Positional fatty acid composition in total and acetone-precipitated amniotic fluid lecithin.

The fatty acid composition of lecithin has been determined separately at the 1- and 2-position in 22 samples of amniotic fluid. Palmitic acid (16:0) was the main fatty acid present at both carbon atom positions throughout the last trimester. Myristic acid (14:0) was present in very small amounts only. There was no evidence of 1-palmitoyl/2-myristoyl lecithin being present in more than trace amounts before 35 weeks of gestation. The positional specificity of the methylation pathway is therefore doubtful. Acetone precipitation of lecithin did not alter the observed fatty acid composition at either position, indicating that this procedure precipitates the different lecithin subspecies in equal proportion, without any preference for disaturated species. This method therefore fails to isolate a lecithin fraction with a specific fatty acid composition.