Phospholipid acylation by mouse sciatic nerve microsomes

Dietary oleic acid not used during brain development and in adult in rat, in contrast with sciatic nerve

In order to determine exactly the effect on the nervous system of concentration of dietary oleic acid on the fatty acid composition of different part of the nervous system, triglycerides were synthesized using chemical and enzymological methods. The dose-effect was determined using an experimental protocol with seven groups of rats who received a diet in which the oleic acid level varied from 0 to 6000 mg per 100 g diet, but the other ingredients were identical (in particular the essential fatty acids, linoleic and alpha-linolenic acid). Rats were fed the diets from two weeks before mating, and their pups were sacrificed aged either 21 or 60 days. When the level of oleic acid in the diet was increased, the main modifications observed in 21-day-old deficient animals were as follows. (i). For 18:1(n-9), in liver, plateau was reached at about 4 g oleic acid per 100 g diet. Below this level, the higher the dose the greater the response. In whole brain, brain myelin, and nerve endings (but not sciatic nerve) the oleic acid level remained optimal and constant whatever the level of oleic acid in the diet. (ii). 16:1(n-7) concentration decreased in liver and in sciatic nerve, but not in nervous tissue. (iii). In 60-day-old animals, results were generally similar to those in 21-day-old animals.

Acyl-CoA:lysophosphatidylcholine acyltransferase activity in bovine retina rod outer segments

In the present paper the properties of acyl-CoA:lysophosphatidylcholine acyltransferase activity associated with rod outer segments (ROS) have been studied. Under adequate experimental conditions, ROS acyl-CoA:lysophosphatidylcholine acyltransferase activity presented a maximum at pH 7.0. The enzyme was able to incorporate as much as 60% of the label offered as [1-14C]oleoyl-CoA into phosphatidylcholine after 5 min of incubation. The use of varying concentrations of oleoyl-CoA and 46 microM lysophosphatidylcholine gave an apparent K(m) value for oleoyl-CoA of 100 microM and a Vmax value of 153 nmol x h-1 x (mg protein)-1. The use of varying concentrations of lysophosphatidylcholine and 100 microM oleoyl-CoA gave an apparent K(m) value for lysophosphatidylcholine of 27 microM and a Vmax value of 155 nmol x h-1 x (mg protein)-1. The enzyme was inhibited by 25% when ROS membranes were incubated in the presence of 10 mM MgCl2. The acyltransferase was able to incorporate other acyl-CoAs (palmitoyl-CoA and arachidonoyl-CoA) into ROS phospholipids and to acylate other lysophospholipids but less efficiently than lysophosphatidylcholine. Lysophoshatidylcholine was preferentially acylated with arachidonic acid followed by oleic acid and, less efficiently, with palmitic acid. The high specific activity of acyl-CoA lysophosphatidylcholine acyltransferase found in purified ROS compared to the activity found in other subcellular fractions of the bovine retina suggests that this enzymatic activity is native to the ROS.

Pathways of incorporation of fatty acid into glycerolipids of the murine peripheral nervous system in vivo: alterations in the dysmyelinating mutant trembler mouse

In vivo glycerolipid metabolism was studied in sciatic nerves of normal and Trembler mice. The results showed that two kinetically independent pathways were implicated in the labeling of diacylglycerophospholipids from [3H]palmitate: the Kennedy pathway and a 'direct acylation' pathway. In normal nerves, 45% of the glycerophospholipids were labeled, with a rate constant k3 = 3.9 x 10(-3) min-1, from phosphatidic acid and diacylglycerol intermediates, themselves formed with a rate constant of k1 = 0.24 min-1 from a free 3H-fatty acid pool, FFA1, that represents 45% of the total injected label. The remaining 55% of the glycerophospholipids were labeled from a kinetically distinct free 3H-fatty acid pool, FFA2, with a rate constant of k4 = 9.8 x 10(-2) min-1, via a process that does not implicate a detectably labeled metabolic intermediate ('direct acylation'). Glycerophospholipid labeling via the Kennedy pathway in the Trembler mouse sciatic nerves was reduced to 75% of the normal level, while labeling via the 'direct acylation' pathway was increased 1.4-fold. The values of the rate constants for free 3H-fatty acid utilisation (k1 and k4) were both increased about 2.5-fold, while that of glycerophospholipid formation from diacylglycerol (k3) was close to normal.

Acylation of endogenous acyl acceptors by mouse sciatic nerve microsomes

Phospholipid (chiefly phosphatidylcholine) labeling from radioactive acyl-CoAs by mouse sciatic nerve microsomes is observed in the absence of added acyl acceptors. The maximal acylation (ca 10% of administered) for 10 micrograms microsomal proteins is observed at relatively low amounts of oleoyl-CoA (0.2-0.3 nmol) and decreases as the acyl-CoA amount increases. Labeled lysophosphatidylcholine (almost exclusively esterified at position 2) is also observed, particularly when the [1-14C]oleoyl-CoA concentration is higher than 0.2-0.3 nmol/50 microliters. The labeled acyl group is mainly inserted in position 2] of the glycerophosphorylcholine. With 0.15 nmol labeled oleoyl-CoA, phosphatidylcholine acylation increases as a function of the protein amount and reaches 25% of the added label at 40 microgram proteins. It is evaluated that, in the presence of 10 microgram proteins, 2% of the microsomal phosphatidylcholine molecules are acylated from 0.1 nmol acyl-CoA. The acylation mechanism seems to involve an acyl exchange between acyl-CoA and phosphatidylcholine.

Cerebroside formation in the peripheral nervous system of normal and Trembler mice

The formation of cerebrosides by the galactosylation of ceramides was investigated in a microsomal fraction prepared from sciatic nerves of normal and Trembler mice. In the control, cerebroside synthesis is observed in the presence of uridine-diphosphate-galactose both from endogenously synthesized [1-14C]stearoyl-sphingosine (C18-ceramide), and from [1-14C]lignoceroyl-sphingosine (C24-ceramide). Cerebroside formation is also demonstrated by studying the galactosylation of exogenous ceramides with UDP[1-14C]-galactose. In the mutant, only trace amounts of labeled cerebrosides are formed from labeled stearoyl-sphingosine, whereas with lignoceroyl-sphingosine, no cerebroside synthesis is detected under conditions allowing their formation in the control. However, a higher rate of synthesis of short acyl chain-glucosyl ceramides is observed in the Trembler samples.

Peripheral nerve sphingomyelin and cerebroside are both formed via two metabolically and kinetically distinct pathways in vivo

We have studied the labeling kinetics of peripheral nerve sphingolipids in vivo. The kinetic analysis of the labeling profiles observed for the various sphingolipids demonstrated that 90% of cerebrosides, but only 30% of sphingomyelin, were synthesized via a de novo synthesized ceramide intermediate following the injection of 1-4 pmol [3H]palmitate into mouse sciatic nerves. The remaining sphingolipid labeling (30% of the total) was due to direct acylation events, using free fatty acids originating from a pool different from those implicated in the de novo ceramide pathway. Direct acylation events ceased within 1 h following substrate administration, while labeling via the ceramide pathway continued through 5 h. The results provide the first in vivo demonstration that the formation of cerebrosides and sphingomyelin in peripheral nerves in situ can be simultaneously assured via two metabolically and kinetically distinct pathways that employ different fatty acid pools.