Volume 272, Number 31, Issue of August 1, 1997 pp. 19214-19219
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Activation, Inhibition, and Regiospecificity of the Lysophospholipase Activity of the 85-kDa Group IV Cytosolic Phospholipase A₂

(Received for publication, May 1, 1997, and in revised form, May 28, 1997)

From the Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, California 92093-0601

The 85-kDa Group IV calcium-dependent cytosolic phospholipase A₂ (cPLA₂) catalyzes the hydrolysis of palmitoylglycero-3-phosphocholine to palmitic acid and glycero-3-phosphocholine. Palmitoylglycero-3-phosphocholine exists as a 9:1 equilibrium mixture of the sn-1 and sn-2 isomers, with the fatty acid predominately at the sn-1 position. We have monitored this reaction by ³¹P NMR to determine which palmitoylglycero-3-phosphocholine isomer is processed by cPLA₂. When both lysophospholipid isomers are present in a 1:1 mixture under conditions in which acyl migration is minimized, cPLA₂ rapidly consumes both isomers. However, 1-palmitoylglycero-3-phosphocholine is consumed seven times faster than the 2-palmitoylglycero-3-phosphocholine isomer. We have previously reported that this lysophospholipase reaction is accelerated in the presence of glycerol. We now find that this apparent increase in activity is accounted for, in part, by glycerol acting as an alternative acceptor for the cleaved fatty acid, as is the case for this enzyme's phospholipase A₂ (PLA₂) activity. In contrast, dioleoylglycerol, which accelerates the PLA₂ activity, does not act as an acceptor in either the lysophospholipase or the PLA₂ reaction, but can affect enzyme activities by altering substrate presentation. We also show that a known inhibitor of the PLA₂ activity of cPLA₂ is able to inhibit its lysophospholipase activity with a similar IC₅₀ to its PLA₂ activity. However, the effect of inhibitors is dependent on the manner in which they are presented to the enzyme.

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