Evidence for a common binding-site for omeprazole and n-ethylmaleimide in subunit a of chromaffin granule vacuolar-type H⁺-ATPase

Vacuolar-type H⁺-ATPase from adrenal chromaffin granules wasfound to be sensitive to omeprazole, a known gastric H⁺/K⁺-ATPase inhibitor, the concentration required for 50 % inhibition being 80 μM freshly-prepared and 12 μM acid-treated reagent. ATP and ADP protected the enzyme from inhibition by omeprazole. The activity of the inhibited enzyme was restored by the addition of reduced glutathione. Omeprazole protected the enzyme from inhibition by N-ethylmaleimide and its binding to the subunit A. As subunit A has a nucleotide binding site(s) and as a cysteine residue is involved in the inhibition by N-ethylmaleimide, these results suggested that the two sulfhydryl reagents bind to the same cysteine residue near the nucleotide binding domain in the subunit A, resulting in inactivation of vacuolar-type H⁺-ATPase.