Journal of Lipid Research, Vol. 21, 617-624, July 1980
Copyright © 1980 by Lipid Research, Inc.

The action of human and rabbit serum phospholipase A₂ on Escherichia coli phospholipids

Lesley Kaplan-Harris , Jerrold Weiss , Carolyn Mooney , Susan Beckerdite-Quagliata , and Peter Elsbach

Department of Medicine, New York University School of Medicine, New York, NY 10016

We have compared the properties of phospholipase A (E.C. 3.1.1.4) activity in whole human and rabbit serum toward the phospholipids of Escherichia coli. Using as substrate E. coli labeled during growth with either [1-¹⁴C]-palmitic acid or [1-¹⁴C]oleic acid, and then autoclaved to inactivate E. coli phospholipases and to render the labeled phospholipids accessible to exogenous phospholipases, we show that the deacylating activity in both human and rabbit serum is almost exclusively of the A₂ type. Rabbit serum is at least 20-fold more active than human serum. Activity in both sera is maximal at physiological Ca²⁺ concentrations (2 mM) and is abolished by ethylenediaminetetraacetic acid. To examine hydrolysis of intact (unautoclaved) E. coli treated with 25% serum, use was made of a phospholipase A-deficient E. coli strain (E. coli S17), thereby eliminating the possible contribution of bacterial phospholipases to degradation. Human and rabbit serum are about equally bactericidal toward E. coli and cause comparable structural damage. However, only rabbit serum produces substantial hydrolysis of the phospholipids of intact E. coli S17. Heated (56°C, 30 min) rabbit serum is non-bactericidal and retains phospholipase A₂ activity toward autoclaved, but not intact E. coli. The ability of heated serum to degrade phospholipids of intact E. coli S17 is restored, however, by adding 25% normal human serum, which is bactericidal. In this combination, doses of heated rabbit serum containing as much phospholipase A₂ activity (toward autoclaved E. coli) as is present in 25% unheated rabbit serum, produce roughly the same extent of hydrolysis of intact E. coli as does normal rabbit serum alone. Low doses with a phospholipase A₂ activity comparable to that of normal human serum elicit little or no hydrolysis. These findings indicate that hydrolysis of the phospholipids of intact E. coli S17 by serum occurs when: 1) the serum is bactericidal, and 2) when sufficient phospholipase A₂ is present. The difference in phospholipid hydrolysis that accompanies killing of E. coli by human or rabbit serum appears to reflect, therefore, the different amounts of phospholipase A₂ activity in the two sera. Phospholipid degradation is not required for the bactericidal action of serum. Bacterial phospholipid breakdown may be important, however, in the overall destruction and digestion of invading bacteria by the host.—Kaplan-Harris, L., J. Weiss, C. Mooney, S. Beckerdite-Quagliata, and P. Elsbach. The action of human and rabbit serum phospholipase A₂ on Escherichia coli phospholipids.

Supplementary key words bacterial phospholipid • bactericidal action

Submitted on November 13, 1979
Revised on February 13, 1980

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