Activation Mechanism of Phospholipase

Phospholipase refers to a number of enzymes that catalyze the hydrolysis of specific ester bonds in phospholipids. The individual enzymes are categorized by the bond they hydrolyze and as carboxylic acid esterases or phosphodiesterases. Phospholipase A1 (p. A1) and phospholipase A2 (p. A2) are classified as carboxylic acid esterases and phospholipase C (p. C) and phospholipase D (p. D) are classified as phosphodiesterases.

Phospholipases are of the family of enzymes called hydrolases which use water to catalyze the degradation of biological molecules to their component parts; phospholipases use water to degrade phospholipid molecules. Phospholipase activities are found to occur at a carbon-oxygen or phosphate-oxygen bond and they are defined by the bond that is broken during catalysis.

There are different phospholipases which are defined by the position they attack on the phospholipid molecule. Phospholipases are important enzymes in lipid metabolism and signal transduction; hydrolysis products are involved in the mobilization of intracellular calcium (inositol trisphosphate), regulation of protein phosphorylation (diacylglycerol), disruption of membrane integrity (lysophospholipid), activation and aggregation of platelets (platelet-activating factor), and the onset of inflammation (arachidonic acid metabolites, prostaglandins and leukotrienes).

Much information is available regarding phospholipases A2, C, an

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ansferred to an alcohol acceptor. The phospholipase A1 of a rat liver plasma membrane was found to be released by heparin and catalyzed a transacylation between two molecules of monoacylglycerol. These observations suggest that this enzyme is an ectoenzyme with broad substrate specificity. There is uncertainty regarding substrate specificity of this enzyme. It has been observed that a protease decreased the capacity of purified phospholipase A1 to degrade triglyceride. It has been noted that the enzyme's capacity to hydrolyze triglyceride is more susceptible to pepsin cleavage than its capacity to degrade monoglyceride or phospholipid. An important physiologic role of this enzyme includes that it is involved in lipoprotein catabolism; an injection of antibody to the enzyme into rats demonstrated this lipoprotein metabolism. Both low-density lipoprotein and high-density lipoprotein phospholipid were found to increase, which lead investigators to conclude that the plasma membrane phospholipase A1 functions as a phospholipase. Cholesterol also accumulated indicating that its uptake by the liver required phospholipid hydrolysis catalyzed by the phospholipase A1. These findings propose that this enzyme regulates cholestero
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