علت عدم نياز به يون mg2+ در آنزيم فسفاتيدات فسفوهيدروالز غشايي نسبت به فرم سيتوزولي آن در كبد موش صحرايي‌

Rat Membrane Phosphatidate Phosphohydrolase and the Reason for its Rat Membrane Phosphatidate Phosphohydrolase and the Reason for its Mg+ Independence

Introduction: phosphatidate phosphohydrolase (PAP) catalyzes the dephosphorylation of phosphatidic acid to yield Pi and diacylglycerol. Two forms of PAP in rat hepatocyte have been reported. A cytosolic form (PAPi) that is responsible for glycerolipid metabolism and requires Mg1+ for its activity. Another form (PAP2) is primarily involved in lipid signaling pathways which dose not need Mg +. It has two isoforms; PAP 2a and PAP2b. Little information is known about enzymological characterization of PAP2 especially its substrate. We investigated the enzyme behavior against Mg2+ structure- breaking agents (urea and guanidine HC1) with respect to its substrate. Materials & Methods: PAP2b was purified from rat hepatocyte membrane using a multi - stage chromatography. The enzyme activity was determined against La (lamellar) and Hn (Hexagonal) forms of substrate in the presence of triton X-100. The effect of Mg2+concentration and urea and guanidine HC1 (structure- breaking agents) were examined on La to Hn phase transition and enzyme activity. Results: PAP2b consumes La form of phosphatidate. Cations such as Mg result in Hn from La phase of substrate and therefore reduce enzyme activity. Urea and guanidine HC1 increase enzyme activity due to prevention of Hn formation. Both can also reduce the effect of cations on Lato Hn transition of phosphatidate. Conclusion: Since PAP2b consumes La form of substrate parameters can induce La to Hn phase transition which result in low enzyme activity. In contrast, factors such as urea and guanidine HC1 increase activity due to prevention of La to Hn phase transition . As Mg2+ stimulates Hn formation and the enzyme needs La form substrate, it can be concluded that, Not only does the enzyme need no Mg2+,but it is controlled by it too.