Involvement of both a Zn2+ site and an anionic binding site in the selective inhibition of a Zn2+-glycerophosphocholine cholinephosphodiesterase by thiols and tellurites

Abstract

Inhibition of a Zn(2+)-glycerophosphocholine cholinephosphodiesterase by thiols or tellurites were examined mechanistically. Inactivation of the phosphodiesterase by thio-carboxylates, which was due to the removal of Zn2+ in the catalytic site, was enhanced by introduction of an amino group in the structure of thiols, suggesting the presence of an anionic site adjacent to a Zn2+ site. In support of the suggestion, it was found that thiols, associable with both a Zn2+ site and an anionic site, were more potent reversible inhibitors; dimethylaminoethanethiol (Ki, 17 microM), diethylaminoethanethiol (Ki, 1.2 microM) and thiocholine (Ki, 2.6 microM). Meanwhile, the inhibition of the phosphodiesterase by tellurites is ascribed to the binding of tellurite anions to a Zn2+ site, based on the protective action of tellurite anions against the inactivation of the enzyme by EDTA. Moreover, the inhibition of the phosphodiesterase by tellurites was prevented by phosphate ions, which expressed the protective effect against EDTA inactivation. In further support, it was observed that tellurite and thiocholine appeared to interact with active site in an additive manner, in contrast to a synergistic action between tellurites and quaternary ammonium compounds such as acetylcholine or choline.