Influence of the physical state of the membrane on the enzymatic activity and energy of activation of protein kinase C α

The activation of protein kinase C alpha was studied by using a lipid system consisting of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero(POPS) (molar ratio 4:1) and different proportions of 1-palmitoyl-2-oleoyl-sn-glycerol (POG), The phase behavior of the lipidic system was characterized by using differential scanning calorimetry and P-31 NMR, and a phase diagram was elaborated. The results suggested the formation of two diacylglycerol/phospholipid complexes, one at 15 mol % of POG and the second at 30 mol % of POG. These two complexes would define the three regions of the phase diagram: in the first region (concentrations of POG lower than 15 mol %) there is gel-gel immiscibility at temperatures below that of the phase transition between Cl and pure phospholipid, and a fluid lamellar phase above of the phase transition. In the second region (between 15 and 30 mol % of POG), gel-gel immiscibility between C-1 and C-2 with fluid-fluid immiscibility was observed, while inverted hexagonal Hn and isotropic phases were detected by P-31 NMR. In the third region (concentrations of FOG higher than 30 mol %), gel-gel immiscibility seemed to occur between CZ and pure FOG along with fluid-fluid immiscibility, while an isotropic phase was detected by P-31 NMR. When PKC alpha activity was measured, as a function of FOG concentration, maximum activity was found at FOG concentrations as low as 5-10 mol %; the activity slighty decreased as FOG concentration was increased to 45 mol % at 32 degrees C (above T-c) whereas activity did not change with increasing concentrations of POG at 5 degrees C (below T-c). When the activity was studied as a function of temperature, at different POG concentrations, and depicted as Arrhenius plots, it was found that the activity increased with increasing temperatures, showing a discontinuity at a temperature very close to the phase transition of the system and a lower activation energy at the upper slope of the graph, indicating that the physical state of the membrane affected the interaction of PKC alpha with the membrane.