In vitro synthesis and transbilayer movement of phosphatidylethanolamine molecules labelled with different fatty acids in chick brain microsomes

Effects of snake venom phospholipase A2 toxins (beta-bungarotoxin, notexin) and enzymes (Naja naja atra, Naja nigricollis) on aminophospholipid asymmetry in rat cerebrocortical synaptosomes

The effects of snake venom phospholipase A2 (PLA2) toxins (beta-bungarotoxin, notexin) and PLA2 enzymes (Naja nigricollis, Naja naja atra) on aminophospholipid asymmetry in rat cerebrocortical synaptic plasma membranes (SPM) were examined. Incubation of intact synaptosomes with 2 mM 2,4,6-trinitrobenzene sulfonic acid (TNBS) for 40 min, under non-penetrating conditions, followed by SPM isolation, allowed us to calculate the percentage of phosphatidylethanolamine (PE) and phosphatidylserine (PS) in the outer leaflet of the SPM, while incubation with disrupted synaptosomes provided total labeling values with the difference representing labeling of the inner leaflet. We found that 30% of the PE and 2% of the PS were in the outer leaflet, with 54% of the PE and 80% of the PS in the inner leaflet; 16% of the PE and 18% of the PS was inaccessible to TNBS. PLA2 toxins and enzymes increased in a concentration-dependent manner the percentage of PS and, to a lesser extent, the percentage of PE in the outer leaflet of the SPM, due to a redistribution from the inner to the outer leaflet. There was no correlation between the PLA2 enzymatic activities and the increased percentage of PS in the outer leaflet of the SPM induced by the PLA2 toxins and enzymes. Alteration of aminophospholipid asymmetry does not explain the greater presynaptic specificity and potencies of the PLA2 toxins as compared to the PLA2 enzymes, but may be associated with the increased acetylcholine release from synaptosomes induced by both the toxins and enzymes.

Polyunsaturated fatty acid incorporation into plasmalogens in plasma membrane of glioma cells is preceded temporally by acylation in microsomes

Plasmalogens (1-O-alk-1'-enyl-2-acyl-sn-glycero-3-phosphoethanolamine) are major phospholipids in many tissues and cells, particularly of neural origin. Using cultured C6 glioma cells and subcellular fractions isolated on Percoll gradients we investigated selectivity for esterification of several polyunsaturated fatty acids (PUFA) in the sn-2 position of plasmalogens compared to [1-14C]hexadecanol, representative of de novo synthesis of the ether-linked sn-1 position. In whole cells at a final concentration of 105 microM PUFA, 2-4 nmol plasmalogen/mg protein was labeled in 4 h and 10-14 nmol in 24 h, representing 8-15% and 35-50%, respectively, of initial plasmalogen mass. Incorporation of label from hexadecanol was lower than PUFA incorporation (20:5(n-3) greater than 20:4(n-6) greater than 18:3(n-3) much greater than 18:2(n-6)) suggesting deacylation-reacylation at the sn-2 position. Plasmalogens accounted for 50% of total cell ethanolamine phospholipids and 75% in plasma membrane. Using a novel, improved method for extraction of subcellular fractions containing Percoll, plasma membrane also was enriched in plasmalogen relative to microsomes (107.4 +/- 5.2 vs. 40.0 +/- 2.9 nmol/mg protein). Selectivity for esterification at the sn-2 position of plasmalogens with respect to chain length and unsaturation of the fatty acyl chain was similar in both subcellular fractions and reflected that of whole cells. Labeling of plasma membrane with PUFA and fatty alcohol lagged behind that of microsomes. Chase experiments in cells prelabeled with [1-14C]18:3(n-3) for 2 h showed no significant reduction of label in plasmalogen of any subcellular fraction although accumulation of label in the microsomal fraction was slowed initially. Reduction of plasmalogen label (40-50%) did occur in microsomes and plasma membrane when cells prelabeled for 24 h were switched to chase medium with or without chase fatty acid. Our data suggest that esterification of PUFA to plasmalogen may occur at the endoplasmic reticulum with subsequent translocation to plasma membrane resulting in accumulation of relatively stable pools of plasmalogen that are not readily accessible for deacylation-reacylation exchange with newly appearing PUFA. Alternatively, deacylation-reacylation may occur in a more stable phospholipid pool within the plasma membrane but would involve a slower process than at the endoplasmic reticulum.