ZINC AND BARIUM INHIBIT THE PHOSPHOLIPASE-A(2) FROM NAJA-NAJA-ATRA BYDIFFERENT MECHANISMS

The mode of inhibition of the phospholipase A(2) (PLA(2)) enzyme from the Chinese cobra (Naja naja atra) by Zn2+ is qualitatively different from inhibition by Ba2+. Inhibition by Ba2+ shows the kinetic characte ristics of a conventional competitive inhibitor acting to displace Ca2 + from a single essential site, but Zn2+ has the paradoxical property of being more inhibitory at high than at low Ca2+ concentration. Kinet ic analysis of the Ca2+-dependence of enzymic activity shows a bimodal response, indicating the presence of two Ca2+-binding sites with affi nities of 2.7 mu M and 125 mu M respectively, and we propose that thes e can be identified with the two Ca2+-binding sites revealed by crysta llographic analysis [White, Scott, Otwinowski, Gleb and Sigler (1990) Science 250, 1560-1563]. The results are consistent with the model tha t the enzyme is activated by two Ca2+ ions, one that is essential and can be displaced by Ba2+, and one that modulates the activity by a fur ther 5-10-fold and which can be displaced by Zn2+. An alternative mode l is also presented in which the modulating Zn2+-binding site is a phe nomenon of the lipid/water interface.