Activation of long chain fatty acids by subcellular fractions of rat liver. 3. Effect of ethylenic bond position on acyl-CoA formation of cis-octadecenoates

Dissimilar properties of vaccenic versus elaidic acid in beta-oxidation activities and gene regulation in rat liver cells

Vaccenic acid (trans-11-C(18:1)) chemically resembles elaidic acid (trans-9-C(18:1)) which is assumed to increase the risk of cardiovascular diseases, and thus could exert similar effects. Possible different oxidation rates of vaccenic versus elaidic acid were checked in muscles and liver, and through related gene expression in normal rat liver cells. In hepatic mitochondria, carnitine palmitoyltransferase (CPT) I exhibited comparable activity rates with both trans-isomers. CPT II activity was 30% greater (P < 0.05) with vaccenic than with elaidic acid as nonesterified fatty acids (NEFAs) or acyl-CoAs. Activity of the first beta-oxidation step was similar between the isomers in all the tissue slices and liver extracts assayed. Respiration rates were comparable with both trans-isomers as NEFAs in various liver extracts, but were 30% greater (P < 0.05) with vaccenoyl-CoA than with elaidoyl-CoA in liver mitochondria. Vaccenic acid was oxidised 25% more (P < 0.05) by liver peroxisomes than elaidic acid. In hepatocytes cultured with trans- and corresponding cis-C(18:1) isomers, gene expression of CPT I, hydroxyacyl-CoA dehydrogenase and hydroxymethylglutaryl-CoA synthase was at least 100% increased (P < 0.05), but was unchanged with vaccenic acid, relative to controls. In conclusion, the position and geometry of the double bonds in acyl chains are suggested to confer on vaccenic and elaidic acid specific biochemical properties that might differently affect their fates in tissues.

Desaturation of positional and geometric isomers of monoenoic fatty acids by microsomal preparations from rat liver

A range of cis- and trans-monoenoic fatty acids was tested as substrates for desaturation in microsomal preparations from rat liver. Trans-monoenoic acids were generally desaturated in the delta 9 position to the same extent as stearic acid. Acids with delta 7-trans- and delta 11-trans-olefinic unsaturation produced delta 7-trans, 9-cis- and delta 9-cis, 11-trans-conjugated dienoic acids, respectively, but the delta 8-trans-and delta 10-trans-monoenoic acids did not give delta 8,9- or delta 9,10-allenes. Of the cis-monoenoic acids examined, only those with double bonds at or beyond the delta 14 position gave any measurable delta 9 desaturation. When delta 9 desaturation of long chain saturated acids was inhibited by adding sterculic acid, these saturated acids were desaturated at the delta 5 and delta 6 positions. Many of the monoenoic acids tested were also desaturated at the delta 5 and/or delta 6 positions, although the percentage conversions were always low. delta 9-cis, 11-trans-, delta 9-cis, 12-trans- and delta 9-cis, 13-trans-dienoic acids, produced in situ by delta 9 desaturation of the corresponding monoenoic acids, were extensively desaturated in the delta 6 position. These results are discussed in terms of: (a) the various models proposed to explain the substrate specificities of the desaturases, and (b) the metabolism of unnatural fatty acids ingested from dietary sources.

Incorporation of cis- and trans-octadecenoic acids into the membranes of rat liver mitochondria

The incorporation of dietary isomeric fatty acids into the membranes of liver mitochondria was investigated. Three groups of rats were fed diets containing 3% sunflower seed oil plus 15%, 20%, or 25% partially hydrogenated arachis oil. A fourth group was fed 25% partially hydrogenated arachis oil, but no sunflower seed oil. All diets were given for 3, 6, or 10 weeks. After 10 weeks, the content of trans fatty acids in the lipids of the mitochondrial membranes was 15--19% of the total fatty acids. The composition of the trans- and the cis-octadecenoic acids in the lipids of the mitochondrial membranes was similar for all groups supplemented with sunflower seed oil (SO), irrespective of time and dietary level of partially hydrogenated arachis oil (HAO). The cis 18:1 (n-8), which was a major isomer of the partially hydrogenated arachis oil, was almost excluded from the mitochondrial fatty acids. Likewise, the content of trans 18:1 (n-8) was considerably lower in the mitochondrial lipids than in the diet. On the contrary, the content of trans 18:1 (n-6) was higher in the mitochondrial lipids than in the diet. In the group fed without sunflower seed oil, isomers of linoleic acid and arachidonic acid were observed in the lipids of mitochondrial membranes.

beta-Oxidation of the coenzyme A esters of elaidic, oleic, and stearic acids and their full-cycle intermediates by rat heart mitochondria

beta-Oxidation rates for the CoA esters of elaidic, oleic and stearic acids and their full-cycle beta-oxidation intermediates and for the carnitine esters of oleic and elaidic acids were compared over a wide range of substrate and albumin concentrations in rat heart mitochondria. The esters of elaidic acid were oxidized at about half the rate of the oleic acid esters, while stearoyl-CoA was oxidized equally as rapid as oleoyl-CoA. The full-cycle beta-oxidation intermediates of elaidoyl-CoA (trans-16 : 1 delta 7, -14 : 1 delta 5, and -12 : 1 delta 3) were found to be oxidized at rates nearly equal to those for the corresponding intermediates of oleoyl-CoA. Therefore, after the first cycle of beta-oxidation, oleoyl-CoA and elaidoyl-CoA are oxidized at nearly equal rates. The activity of fatty acyl-CoA dehydrogenase was higher with elaidoyl-CoA and its full-cycle intermediates as substrates than with the corresponding cisisomers. It was concluded that the slower oxidation rate of elaidic acid is not due to slower oxidation of any of its full-cycle beta-oxidation intermediates, nor to slower activity of fatty acyl-CoA dehydrogenase, nor to outer mitochondrial carnitine acyltransferase. Possible explanations to account for the slower oxidation rate of elaidic acid are discussed.

Double bond position affects metabolism of cis-Octadecenoates

The metabolic fate of cis-positional isomers of octadecenoates has been compared to that of naturally occurring oleic acid (cis-delta9). Radioactive mixtures of tritium-labeled positional octadecenoate isomer and oleic acid-10(-14)C were administered to laying hens, and their eggs were analyzed for the isotopic ratios (3H/14C) incorporated into total egg lipid, triglycerides, and phospholipids. Variations in the isotopic ratios indicated the comparative metabolic utilization of cis-positional isomers delta8 through delta12. Incorporation into egg lipid fractions is as follows: triglycerides: delta9 greater than delta8, delta9 greater than delta10, delta9 greater than delta11, delta9 greater than 12; phospholipid: delta9 greater than delta8, delta9 greater than delta10, delta9 less than delta11, delta9 less than delta12.

In vitro incorporation of isomeric cis-octadecenoic acids by rat liver mitochondria

The metabolic fate of positional isomers of cis-octadecenoic acids was compared to that of oleic acid, elaidic acid and stearic acid in rat liver mitochondria. The positional isomers as well as elaidic acid and stearic acid were labelled with 3H and they were incubated in pairs with [1-14C]oleic acid. 3H/14C ratios were determined for the administered mixtures and for the isolated lipid classes. The results suggested that all isomers were readily incorporated into the membraneous structure of mitochondria. Those with the double bond in the middle of the acyl chain resembled oleic acid and they were preferentially incorporated in cholesterol esters, triacylglycerols and in the 2-position of triacylglycerols and of phosphatidylcholines. Those with the double bond away from the middle of the chain were metabolically distinct from oleic acid and behaved like trans fatty acids. They were rapidly taken up by mitochondria. They were preferentially incorporated in phospholipids and they occupied the 1-position in phosphatidylcholines and the 1- and 3-positions in triacylglycerols.