A NOVEL PHOSPHOLIPASE A(2) ACTIVITY IN SALIVA OF THE LONE STAR TICK, AMBLYOMMA-AMERICANUM (L.)

Saliva from female lone star ticks, Amblyomma americanum, contained a novel phospholipase A(2) (PLA(2)) activity that hydrolyzed C-14- arach idonate from C-14-arachidonyl phosphatidylcholine. The tick saliva PLA (2) (ts-PLA(2)) was active over a broad pH range (4.5-11.5) with two d istinct pH optima of pH 5.5 and 9.5. Though extracellular PLA(2)s are reported to be activated by millimolar Ca2+, ts-PLA(2) was sensitive t o submicromolar Ca2+ and was half-maximally activated by 3.5 mu M Ca2. Tick saliva contains >500 mu M Ca2+ and the feeding lesion in the ho st is expected to contain millimolar Ca2+. Saliva exhibited a single p eak of PLA, activity corresponding to a molecular weight of 55.7 +/- 1 .3 kDa by size exclusion chromatography. The ts-PLA(2) was unaffected by a variety of compounds known to inhibit either secreted or cytosoli c PLA(2)s from other sources. However, ts-PLA(2) was inhibited by the substrate analog, oleyloxyethyl phosphorylcholine (IC50 = 1.4 mu M), a nd the end product, arachidonic acid (IC50 = 38 mu M). Low concentrati ons of dithiothreitol did not greatly affect ts-PLA(2) but activity wa s reduced al higher concentrations. The PLA(2) activity found in A. am ericanum salivary glands showed many similarities to ts-PLA(2), but al so some distinct differences. Secreted at the tick-host interface, ts- PLA(2) is thought to play an important, but unknown, role during the p rolonged tick feeding. (C) 1997 Academic Press.