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The hydrolytic activity of secretory phospholipase A2 (sPLA2) toward mammalian cells depends on the health of the cell. During the process of programmed cell death, changes occur in the cell membrane that render it susceptible to hydrolytic attack. Various isozymes of sPLA2 respond differently to cell death depending on the timing and origin of the process. This study used flow cytometry to classify subpopulations of S49 lymphoma cells during several modes of programmed death depending on their vulnerability to sPLA2 and identify the basis for isozyme specificity. Most death stimuli (thapsigargin, dexamethasone, actinomycin D, paclitaxel, and methotrexate) caused a reduction in membrane lipid-neighbor interactions detected as increased binding of the fluorescent dye merocyanine 540 and modest permeability to a vital stain, propidium iodide. In each of these cases, all sPLA2 isozymes tested (snake venom and human groups IIa, V, and X) displayed enhanced ability to hydrolyze the cell membrane. In contrast, cells exposed to a calcium ionophore showed the increase in merocyanine 540 binding without accompanying permeability to propidium iodide. Under these conditions, only the snake venom and human group X enzymes hydrolyzed cells that were dying. Lastly, the human group IIa enzyme, although most active of the isoforms tested toward anionic artificial bilayers, hydrolyzed dying cells at a rate that was only 1/100 that of the other isozymes. These results suggested that each of these human isozymes fills different physiological roles in responding to cell death and provides probable molecular explanations for the distinctions. |
