COEXISTENCE OF 2 BIOCHEMICALLY DISTINCT PHOSPHOLIPASE A(2) ACTIVITIES IN HUMAN PLATELET, MONOCYTE, AND NEUTROPHIL

The cell-associated phospholipase A(2) (PLA(2)) activities of the human platelet, neutrophil, and monocyte were simultaneously characterized, utilizing the biochemical differences observed between the 14 kDa (kilodalton), type II PLA(2) isolated from inflammatory human synovial joint fluid (HSF) and the arachidonic acid (AA) specific, 85-kDa high molecular mass (HMM) PLA(2) isolated from the cytosol of a U937 monocytic cell line. The HSF PLA(2) can be distinguished from the HMM PLA(2) by its resistance to acid treatment, sensitivity to a sulfhydryl reducing agent, lack of preference for the fatty acid on the sn-2 position of phospholipid substrate, and inhibition by the C-7 phosphonate-phospholipid transition-state PLA(2) inhibitor. Evaluation of all three cell types revealed that HMM-like PLA, activity was found predominantly in the cytosolic fractions, although detection in neutrophil cytosol required more concentrated preparations and the use of high specific activity [H-3]AA-labeled Escherichia coli. HSF-PLA(2)-like activity was measured in microsomal and cytosolic fractions of all three cell types, but was found in neutrophil cytosol only after treatment with acid. Further HMM-PLA(2)-specific interfering agents in neutrophil cytosol were observed and exemplifies one problem in assigning the existence of this enzyme in crude broken cell preparations using activity measurements alone. The role that these two enzymes play in eicosanoid production of the respective cell types remains to be studied.