpha-helical and beta-sheet structures. Addition of 10 mM KCl to a suspension in 2H2O does not significantly affect the amide I band, indicating that the E1-E2 conformational transition of the enzyme, induced by K+, does not involve a gross change in protein secondary structure. Analysis of the amide II band in the spectra of suspensions in 2H2O shows that inhibition of the enzyme with omeprazole increases the rate of 1H-2H exchange, indicating an increase in conformational flexibility. Furthermore, an additional feature at 1628 cm-1 in the spectra of the inhibited samples in 2H2O could either support a conformational change or arise from a vibrational mode of omeprazole in its enzyme-bound form. The frequency of the band due to the symmetric stretching vibrations of the methylene groups of the lipid acyl chains increases steadily with increasing temperature indicating that there is no co-operative melting process in the lipid component of the membrane over the temperature range 9-50 degrees C. For comparison, FT-IR studies on aqueous suspensions of Na+/K+-ATPase were also carried out. These show that the protein components in the Na+/K+- and H+/K+-ATPases have similar secondary structures.

Effects of NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid) on chloride transport in intestinal tissues and the T84 cell line

NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid) has been reported to block Cl- channels in isolated rabbit nephrons with high potency (IC50 = 80 nM). The effects of this compound on Cl(-)-mediated transport processes in intestinal tissues have been studied using agonist-stimulated short-circuit current (T84) in Ussing chamber experiments and 36Cl- fluxes in monolayers of a colonic cell line (T84). NPPB inhibited PGE1-stimulated Isc in rabbit distal colon and ileum at concentrations in the range 20 to 100 microM. However, NPPB at the same concentrations also inhibited glucose-stimulated Isc in rabbit ileum, suggesting that its effects were not restricted to those on Cl- transport. Consistent with this, exposure of rabbit distal colon to 100 microM NPPB was found to reduce endogenous ATP levels by 69%, implying that, at these concentrations, NPPB could impair active transport processes by an effect on cellular energy metabolism. Clear evidence for a direct effect of NPPB on epithelial chloride channels was found in studies on Cl- fluxes in T84 cell monolayers. NPPB inhibited VIP-stimulated Cl- uptake into T84 cells with an IC50 of 414 microM. NPPB (1 mM) also inhibited Cl- efflux from pre-loaded cells confirming its effect as a weak Cl- channel blocker in this system.