Control by ganglioside GD1a of phospholipase A2 activity through modulation of the lamellar-hexagonal (HII) phase transition.

Ganglioside GD1a has a dual effect on the formation of the HII phase in mixtures of dioleoylphosphatidyl choline (DOPC)-dioleoylphosphatidylethanolamine (DOPE) (1:5). Below 1 mol% ganglioside in the mixture the formation of the HII phase is facilitated as indicated by a decrease of the lamellar-HII phase transition temperature; above 1 mol% the presence of ganglioside GD1a opposes formation of the HII phase and causes an increase of the phase transition temperature; the latter is completely abolished at 3 mol% GD1a or above. The rate of activity of phospholipase A2 against these mixtures showed a temperature variation in coincidence with the establishment of the lamellar-hexagonal II phase transition. In the presence of GD1a at 0.5 and 1 mol% in the mixture, the maximum activity shifted in correspondence with the ganglioside-induced displacement of the lamellar-hexagonal II phase transition. The temperature variation of the enzymatic activity did not show any maximum for pure DOPC of for DOPC-DOPE (1:5) mixtures containing 3 mol% GD1a or above, in which no lamellar-hexagonal II phase transition occurs. The effects of GD1a are probably due to the geometrical features of the ganglioside molecule that allow a composition-dependent compensation of the structural defects required for the formation of the HII phase which are detected by phospholipase A2.