Cytoplasmic location of linoleoyl-CoA desaturase in microsomal membranes of rat liver

Polyunsaturated Fatty Acid Desaturation Is a Mechanism for Glycolytic NAD+ Recycling

The reactions catalyzed by the delta-5 and delta-6 desaturases (D5D/D6D), key enzymes responsible for highly unsaturated fatty acid (HUFA) synthesis, regenerate NAD⁺ from NADH. Here, we show that D5D/D6D provide a mechanism for glycolytic NAD⁺ recycling that permits ongoing glycolysis and cell viability when the cytosolic NAD⁺/NADH ratio is reduced, analogous to lactate fermentation. Although lesser in magnitude than lactate production, this desaturase-mediated NAD⁺ recycling is acutely adaptive when aerobic respiration is impaired in vivo. Notably, inhibition of either HUFA synthesis or lactate fermentation increases the other, underscoring their interdependence. Consistent with this, a type 2 diabetes risk haplotype in SLC16A11 that reduces pyruvate transport (thus limiting lactate production) increases D5D/D6D activity in vitro and in humans, demonstrating a chronic effect of desaturase-mediated NAD⁺ recycling. These findings highlight key biologic roles for D5D/D6D activity independent of their HUFA end products and expand the current paradigm of glycolytic NAD⁺ regeneration.

Liver delta 5 and delta 6 desaturase activity differs among laboratory rat strains

This study was designed to examine the variations among rat strains in hepatic fatty acid desaturase activities and to determine the correlations between the activities of these enzymes and the levels of each microsomal fatty acid. Wistar rats from two different sources as well as Long-Evans and Sprague-Dawley rats were selected to assess, under standard and identical experimental conditions, the liver delta 5 and delta 6 desaturase activities. Both desaturase activities were significantly reduced by 56% in Sprague-Dawley rats when compared to BB-Wistar control rats, whereas intermediate reduced values were detected in Wistar (CR) and Long-Evans strains. The activities of delta 5 and delta 6 desaturases were significantly and positively correlated with each other. However, no significant correlations were detected between either delta 5 or delta 6 desaturase activities and levels of any of their fatty acid substrates or any other of the major microsomal fatty acids. Fatty acid composition of microsomal total lipids showed strain dependency. A positive correlation was detected between the microsomal levels of the two major final products of both desaturases, namely 20:4n-6 and 22:6n-3. In general, the sum of n-3 or n-6 fatty acids but not the ratio of one to the other, varied among rat strains. The study demonstrated that delta 6 and delta 5 desaturase activities are strain-related. The data also suggested that (i) the desaturation activity should be measured and not predicted from the fatty acid composition and (ii) different rat strains should be used for lipid metabolic studies before conclusions are drawn for rats in general.

The effect of dietary sterol on the activity of fatty acid desaturases isolated from Tetrahymena setosa

Tetrahymena setosa has a nutritional requirement for micro amounts of sterol, a requirement which is also satisfied by relatively large amounts of either intact phospholipids or a mixture of unsaturated fatty acids normally found in these ciliates. Three microsomal fatty acyl-CoA desaturases have been isolated from T. setosa and partially characterized. These enzymes which can account for the formation of the majority of the ciliate's unsaturated fatty acids, include: a delta 9, a delta 12 and a delta 6 desaturase which catalyze the transformation of stearoyl-CoA to oleic acid, of oleoyl-CoA to linoleic acid and of linoleoyl-CoA to gamma-linolenic acid, respectively. The stearoyl CoA desaturase required NAD (or NADP), ATP and free CoA; the delta 6 and delta 12 desaturases required NADP, but not ATP or CoA. Cellular levels of the three desaturases were highest in mid-logarithmic phase cells and lowest in stationary phase cells. In order to determine if there was a relationship between the sterol requirement and the ability of the organism to desaturate, T. setosa was grown in a synthetic medium supplemented with either cholesterol or a phospholipid which permits growth in the absence of cholesterol, or with both phospholipid and cholesterol. Cells grown with phospholipid alone had only half as much stearoyl-CoA and oleoyl-CoA desaturase activity as cells of identical culture age grown either on cholesterol alone or on cholesterol plus phospholipid.

Modification of fatty acid composition of membrane phospholipid in hepatocyte monolayer with n-3, n-6 and n-9 fatty acids and its relationship to triacylglycerol production

The objective of these studies with rat hepatocytes in primary culture was to establish that: (a) membrane phospholipids would become enriched with the specific fatty acid supplemented to the media and (b) hepatocyte monolayer triacylglycerol synthetic rates were dependent on the type of fatty acid enrichment of the membrane phospholipids. Hepatocytes cultured in the absence of media lipid developed a phospholipid fatty acid composition which is indicative of an essential fatty acid deficiency. The extensive rise in 18:1(n - 9) content indicated that delta 9-desaturase was active. The fatty acid composition of phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol in the microsomal- and mitochondrial-enriched fractions was highly dependent upon the type of fatty acid supplemented to the medium. Incorporation of fatty acids into phospholipids was rapid, and a new steady-state in fatty acid composition was achieved within approx. 36 h. Changes in the fatty acid composition of these hepatocyte phospholipid subclasses resulting from media supplementation with 18:2/20:4(n-6) or 20:5(n-3) were similar, but not identical, to changes which occurred in vivo as a result of consuming diets rich in 18:2(n-6) or 20:5(n-3). Hepatocyte lipogenesis was highly dependent upon the type of fatty acid supplemented to the medium. Prior conditioning with 16:0 increased triacylglycerol synthesis and secretion. Secretion of triacylglycerol was reduced by polyenoic fatty acid enrichment with 20:5(n-3) greater than 20:4/18:2(n-6). The suppression of triacylglycerol synthesis by 20:5(n-3) was due to an increased (P less than 0.05) diacylglycerol specific activity, which indicates that 20:5(n-3) suppression of hepatic triacylglycerol production may be caused in part by the inhibition of diacylglycerol acyltransferase.

Biochemical characterization of acetyl-CoA:1-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyltransferase in rat spleen microsomes

Acetyl-CoA:1-alkyl-2-lyso-sn-glycero-3-phosphocholine (lyso-PAF) ultrasonic disruption in the presence of 25% glycerol from rat spleen microsomes. About 26% of the enzymatic activity was recovered in the 225,000 X g supernatant by this treatment, although the specific activity was slightly decreased compared with the original microsomes. The solubilized enzyme was remarkably susceptible to various kinds of metal ions. Sulfhydryl reagents such as p-chloromercuribenzoate and N-ethyl-maleimide significantly inhibited the enzyme reaction, suggesting that the enzyme is an SH enzyme. Based on the sedimentation pattern in sucrose density centrifugation, the isoelectric point, the kinetic characteristics and the sensitivity to tryptic digestion of microsomes, it appears that acetyl-CoA:lyso-PAF acetyltransferase does not differ from the acetyltransferase responsible for the transfer of acetate from acetyl-CoA to 1-acyl-2-lyso-sn-glycero-3-phosphocholine.