Properties of Purified Detergent-Resistant Phospholipase A of Escherichia coli K-12

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Abstract
A crude preparation of membrane-bound phospholipase A (detergent-resistant) in Escherichia coli K-12 cells was found to be quite stable or even apparently activated on incubation at 100°C, but became strikingly thermolabile when it was highly purified and Triton X-100 was removed from the purified enzyme preparation. The rate of inactivation showed a biphasic temperature dependence: inactivation was rapid at 37°C and also above 70°C. Inactivation above 70°C changed the mobility of the enzyme on sodium dodecyl sulfate/polyacrylamide gel electrophoresis, but inactivation at 37°C did not affect the electrophoretic mobility. Triton X-100 effectively protected the enzyme against inactivation at 37°C. The concentration required for the protection of the enzyme was more than its critical micelle concentration. Phospholipids, such as phosphatidylethanolamine, phosphatidylglycerol, cardiolipin, phos-phatidylcholine, lysophosphatidylethanolamine, and lysophosphatidylcholine, also protected the enzyme against inactivation at 37°C. These results suggest that the binding of hydro-phobic compounds stabilizes the enzyme.