Previous studies at the Rowett Institute showed that novel branched-chain fatty acids are present (up to 15%) in the adipose tissue triglycerides of sheep fed on diets containing a high content of a carbohydrate-rich cereal, such as barley. Rumen bacterial fermentation of this carbohydrate produces considerable amounts of propionate which apparently exceeds the animals' hepatic capacity to utilise it for gluconeogenesis or oxidation. For these processes propionate must be converted to succinate via methylmalonate by a vitamin B12-dependent enzyme. It was postulated that, in cereal-fed sheep, the methylmalonate which accumulates is incorporated into fatty acids synthesised de novo, thereby giving rise to branched-chain components. In the present work the above observations were extended and the results and conclusions are summarised below. 1. Lipids of kidney, liver and brain obtained from barley-fed sheep contained 6.7%, 4.3% and 0.7% respectively of branched-chain fatty acids. The carcass lipids of a neonatal lamb, born to a ewe which had been fed on barley throughout pregnancy also contained small amounts (0.6%) of these acids. 2. A comparison of the hepatic capacities of barley-fed sheep and grass-fed sheep to oxidise propionate in vitro revealed that liver of barley-fed sheep had the greater oxidative capacity. Similarly, it was shown that the livers of barley-fed sheep had a greater ability to activate and to oxidise methylmalonic acid. 3. Whereas methylmalonyl CoA behaved as a classical competitive inhibitor towards fatty acid synthatase prepared from adipose tissue of grass-fed sheep and from chicken liver, it did not do so towards the synthetase from the adipose tissue of barley-fed sheep. This non-classical inhibition was associated with the concurrent utilisation of methylmalonyl CoA for synthesis of branched-chain fatty acids. 4. The incorporation of [2-14C]-methylmalonic acid and [2-14c]-methylmalonyl CoA into monomethyl branched-chain fatty acids was demonstrated. Some of these (novel) acids were presumptively identified by capillary column gas chromatography. 5. In other, complementary, studies it was found that the liver lipids of a wild baboon contained very small proportions of branched-chain acids, apparently derived from methylmalonate. The proportions were increased in vitamin B12-depleted animals, especially following the administration of a vitamin B12 analogue which was antagonistic to vitamin B12 in its metabolic effects.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:593398 |
| Date | January 1975 |
| Creators | Scaife, J. R. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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