The regulation of hepatic acetate-1-C14 metabolism by short-chain fatty acids

Carnitine and derivatives in rat tissues

1. Free carnitine, acetylcarnitine, short-chain acylcarnitine and acid-insoluble carnitine (probably long-chain acylcarnitine) have been measured in rat tissues. 2. Starvation caused an increase in the proportion of carnitine that was acetylated in liver and kidney; at least in liver fat-feeding had the same effect, whereas a carbohydrate diet caused a very low acetylcarnitine content. 3. In heart, on the other hand, starvation did not cause an increase in the acetylcarnitine/carnitine ratio, whereas fat-feeding caused a decrease. The acetylcarnitine content of heart was diminished by alloxan-diabetes or a fatty diet, but not by re-feeding with carbohydrate. 4. Under conditions of increased fatty acid supply the acid-insoluble carnitine content was increased in heart, liver and kidney. 5. The acylation state of carnitine was capable of very rapid change. Concentrations of carnitine derivatives varied with different methods of obtaining tissue samples, and very little acid-insoluble carnitine was found in tissues of rats anaesthetized with Nembutal. In liver the acetylcarnitine (and acetyl-CoA) content decreased if freezing of tissue samples was delayed; in heart this caused an increase in acetylcarnitine. 6. Incubation of diaphragms with acetate or dl-beta-hydroxybutyrate caused the acetylcarnitine content to become elevated. 7. Perfusion of hearts with fatty acids containing an even number of carbon atoms, dl-beta-hydroxybutyrate or pyruvate resulted in increased contents of acetylcarnitine and acetyl-CoA. Accumulation of these acetyl compounds was prevented by the additional presence of propionate or pentanoate in the perfusion medium; this prevention was not due to extensive propionylation of CoA or carnitine. 8. Perfusion of hearts with palmitate caused a severalfold increase in the content of acid-insoluble carnitine; this increase did not occur when propionate was also present. 9. Comparison of the acetylation states of carnitine and CoA in perfused hearts suggests that the carnitine acetyltransferase reactants may remain near equilibrium despite wide variations in their steady-state concentrations. This is not the case with the citrate synthase reaction. It is suggested that the carnitine acetyltransferase system buffers the tissue content of acetyl-CoA against rapid changes.

Short chain acyl-CoA synthetases in ovine rumen epithelium

The activation of and interactions between physiological concentration of acetate, propionate, and butyrate are reported. A partially purified shortchain acyl-CoA synthetase has been prepared from ovine rumen epithelium and its kinetic properties characterised. These properties suggest the activation of volatile fatty acids by two short-chain acyl-CoA synthetases. One is capable of activating acetateactivating acetate, propionate and butyrate and the other will only accept butyrate as a substrate.

Activation of volatile fatty acids in bovine liver and rumen epithelium. Evidence for control by autoregulation

1. The total capacities of homogenates of bovine liver and rumen epithelium to activate acetate, propionate and butyrate were determined. 2. Activating capacities were assayed by measuring the rate of formation of the corresponding CoA esters. The methods used for determining the concentrations of the CoA esters allowed the CoA esters of acetate, propionate and butyrate to be distinguished. It was thus possible to investigate the effect of the presence of a second volatile fatty acid on the rate at which a given volatile fatty acid was activated. 3. The propionate-activating capacity in rumen epithelium was decreased by about 87% in the presence of butyrate, the acetate-activating capacity in liver was decreased by about 55% in the presence of either propionate or butyrate, and the butyrate-activating capacity in liver was decreased by about 40-50% in the presence of propionate. 4. All three activating capacities in liver appeared to be located in the mitochondrial matrix and membrane. The three activating capacities had similar locations to each other in rumen epithelium as well, although in this case activity was more evenly divided between the mitochondria and the cytoplasm. 5. The relative activating capacities towards the volatile fatty acids in the two tissues, together with the ability of one volatile fatty acid to inhibit the activation of another volatile fatty acid, appear to ensure that butyrate is mainly metabolized in the rumen epithelium and that propionate is metabolized in the liver.

EFFECTS OF ALIPHATIC ALCOHOLS AND FATTY ACIDS ON THE METABOLISM OF ACETATE BY RAT LIVER SLICES

The effects of the addition of normal aliphatic alcohols and fatty acids on the metabolism of acetate by rat liver slices have been investigated, with particular reference to the formation, from acetate-1-C14, of C14O2 and radioactive lipids, proteins, and fatty acids. Whereas the addition of unlabelled acetate causes a fall in the rate of formation of C14O2, the decrease, at low concentrations, is less than that calculated for isotopic dilution. This is probably due to the fact that with an increased concentration of acetate there is an increased rate of acetate oxidation. However, the addition of ethanol causes a larger fall in the rate of formation of C14O2 than would be expected if the alcohol were converted to acetate. This points to some inhibition by ethanol of acetate oxidation, a conclusion borne out by the inhibitory effects of n-alcohols on total CO2 formation (in the absence and in the presence of added acetate) by rat liver slices. The fact, however, that the inhibitory effect of ethanol reaches a maximum at 5 mmolar and is constant to 50 mmolar points against a major inhibition due to ethanol per se. The results would be consistent with the conclusion that ethanol, at low concentrations, is more quickly converted into acetyl-CoA than acetate itself and that its speed of conversion into acetyl-CoA reaches a maximum at about 5 mmolar with rat liver slices. n-Propanol is much more inhibitory than ethanol but its effect is quantitatively identical with that of an equivalent quantity of propionate. Propionate and n-propanol are presumably rapidly converted to propionyl-CoA (or methyl malonyl-CoA), which is the effective inhibitor of acetate oxidation by its competition with acetyl-CoA. Propanol also shows direct inhibitory effects on acetate oxidation in rat liver slices. n-Butanol, n-pentanol, and n-hexanol have inhibitory effects identical with those of equivalent concentrations of corresponding fatty acids. These facts point to the oxidation, in rat liver, of aliphatic alcohols and fatty acids to propionyl-CoA or acetyl-CoA which inhibit C14O2 formation from acetate-1-C14 by competition or isotopic dilution. Alternations of inhibition occur between the odd- and even-numbered carbon alcohols and fatty acids which may be explained by the formation of mixtures of acetyl-CoA and the highly inhibitory propionyl-CoA from the long-chain alcohols and acids. Tribromoethanol is less inhibitory than propanol at equivalent concentrations though it is an effective inhibitor of acetate oxidation. Allyl alcohol is a much more potent inhibitor. Ethanol inhibits incorporation of radioactivity of acetate-1-C14 into fatty acids in rat liver, the inhibition being approximately equal to, or possibly less than, that due to an equivalent quantity of acetate. This may be explained on the basis of isotopic dilution, by acetyl-CoA derived from the alcohol, with radioactive acetyl-CoA undergoing synthesis to radioactive fatty acids. Thus ethanol, at the concentrations investigated, by taking part in fatty acid synthesis, inhibits acetate conversion to fatty acids. Glucose shows the largest effect in stimulating fatty acid synthesis in the liver from acetate, fructose is less effective, and sorbitol shows no effect. The aliphatic alcohols inhibit incorporation of radioactivity from acetate-1-C14 into liver lipids and proteins, propanol and pentanol showing the largest effects. There is an alternation between odd- and even-numbered aliphatic chains. The disappearance, or utilization, of acetate by rat liver slices is inhibited by the aliphatic alcohols, alternations between the odd- and even-numbered chains again being observable. The results are explained as being partly due to isotopic dilution and competition by acetyl-CoA and propionyl-CoA, and partly due to suppressions of acetate metabolism by the alcohols.