Type II phospholipase A2 in human gestational tissues: subcellular distribution of placental immuno- and catalytic...

The aims of this study were to determine the subcellular distribution of Type II phospholipase A2 immunoactivity (irPLA2) and in vitro net PLA2 catalytic activity in human term placenta and to establish the efficacy of previously utilised homogenisation procedures with respect to the quantitative recovery of Type II PLA2 immunoreactive and in vitro net PLA2 catalytic activity. Type II PLA2 immunoactivity and PLA2 catalytic activity recovered in 900 x g supernates prepared from placental tissue (n = 3) homogenised in low ionic strength media (sucrose 0.32 M Hepes 20 mM; phosphate-buffered saline or phosphate-buffered saline containing 3 mM EGTA) was less than 10% of that recovered following homogenisation in high ionic strength medium (ammonium sulphate 10%, w/v). The subcellular distribution of Type II PLA2 immunoactivity and PLA2 catalytic activity was established by the differential centrifugation (10,000, 20,000 and 100,000 x g) of placental homogenates (n = 3). Although Type II PLA2 immunoactivity was equally distributed throughout the particulate subcellular fractions examined, PLA2 catalytic activity increased by comparison in 100,000 x g particulate material. This apparent dissociation between irType II PLA2 and catalytic activity may indicate the presence of other types of PLA2 in this fraction. The data obtained in this study indicate that previous studies which have utilised low ionic strength extractions of human gestational tissue to characterise PLA2 catalytic activity and subcellular distribution have largely excluded the contribution made by Type II PLA2. Consequently, much of the available published data on the role of PLA2 in human parturition is inadequate. A reappraisal of this enzyme's contribution to the biochemical events associated with human pregnancy and labour is required.