| Abstract: | Highly purified chromaffin granule membranes contain high levels (100 nmol/mg protein) of long-chain free fatty acids (Husebye, E.S. and Flatmark, T. (1984) J. Biol. Chem. 259, 15272-15276), as well as lysophosphatidylcholine (268 nmol/mg protein) and lysophosphatidylethanolamine (92 nmol/mg protein). The release of saturated and unsaturated long-chain fatty acids from endogenous phospholipids was 38 and 28 nmol/mg protein per h, respectively, at 37 degrees C and pH 7.5 (alkaline pH optimum). p-Bromophenacyl bromide inhibited the release of palmitate and oleate by 88 and 65%, respectively. The deacylation of membrane phospholipids was not significantly affected by micromolar free Ca2+. Based on experiments with pancreatic phospholipase A2, stearate and arachidonate were found to be suitable markers for deacylation at the sn-1 and sn-2 positions, respectively. Experiments with exogenously added labeled phosphatidylcholines confirmed that chromaffin granule ghosts contain a phospholipase A2 activity (alkaline pH optimum). The preparations also revealed a phospholipase A1 activity (acid pH optimum). Finally, the ghosts contain a lysophospholipase activity (alkaline pH optimum), that accounts for the major part of the deacylation of membrane phospholipids, notably the release of saturated fatty acids (stearate and palmitate). It is unlikely that the high content of lysophospholipids is an artifact of the procedure by which the granule ghosts are isolated. |
