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.