Elucidation of the Catalytic Function and the Inhibition mechanism of Phospholipase A_2

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1. Effects of Ca^<2+> and pH on the kinetic parameter for the hydrolysis of monodispersed 1,2-dihexanoy1-sn-glycero-3-phosphorylcholine, catalyzed by Group I and II phospholipases A_2 (PLA_2s) , were studied by the pH-stat assay method in the absence or presence of carbonic amidetype, oxazolidinone-type, and sulfonic amide-type substrate analogs. The Ca^<2+> dependency and participation of the catalytic group His 48 in the binding of genuine substrate to both types of PLA_2s were found to be very similar to those of the oxazolidinone-type substrate analog, but differed greatly from the carbonic amide-type and sulfonic amide-type substrate analogs. This finding suggests that the binding mode of oxazolidinone-type substrate analog is very similar to that of the genuine substrate.2. Chemical modification and inactivation of Group I and II PLA_2s were investigated by the use of a manoalide (MLD) -analog. It was found that Lys-56 of bovine pancreatic PLA_2s was modified by MLD-analog and that this modification was responsible for enzyme inactivation. It was indicated that the inactivation of pancreatic and N.naja atra PLA_2s originated from the modification of Lys residues at the interfacial recognition site, and that the inactivation of P.australis, T.flavoviridis and V.russelli russelli PLA_2s arose from the modification of Lys residues at the catalytic site, interfacial recognition site and regions outside both sits. The inactivation of A.halys blomhoffii PLA_2s was assumed to be due to the modification of Lys residues outside the two sites described above. We thought that the modification of the Lys residues outside the above two sites give rise to conformational changes leading to inactivation