EFFECT OF PHOSPHATIDYLCHOLINE STRUCTURE ON THE ADENYLATE-CYCLASE ACTIVITY OF A MURINE FIBROBLAST CELL-LINE

To determine which structural characteristics of membrane phospholipid s influence adenylate cyclase activity, we measured basal and sodium f luoride- or forskolin-stimulated activity in a murine fibroblast cell line, i.e., Balb/c3T3 cells grown in media supplemented with fetal cal f serum (FCS), lipid-depleted FCS (LD-FCS) or LD-FCS complexed with di fferent phosphatidylcholine (PC) molecular species. Cells grown in the presence of LD-FCS showed a substantial decrease in their basal and N aF-stimulated adenylate cyclase activities; however, their forskolin-s timulated activity was not altered, suggesting that the enzyme's catal ytic site is not affected by changes in membrane lipids. Media supplem ented with different LD-FCS/PC complexes were shown to prevent the LD- FCS-mediated reduction of basal and NaF-stimulated adenylate cyclase a ctivity to different extents. Addition of cis-9-16:1/cis-9-16:1, cis-9 -18:1/cis-9-18:1 or cis-9-18:1/cis-9,12-18:2 sn-glycerophosphocholine (GPC) completely restored adenylate cyclase activity, while cis-11-18: 1/cis-11-18:1 GPC was not effective and only a partial recovery was ob served with 16:0/16:0, 16:0/cis-9-18:1 and trans-9-18:1 GPC. Consideri ng the structural features of these seven PC molecular species, the fi ndings suggest that an optimal lipid environment is conferred to the e nzyme by the presence of two cis double bonds, each located in DELTA9 position of the PC acyl chains. The limited effect of cis-9-16:1/cis-9 -18:1 GPC and cis-9-18:1/cis-9-16:1 GPC suggests that an equal length of the terminal hydrocarbon chains extending beyond the DELTA9 double bonds is also important. Moreover, complete restoration of adenylate c yclase activity in cells exposed to 16:0/cis-9,12-18:2 GPC suggests th at two cis-9,12 double bonds located on the same chain are as effectiv e as two cis-9 double bonds each located on two different chains of PC . As the four double bonds of 16:0/cis-5,8,11,14-20:4 GPC had no effec t, a mere increase in the number of double bonds seems insufficient to build an optimal lipid microenvironment for the enzyme.