CHARACTERIZATION OF CALCIUM-INDEPENDENT CYTOSOLIC PHOSPHOLIPASE-A(2) ACTIVITY IN THE SUBMUCOSAL REGIONS OF RAT STOMACH AND SMALL-INTESTINE

This study was undertaken to compare the calcium-independent phospholipase A(2) (PLA(2)) activities in the cytosols of twelve rat tissues and to determine whether their activities were distinct. 1-O-Alk-1'-enyl-2-[C-l4]-oleoyl-sn-glycero-3-phosphocho-line (PlsC) and 1-O-ALk-1'-enyl-2-[C-14]oleoyl-sn-glycero-3-phosphethanolamine (PlsE) were synthesized and used as substrates, instead of phosphatidyl compounds, to exclude hydrolysis by cytosolic PLA(1) activity that could be present in some of the cytosolic preparations. For each tissue, we examined substrate specificity, pH optimum, and effect of adenosine triphosphate (ATP) and ATP analogues. PLA(2) activity was detected in eleven out of the twelve issues examined. Based on substrate specificity and pH optimum, cytosolic calcium-independent PLA(2) were classified in three groups. The first group, which included PLA(2) from small intestine, stomach and spleen, had the highest specific activity with PlsC as substrate (1253, 309 and 75 nmol/mg protein/hour, respectively) and an optimal pH at 6.5. Activity with PlsE as substrate was much lower (20-37%) than with PlsC. The second group of PLA(2) activities included the cytosolic activities from thymus, lung, liver and pancreas that showed lower specific activities for both substrates (14-23 nmol/mg protein/hour with PlsC) and had a broader optimal pH range of 6.1 to 7.5. The cytosols from brain, kidney, heart and muscle comprised the third PLA(2) group that was found to have a higher specific activity with PlsE (5-20 nmol/mg protein/hour) than PlsC and an optimal pH range from 7.4 to 7.9. Since the highest specific activity was found in the cytosol from small intestine, this PLA(2) was examined further. PLA(2) activity was found to be equally distributed in the cytosol of the submucosal portion of duodenum, jejunum and ileum with an optimal pH of 6.1 and a 5-fold higher activity with PlsC than PlsE as substrate. Moreover, this PLA(2) activity was inhibited by treatment with detergents. These results indicate the presence in the submucosal portion of the intestine of a calcium-independent cytosolic PLA(2) with a high specific activity toward PlsC and properties distinct from those described for the PLA(2) found in the intestinal brush-border.