Inhibition of LDL-associated phospholipase A activity in human plasma by albumin.

LDL has been previously shown to exhibit activity of phospholipase A1 and phospholipase A2 as measured by the hydrolysis of NBD caproic phosphatidylcholine (C6-NBD-PC), which is considered to represent the phospholipase activity towards short chain or oxidized fatty acyl chains/3/. In the present study we show that in whole plasma the LDL-associated phospholipase A activity, measured by the hydrolysis of C6-NBD-PC, is inhibited by albumin due to its binding of the substrate. The inhibition depends on the molar ratio between the substrate, albumin and the enzyme. C6-NBD-PC and other synthetic phospholipid analogues have been used previously to determine plasma phospholipase A2 activity in various pathological states. This study suggests that when using this kind of substrate to measure plasma PLA activity, the choice of substrate and the experimental conditions should be carefully considered. Determination of the appropriate ratio between the three reaction components--enzyme, substrate and albumin--is required for reliable and consistent determination of plasma phospholipase activity.