Reversible activation of secretory phospholipase A2 by sulfhydryl reagents

Secretory phospholipase A2s (sPLA2s) have been implicated in physiological and pathological events, but the regulatory mechanism(s) of their activities in cells remains to be solved. Previously, we reported that phenylarsine oxide (PAO), a sulfhydryl reagent, stimulated arachidonic acid (AA) release in rat pheochromocytoma PC12 cells. In this study, we examined the effects of thimerosal, another sulfhydryl reagent, to clarify the sulfhydryl modification and activation of sPLA2 molecules in cells. Like PAO, thimerosal-stimulated AA release in an irreversible manner and the responses were not additive. Dithiol compounds such as dithiothreitol inhibited AA release from both the thimerosal- and the PAO-treated cells, and monothiol compounds (l-Cys and glutathione) decreased the thimerosal response. Both sulfhydryl reagents stimulated AA release from the HEK293T cells expressing human sPLA2X, and stimulated the sPLA2 activities of bee venom sPLA2 and the soluble fraction of sPLA2X-expressing cells. Our results suggest that the sPLA2s in cells are inactive and modification of disulfide bonds in the molecules can be a trigger of sPLA2 activation in cells. Sulfhydryl reagents are useful tools for studying the regulatory mechanism(s) of sPLA2 activity in cells.