臺灣飯匙倩蛇毒中磷脂?甲之化學修飾

Abstract

Phospholipase A2 from the venom of Formosan Cobra (Naja naia atra) has been inactivated by p-bromophenacyl bromide (BPB) and histidine-47 is proposed to be the primary site of modification, based on amino acid analysis. The inactivation by BPB showed many aspects of active site properties so that His-47 is suggested to be in this region. Other alkylating agents (iodoacetic acid, iodoacetamide, and methyl iodide) failed to affect the enzymic activity. Diethylpyrocarbonate-(Ethoxyformic anhydride) and tetranitromethane inactivated the PLA by modification of one or more tyrosine residues. Sulfenylation of tryptophan residues by 2'-nitrophenylsulfenyl chloride also inactivated this enzyme, suggesting that both aromatic residues are essential for the enzymic activity. Methionine residue is hard to be oxidized by Chloramine-T. Other residues are discussed based on the previous results. Finally, several models are compared and summarized as follows: the enzyme binds one calcium ion and a conformational change occurs to expose some functional groups to effect substrate binding. Ca2+-enzyme complex will then bind to micelle produced by phospholipid sad sodium deoxycholate or other detergents. Catalysis occurs through many driving forces and stabilizing forces on substrate molecule with the release of fatty acid and lysophosphatide.