Vaccenic and elaidic acid equally esterify into triacylglycerols, but differently into phospholipids of fed rat liver cells.

Elaidic acid (trans-9-C18:1 or trans-9) is assumed to exert atherogenic effects due to its double bond configuration. The possibility that trans-9 and vaccenic acid (trans-11-C18:1 or trans-11), its positional isomer, were biochemically equivalent and interchangeable compounds, was investigated by reference to their cis-isomers through esterification-related activities using rat liver cells and subcellular fractions. In hepatocytes, both trans-C18:1 were incorporated to the same extent in triacylglycerols, but trans-9 was more esterified than trans-11 into phospholipids (P < 0.05). Glycerol-3-phosphate acyltransferase activity in microsomes was lower with trans-11 than with trans-9, while this activity in mitochondria was ~40% greater with trans-11 than with trans-9 (P < 0.05). Activity of 2-lysophosphatidic acid acyltransferase in microsomes was of comparable extent with both trans isomers, but activity of 2-lysophosphatidylcholine acyltransferase was significantly greater with trans-9 than with trans-11 at P < 0.01. Lipoproteins secreted by hepatocytes reached equivalent levels in the presence of any isomers, but triacylglycerol production was more elevated with trans-11 than with trans-9 at P < 0.05. Cholesterol efflux from previously labelled hepatocytes was lower with trans-11 than with trans-9. When these cells were exposed to either trans-C18:1, the gene expression of proteins involved in fatty acid esterification and lipoprotein synthesis was unaffected, which indicates that the biochemical differences essentially depended on enzyme/substrate affinities. On the whole, vaccenic and elaidic acid were shown to incorporate cell phospholipids unequally, at least in vitro, which suggests they can differently affect lipid metabolic pathways in normal cells.

Effects of tetradecylthiopropionic acid and tetradecylthioacrylic acid on rat liver lipid metabolism.

Studies of effects of 4-thia-substituted fatty acid analogues on rat liver lipid metabolism are described. With isolated hepatocytes tetradecylthiopropionate was shown to divert [1-14C]oleate from beta-oxidation into esterification, the total amount of [1-14C]oleate metabolized remaining unchanged. Tetradecylthiopropionyl-CoA was a good substrate for mitochondrial carnitine palmitoyltransferases I and II (EC 2.3.1.21), acyl-CoA oxidase (EC 1.3.3.6), for the microsomal (but not mitochondrial) glycerophosphate acyltransferase (EC 2.3.1.15), and for long-chain acyl-CoA dehydrogenase (EC 1.3.99.3). In isolated hepatocytes, its 4-thia-trans-2-enoic derivative, tetradecylthioacrylate, inhibits both beta-oxidation of, and incorporation of, [1-14C]oleate into lipids. In rat liver mitochondria tetradecylthiocrylate inhibited beta-oxidation. The degree of inhibition was not markedly increased by preincubation with tetradecylthioacrylate. Tetradecylthioacrylyl-CoA was a poor substrate for carnitine palmitoyltransferase I, and inhibited carnitine palmitoyltransferase II, microsomal glycerophosphate acyltransferase and acyl-CoA oxidase. It is concluded that the inhibitory effects of tetradecylthiopropionyl-CoA are expressed intramitochondrially, whereas primary sites of inhibition by tetradecylthioacrylyl-CoA are extramitochondrial.

The response of brown adipose tissue mitochondrial glycerolphosphate acyltransferase to cold-exposure in hypothyroidism, after adrenalectomy and after treatment with cycloheximide.

1. Exposure to cold has previously been shown to considerably increase the activity of the mitochondrial form of glycerolphosphate acyltransferase (GPAT) in brown adipose tissue (A. C. Darnley, C. A. Carpenter and E. D Saggerson, Biochem. J. 253, 351-355, 1988; J. R. D. Mitchell and E. D. Saggerson. Biochem. J. 277, 665-669, 1991). 2. Both adrenalectomy and chemically-induced hypothyroidism increased mitochondrial GPAT activity in rats maintained at 21 degrees C. This increase was similar to that caused by exposing rats to the cold (4 degrees C) for three days. Whereas exposure of hypothyroid rats to cold (4 degrees C) resulted in a further increase in GPAT activity, no further increase in activity was observed after exposure of adrenalectomized rats to the cold. 3. Administration of triiodothyronine (T3) to rats maintained at 21 degrees C had no effect on mitochondrial GPAT activity. 4. Prior treatment with cycloheximide abolished 60-70% of the increase in GPAT activity caused by cold-exposure.