Formation Of Lecithin Research Articles

Metabolism of labeled lysolecithin, lysophosphatidyl ethanolamine and lecithin in the rat

Labeled serum phospholipids were injected intravenously to rats and their disappearance from the circulation was studied. The half-life of total plasma phospholipids was found to be 47 min. In the disappearance curve of lysolecithin, two components were found. The half-life derived from the initial slope ranged from 6 to 11 min. Lysophosphatidyl ethanolamine had a disappearance curve similar to that of lysolecithin. The lysophosphatides which disappear from the blood stream were recovered in organs such as the liver, small intestine, skeletal muscles, lungs, kidneys and heart. In the liver and small intestine a rapid and complete conversion of lysolecithin to lecithin and of lysophosphatidyl ethanolamine to phosphatidyl ethanolamine was observed, while in the other organs these reactions proceeded at a slower rate. The pathways of formation of lecithin from lysolecithin were studied in the liver, intestine, lungs and kidneys. Using lysolecithin labeled with [ 32P]- and [1- 14C]-palmitic acid it was shown that in vivo the acylation reaction, leading to the formation of lecithin with a 32P 14C ratio similar to that of the lysolecithin, is the predominant one. The origin of plasma lysolecithin has been studied in rats injected with [ 32P]-lecithin or β-[1- 14C]linoleoyl lecithin. The labeled lecithin was predominantly taken up by the liver. Up to 10% of the radioactivity remaining in the plasma was found in lysolecithin, 30–150 min after injection. At early time intervals after injection of β-[1- 14C]linoleoyl lecithin, labeled cholesterol ester was found in the serum, but little or none in the liver. It has been concluded that the fatty acid transferase reaction between lecithin and free cholesterol, described in vitro, is also operative in vivo and that it contributes to the formation of plasma lysolecithin. A scheme for a lysolecithin-lecithin cycle in the intact rat is proposed.

Metabolism of phospholipids by polymorphonuclear leukocytes

By incubating homogenates of polymorphonuclear leukocytes obtained from rabbit-peritoneal exudates with various 32P-labeled phosphoglycerides the following reactions were identified: Lecithin → Lysolecithin and fatty acid (1) Lysolecithin → Glycerylphosphorylcholine and fatty acid (2) Lysolecithin → Lecithin (3) The mechanism of the last reaction appears to be the recently postulated (Erbland and Marinetti) pathway: 2 lysolecithin → lecithin and glycerylphosphorylcholine. This conclusion is based upon: (a) [ 32P]lysolecithin accumulation occurred without appreciable incorporation of [ 14C]linoleic acid. (b) Lysolecithin carrying 32P in the phosphorylcholine moiety and 14C in the fatty acid yielded lecithin with a 14C/ 32P ratio 2 times that of the original lysolecithin. (c) ATP and CoA did not stimulate [ 32P] lecithin formation nor incorporation of [ 14C]linoleic acid. Lysolecithin is a membrane-lytic agent. Its production and removal by conversion to glycerylphosphorylcholine and lecithin may therefore play a role in the lysis and renewal of cellular membranes.

The metabolism of lysolecithin in rat-liver particulate systems

The following reactions occur in the rat-liver preparations studied, lysolecithin → glycerophosphorylcholine + fatty acid (1) lysolecithin + acyl-CoA → lecithin (2) 2 lysolecithin → lecithin + glycerophosphorylcholine (3) In enzyme systems containing microsomes, lecithin synthesis from lysolecithin occurs primarily by Reaction 2 which is ATP and CoA dependent. In the particlefree cytoplasmic fluid Reaction 1 is predominant. This reaction is not influenced by added ATP or CoA. Triglyceride, diglyceride, phosphatidylethanolamine, inositol phosphatide and cholesterol esters do not act as acyl donors for the esterification of lysolecithin in ratliver homogenates. The formation of lecithin in the microsomal system by Reaction 2 is approx. 6 times greater than the synthesis of lecithin in the cytoplasmic fluid by Reaction 3.

Metabolism of red-cell lipids II. Conversions of lysophosphoglycerides

Lysolecithin is converted by lysed rabbit erythrocytes into lecithin and glycerylphosphorylcholine. The formation of lecithin by two reactions was demonstrated: by a transacylation requiring the addition of ATP and CoA, and by a dismutation of lysolecithin, not involving the incorporation of exogenous fatty acid but accompanied by the formation of glycerophosphorylcholine. under conditions favorable for fatty acid incorporation the first-mentioned pathway was found to be predominant. In addition, lysolecithin appears to be degraded by a lysophospholipase (EC 3.1.1.5).

Fatty acid esterification and chylomicron formation during fat absorption: 2. Phospholipids

Studies were conducted on the over-all fatty acid specificity of the mechanisms involved in chylomicron lecithin formation during fat absorption in the rat. Synthetic mixtures of fatty acids containing trace quantities of C14-labeled fatty acids were administered by gastric intubation to rats with cannulated thoracic ducts. The chylomicron lipids were first separated on silicic acid columns into cholesterol ester, glyceride, and phospholipid fractions. The results obtained with the sterol ester and glyceride fractions are described in the accompanying paper. The phospholipids were rechromatographed to obtain pure lecithin, and determinations were made of the total fatty acid radioactivity, total mass, and distribution of mass and radioactivity, among the several fatty acids of lecithin, by gas-liquid chromatography. Different fatty acids fed to the rat were not incorporated to equal extents into chylomicron lecithin. The incorporation showed a marked relative specificity for stearic acid and a lesser specificity for linoleic acid. Oleic acid was incorporated least of all into lecithin. The incorporation of different dietary fatty acids varied with the nature of the diet, but the addition of fatty acids from endogenous sources was of such magnitude as to make this change less evident, so that the over-all fatty acid pattern of lecithin was relatively constant and independent of the composition of the diet. The endogenous contribution to lecithin fatty acids was greater than the endogenous contribution to the sterol ester or triglyceride fatty acids of the same chylomicron sample. This endogenous contribution to lecithin also varied from fatty acid to fatty acid, being greatest with palmitic acid and least with oleic acid. The general features of chylomicron formation are discussed in detail. Lecithin carried a relatively large share of stearic acid, and relatively vcry little oleic acid, within the chylomicron.

LIPID SYNTHESIS IN RAT LIVER PARTICLES: DIVERSION OF SYNTHESIS FROM TRIGLYCERIDE TO LECITHIN BY THE IN VITRO ADDITION OF CYTIDINE DIPHOSPHATE CHOLINE

When particle preparations from rat or chicken liver were incubated in a suitable medium containing α-glycerophosphate-C14, radioactivity was recovered from lecithin, phosphatidyl inositol, phosphatidic acid, and triglyceride. Whole homogenate and various particle preparations catalyzed the dephosphorylation of a number of phosphatidic acids, with the liberation of inorganic P.It is currently believed that liver preparations are capable of catalyzing the esterification of L-α-glycerophosphate, with the formation of L-α-phosphatidic acid, which subsequently may be dephosphorylated to form D-α,β-diglyceride. The diglyceride so formed may then give rise either to lecithin, by combining with phosphorylcholine from cytidine diphosphate choline, or to triglyceride, by combining with fatty acid from fatty acyl coenzyme A. If these reactions occur in liver particle preparations, it should be possible, by the addition in vitro of unlabelled cytidine diphosphate choline, to divert the synthesis of lipid from the formation of triglyceride to the formation of lecithin. In experiments designed to put this hypothesis to the experimental test, such a diversion of lipid synthesis was achieved.

LIPID SYNTHESIS IN RAT LIVER PARTICLES: DIVERSION OF SYNTHESIS FROM TRIGLYCERIDE TO LECITHIN BY THE IN VITRO ADDITION OF CYTIDINE DIPHOSPHATE CHOLINE

When particle preparations from rat or chicken liver were incubated in a suitable medium containing α-glycerophosphate-C14, radioactivity was recovered from lecithin, phosphatidyl inositol, phosphatidic acid, and triglyceride. Whole homogenate and various particle preparations catalyzed the dephosphorylation of a number of phosphatidic acids, with the liberation of inorganic P.It is currently believed that liver preparations are capable of catalyzing the esterification of L-α-glycerophosphate, with the formation of L-α-phosphatidic acid, which subsequently may be dephosphorylated to form D-α,β-diglyceride. The diglyceride so formed may then give rise either to lecithin, by combining with phosphorylcholine from cytidine diphosphate choline, or to triglyceride, by combining with fatty acid from fatty acyl coenzyme A. If these reactions occur in liver particle preparations, it should be possible, by the addition in vitro of unlabelled cytidine diphosphate choline, to divert the synthesis of lipid from the formation of triglyceride to the formation of lecithin. In experiments designed to put this hypothesis to the experimental test, such a diversion of lipid synthesis was achieved.

Lecithin formation by methylation of intact phosphatidyl dimethylethanolamine

S>Results are reported from tracer studies of lecithin formation by methylation of intact phosphatidyl dimethylethanolamine (DME). After incubation of C/sup 14/ labeled DME with rat liver slices, the isotope was found to be present in the DME as well as in the choline moiety of the phospholipids. Results are tabulated from typical experiments. Possible reaction mechanisms involved are discussed. (C.H.)