BIOGENESIS OF PHOSPHATIDES AND TRIGLYCERIDES

The metabolism of phospholipids in mouse brain slices

1. Slices of mouse brain grey matter were incubated with [(32)P]phosphate and [1-(14)C]acetate. Doubly labelled phospholipids were extracted from subcellular fractions prepared from the slices in a mixture of metabolic inhibitors, under conditions where there was negligible change in radioactive labelling during the preparation. Two tissue fractions were studied in detail; one contained a high proportion of mitochondria and the other was mainly microsomal. 2. In all tissue fractions the highest incorporations of both [(32)P]phosphate and [1-(14)C]acetate occurred into phosphatidylcholine. 3. After incubation for 1hr., the (32)P/(14)C ratios for phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid in the mitochondrial fraction were similar to those in the microsomal fraction. 4. The (32)P/(14)C ratios were similar in phosphatidylcholine and phosphatidylethanolamine and much lower than those in phosphatidic acid and phosphatidylinositol.

Phosphatidate phosphatase: activity and properties in fetal and adult rat lung

The purpose of this work is to compare the properties of phosphatidate phosphatase (L-alpha-phosphatidate phosphohydrolase, EC 3.1.3.4) in fetal and adult rat lung and to establish the developmental profile of activity measured under optimal conditions. The maximal pH of 6.0--7.0 and the inhibition by fluoride, Ca2+ and detergents were simialr for both adult and fetal. Phosphatidate phosphohydrolase activity was located in both mitochondria and microsomes. The localizations of marker enzymes indicated that the activity in these subfractions was not a result of cross contaminations. Very low activity was detected in the supernatant fraction and no Mg2+ requirement was demonstrable. The activity in the particulate fraction was about 50% of the adult from 18 day gestation until birth. Following birth, the activity rapidly increased to adult levels. Dipalmitoyl, dioleoyl and diacyl glycerol 3-phosphates are all utilized well as substrates. 1,2-dipalmitoyl-sn-glycerol 3-phosphate was hydrolyzed faster under maximal conditions. The velocity-substrate curves tended to be sigmoidal, particularly when 1,2-dipalmitoyl-sn-glycerol 3-phosphate was the substrate. Estimated apparent Km values of 0.02--0.03 mM were obtained for fetal and adult preparations.

Glyceride lipases in nerve endings of guinea-pig brain and their stimulation by noradrenaline, 5-hydroxytryptamine and adrenaline

1. Combined guinea-pig cortex and cerebellum was shown to contain triglyceride lipase, diglyceride lipase and monoglyceride lipase, which were assayed by the release of [1-(14)C]palmitate from [1-(14)C]palmitoylglycerol esters. Triglyceride lipase and diglyceride lipase were found in all particulate fractions. 2. With osmotically ruptured synaptosomes the rates of release of palmitate from glyceryl tripalmitate and glyceryl dipalmitate were 7-25mumol/h per g of protein and 0.18-0.69mmol/h per g of protein respectively. The logarithm of the rate of hydrolysis of glyceryl monopalmitate increased linearly with the logarithm of protein concentration. The pH optima of triglyceride lipase and diglyceride lipase were between 7 and 8. The pH optimum for monoglyceride lipase was approx. 8. 3. Triglyceride lipase and diglyceride lipase of osmotically ruptured synaptosomes were stimulated by noradrenaline, 5-hydroxytryptamine and adrenaline. Triglyceride lipase of isolated synaptic membranes was stimulated by 0.01-1mm-noradrenaline. Aging of membranes at 0 degrees C decreased activity, which could still be stimulated by noradrenaline. Diglyceride lipase of isolated membranes was stimulated by 1mum-1mm-noradrenaline. The activity of triglyceride lipase in isolated synaptic vesicles was diminished by 1mm-5-hydroxytryptamine.

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.

PHOSPHORYLATION OF DIGLYCERIDES BY RAT BRAIN

The addition of D-α,β-dimyristin was observed to stimulate by three to six times the labelling of phospholipids from radioactive inorganic phosphate (Pi32) by glycolysing homogenates and respiring mitochondria of rat brain. The increase in labelling was confined to the glycerophosphate (GP) isolated by two-dimensional chromatography from the water-soluble hydrolysis products obtained on weak alkaline treatment of the labelled phospholipids. The GP formed under these conditions is presumed to be derived mainly from phosphatidic acid formed by the phosphorylation of the diglyceride. A similar effect was observed for D-α,β-dipalmitin, D-α,β-diolein, and natural diglycerides prepared from either egg lecithin or spinal cord lecithin, but not for D-α-β-distearin. L-α,β-Diolein was much less effective than the D-isomer, suggesting a stereospecificity on the part of the enzymic phosphorylation of diglyceride. Experiments on the effects of the omission of Mg++ and the addition of glycolytic inhibitors on the stimulation of the labelling from Pi32 caused by D-α,β-dimyristin and D-α,β-diolein in the anaerobic homogenate system suggested that the increased phosphorylation caused by added diglycerides was closely coupled to active glycolysis. A comparison of the relative specific activity of the lipid P, following incubation of Pi32 and ATP32 in the anaerobic homogenate system inhibited by fluoride with and without D-α,β-diolein added, showed that the phosphate of the newly formed phosphatidic acid was derived from ATP, suggesting the presence of a D-α,β-diglyceride kinase.

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.