The molecular species of lecithin from erythrocyte and plasma of man and rabbit were determined after conversion of the lecithins into diglycerides by means of hydrolysis with phospholipase C. The resultant diglycerides were separated by thin-layer chromatography on silica impregnated with silver nitrate into 6 or 7 fractions differing with respect to their degree of unsaturation. The positional distribution of the fatty acids in these fractions was determined by hydrolysis with pancreatic lipase and was found to be in agreement with the positional distribution of the fatty acids in the lecithin as ascertained by means of phospholipase A hydrolysis. Using these techniques about 20 molecular species accounting for about 90% of the total lecithin, could be evaluated in the erythrocyte and plasma of man and rabbit.\ud \ud It became clear that qualitatively the molecular species of lecithin in the red cell and the plasma are similar. Quantitatively, however, there were some striking differences to be noted: in man the amount of (dipalmitoyl)- and (di-oleoyl)-lecithin was higher in the corpuscles when compared with plasma. On the other hand (1-palmitoyl-2-linoleoyl)- and (1-palmitoyl-2-arachidonoyl)-lecithin were more abundant in plasma.\ud \ud In rabbit similar differences were found in the make-up of the molecular species of lecithin between the erythrocyte membrane and the surrounding plasma
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