Digestion of phosphatidylcholines, absorption, and esterification of lipolytic products by Aeshna cyanea larvae as studied in vivo and in vitro.

Digestion and absorption of phosphatidylcholine by Aeshna cyanea larvae were studied in vivo and in vitro with the isolated digestive juice and isolated midgut. The experiments were performed with stable ether analogues (1-alkyl-2-acyl-,1,2-dialkyl phosphatidylcholine, and 1-monoalkyl-lysophosphatidylcholine), with radioactive 1,2-diacylphosphatidylcholine alternatively labelled in the acyl- and choline moieties, and with several phosphatidylcholine derivatives (1- [1-14C]acyl- and 1-[3H] alkyl-lysophosphatidylcholine, [1-14C]oleic acid, [2-14C]glycerol, phosphoryl[methyl-14C]choline, and [methyl-14C]choline). Chromatographic analyses of the digestion products revealed that phosphatidylcholine was degraded via two interconnected hydrolytic pathways involving phospholipase C, phospholipase A2, lipase, and alkaline phosphatase. Complete hydrolysis by these pathways yielded the same four end products: free fatty acid, glycerol, choline, and Pi, which were absorbed by the midgut enterocytes. Of the intermediate hydrolysates, lysophosphatidylcholine, monoacylglycerol, and possibly phosphorylcholine were also absorbed. Radiolabelled oleic acid, glycerol, lysophosphatidylcholine and monoacylglycerol (as judged from monoacylglycerol absorption) were incorporated into phospholipids and acylglycerols of the midgut enterocytes and were released into the haemolymph primarily in the form of diacylglycerols. In the case of glycerol ingestion, a small fraction of haemolymph radioactivity was associated with free glycerol and glycerolphosphate. After absorption by the enterocytes, radiolabelled choline was partly oxidized to betaine, partly phosphorylated, and partly incorporated into lyso- and phosphatidylcholine. It was recovered from the haemolymph predominantly as free choline, phosphorylcholine, and betaine.