Studies on the mechanism of action of omeprazole

Abstract

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 pH6.1 compared with pH7.4 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 μg | frsol| protein/ ml) for 30 min at 37° and pH 6.1, omeprazole inhibited the (H + + K +)-ATPase activity with an IC 50 of 3.9 μM. 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 ( ic 50 = 36 μM), transient acidification of omeprazole resulted in the formation of a compound(s) which produced marked inhibition at this pH ( ic 50 = 5.2 μM). 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 ( ic 50 = 186 μM). Acid-degraded omeprazole inhibited the (Na + + K +)-ATPase activity with greater potency ( ic 50 = 19 μM) and was also shown to incorporate into this enzyme preparation.