CA2+-INDEPENDENT PHOSPHOLIPASE-A2 ACTIVITY ASSOCIATED WITH SECRETORY GRANULAR MEMBRANES IN RAT PAROTID-GLAND

Phospholipase A2 activity was detected in a secretory granular fraction (SG) purified by Percoll gradient centrifugation from rat parotid gland using [H-3]phosphatidylcholine (PC) as a substrate. High activity of this enzyme was observed at neutral pH. The enzyme was activated by Triton X-100 and did not require Ca2+ for its activity. In the absence of Ca2+, its apparent K(m) for exogenous PC was 28-mu-M while it was slightly increased by adding 5 mM CaCl2 (73-mu-M). Furthermore, the enzyme was located essentially in a granular membrane fraction separated from granular lysate. The deacylation activities were also detected in other subcellular fractions, which showed a different detergent-susceptibility or pH-dependency from that in SG. These results suggest that secretory granules have membrane-bound phospholipase A2 which has properties different from that found in other organelles.