A model for the molecular mechanism of interfacial activation of phospholipase A2 supporting the substrate theory

Changes occurring in the activity of porcine pancreatic phospholipase A2 upon formation of mixed micelles of sodium cholate and the fluorescent phosphocholines 1,2-di[6-(pyren-1-yl)butanoyl]-sn-glycero-3-phosphocholine or 1-[6-(pyren-1-yl)butanoyl]-2-[6-(pyren-1-yl)hexanoyl]- sn-glycero-3- phosphocholine were studied. A 2-fold enhancement was observed in the activity of phospholipase A2 towards both pyrene phospholipids upon exceeding the critical micellar concentration of the system. Changes in the pyrene excimer/monomer fluorescence emission intensity ratio coincide with the enhancement of phospholipase A2 activity at the critical micellar concentration. Due to the different effects of micellization on the alignment of the pyrene in the two fluorescent probes conformational changes could be assessed. A model describing possible conformations of these pyrene phospholipid molecules below and above the critical micellar concentration is presented and correlated with the interfacial activation of phospholipase A2.