Comparative in vivo and in vitro study of the influence of experimental diabetes on rat liver linoleic acid delta 6- and delta 5-desaturation

Rats with experimental diabetes were administered in vivo a tracer dose of either [1-14C]-linoleic, [1-14C]-gamma-linolenic or [2-14C]-dihomo-gamma-linolenic acid and sacrificed 48 h later. With all three radioactive precursors, the radioactivity incorporated into arachidonic acid was lower in experimental diabetes, compared to nondiabetic rats similarly treated, while the weights of hepatic arachidonic acid were not significantly affected by the diabetic state. Streptozotocin-treated rats were administered moderate or excessive quantities of protamine-zinc-insulin. Streptozotocin diabetes inhibits rat liver homogenate [2-14C]-dihomo-gamma-linolenic acid delta 5-desaturation; only moderate injections of protamine-zinc-insulin restore the in vitro delta 5-desaturation. These results suggest that experimental diabetes, a reported inhibitor of delta 6-desaturation, also causes partial inhibition of delta 5-desaturation in rat liver; this suggests that dihomo-gamma-linolenic acid desaturation, a secondary regulatory step in linoleic acid metabolism, may be restored through an optimum insulin therapy.     

Metabolism of lysolecithin in vivo: effects of hyperlipemia and atherosclerosis in squirrel monkeys

We have studied the effect of long-term hyperlipemia and atherosclerosis in squirrel monkeys on the metabolism of lysolecithin-(14)C (1-palmitoyl-1'-(14)C sn-glycerol 3-phosphorylcholine) in order to explain elevated plasma and arterial concentrations of lysolecithin. The die-away curves of lysolecithin-(14)C from plasma and the timing of appearances of other (14)C-labeled moieties in plasma and other tissues demonstrated a complex pattern of metabolic reactions. There was a rapid equilibration of specific activities of lysolecithin of plasma, liver, and aortic intima plus inner media. The specific activities of lecithin peaked first in liver, then in plasma, and rose slowly in aortic intima plus inner media. The appearance of lecithin-(14)C in heart and skeletal muscle was also slower than in the liver and some other tissues. Triglycerides, and to a lesser extent, cholesteryl esters contained radioactivity. The concentrations of aortic lysolecithin in the atherosclerotic aortas were several times greater than comparable values for control aortas, and the time of equilibration of plasma and aorta lysolecithin-(14)C was much greater for the atherosclerotic group. The quantities of lysolecithin in plasma and in the pool of which the plasma was a part, were increased with hyperlipemia and atherosclerosis, as was the rate of lysolecithin production in the fast pool. Hyperlipemia was also associated with an early increase in plasma lecithin:cholesterol acyltransferase (LCAT) activity in vitro. Furthermore, nutritional hyperlipemia influenced the distribution of lysolecithin-(14)C and lecithin-(14)C between different plasma lipoproteins. The increase in concentrations of lysolecithin in the aorta occurred more slowly than that in plasma after we had induced hyperlipemia in the monkeys.     

Conversion of testosterone and androstenedione to 5 beta-androstanes by adult male hamster liver cytosol

The incubation of [4-14C]testosterone with adult male hamster liver cytosol at pH 6.7 yielded 5 beta-androstane-3 alpha, 17 beta-diol and small quantities of 5 beta-androstane-3 beta, 17 beta-diol, 17 beta-hydroxy-5 beta-androstan-3-one, 3 alpha-hydroxy-5 beta-androstan-17-one and androstenedione. The use of [4-14C]androstenedione as substrate yielded the same 5 beta-metabolites and also testosterone and a trace of epitestosterone. 5 beta-Androstane-3 alpha, 17 beta-diol was the major metabolite at "low" concentrations of substrate but testosterone and 3 alpha-hydroxy-5 beta-androstan-17-one became the major metabolites as the concentration of the substrate was increased. Small quantities of 5 beta-androstane-3,17-dione and 3 beta-hydroxy-5 beta-androstan-17-one were detected at "high" while 5 beta-androstane-3 alpha, 17 alpha-diol was detected at "low" concentrations of androstenedione. NADPH was more effective than NADH except in the formation of the 3 beta-steroids. Furthermore, the 3 beta-steroids were formed in maximum quantities at a lower pH than the other metabolites. The relative production of the metabolites was consistent with their respective spectrophotometrically determined degree of hydroxyl dehydrogenation.     

Propionate mitochondrial toxicity in liver and skeletal muscle: acyl CoA levels

Propionic acidemia occasionally produces a toxic encephalopathy resembling Reye syndrome, indicating disruption of mitochondrial metabolism. Understanding the mitochondrial effect of propionate might clarify the pathophysiology. Liver mitochondria are inhibited by propionate (5 mM) while muscle mitochondria are not. Preincubation is required to inhibit liver mitochondria, suggesting that propionate is metabolized to propionyl CoA. Liver and skeletal muscle mitochondria incubated with [1-14C]propionate contain similar quantities of matrix isotope and release comparable [14C]CO2. However, only liver mitochondria accumulated significant propionyl CoA, which was largely (68%) synthesized from propionate. Carnitine reduced the level of liver matrix propionyl CoA. Inhibition of respiratory control ratios by propionate correlated with propionyl CoA levels. These results support the hypothesis that acyl CoA esters are toxic and that carnitine exerts its protective effect by converting acyl CoA esters to acylcarnitine esters.     

Substantial quantities of the high energy derivative oligophosphoglyceroyl-ATP are located in mitochondria in rat liver

Perfusion with [8-14C]adenosine demonstrated the likely existence in rat liver of oligophosphoglyceroyl-ATP (OPG-ATP). Purification followed by assay with a new specific 3' phosphodiesterase confirmed this. The quantities present were 5-10-fold those found previously and comparable to total soluble nucleotides. OPG-ATP was also purified from the mitochondrial fraction, shown to co-distribute with succinate dehydrogenase and can be co-purified with an enzyme confined to intermembrane space.     

Effects of ethynyloestradiol on the metabolism of [1-14C]-oleate by perfused livers and hepatocytes from female rats

Normal female rats were given 15mug of ethynyloestradiol/kg body wt. for 14 days and were killed on day 15 after starvation for 12-14h. The livers were isolated and were perfused with a medium containing washed bovine erythrocytes, bovine serum albumin, glucose and [1-(14)C]oleic acid; 414mumol of oleate were infused/h during a 3h experimental period. The output of bile and the flow of perfusate/g of liver were decreased in livers from animals pretreated with ethynyloestradiol, whereas the liver weight was increased slightly. The rates of uptake and of utilization of [1-(14)C]oleate were measured when the concentration of unesterified fatty acid in the perfusate plasma was constant. The uptake of unesterified fatty acid was unaffected by pretreatment of the animal with oestrogen; however, the rate of incorporation of [1-(14)C]oleate into hepatic and perfusate triacylglycerol was stimulated, whereas the rate of conversion into ketone bodies was impaired by treatment of the rat with ethynyloestradiol. Pretreatment of the rat with ethynyloestradiol increased the output of very-low-density lipoprotein triacylglycerol, cholesterol, phospholipid and protein. The production of (14)CO(2) and the incorporation of radioactivity into phospholipid, cholesteryl ester and diacylglycerol was unaffected by treatment with the steroid. The net output of glucose by livers from oestrogen-treated rats was impaired despite the apparent increased quantities of glycogen in the liver. The overall effect of pretreatment with oestrogen on hepatic metabolism of fatty acids is the channeling of [1-(14)C]oleate into synthesis and increased output of triacylglycerol as a moiety of the very-low-density lipoprotein, whereas ketogenesis is decreased. The effect of ethynyloestradiol on the liver is apparently independent of the nutritional state of the animal from which the liver was obtained. It is pertinent that hepatocytes prepared from livers of fed rats that had been treated with ethynyloestradiol produced fewer ketone bodies and secreted more triacylglycerol than did hepatocytes prepared from control animals. In these respects, the effects of the steroid were similar in livers from fed or starved (12-14h) rats. Oestrogens may possibly inhibit hepatic oxidation of fatty acid, making more fatty acid available for the synthesis of triacylglycerol, or may stimulate the biosynthesis of triacylglycerol, or may be active on both metabolic pathways.     

Formation of retinoic acid from retinol in the rat

1. The formation in vivo of retinoic acid from microgram quantities of intrajugularly administered [15-(14)C]retinol was demonstrated in the rat. 2. Endogenously formed retinoic acid (about 0.1mug./rat) was found in liver, and to a much smaller extent in intestine, 12hr. after retinol administration. 3. Excretion of some of the endogenously formed retinoic acid occurred in the bile of bile-duct-cannulated rats. 4. Excretion of unaltered retinoic acid in the urine of intact rats did not occur even after the intrajugular administration of preformed retinoic acid.     

Intermediates in fatty acid oxidation

1. Aqueous extracts of acetone-dried liver and kidney mitochondria, supplemented with NAD⁺, CoA and phenazine methosulphate, efficiently convert fatty-acyl-CoA compounds into acetyl-CoA; the process was followed with an O₂ electrode. 2. Label from [1-¹⁴C]octanoyl-CoA appears in acetyl-CoA more rapidly than that from [8-¹⁴C]octanoyl-CoA. 3. Oxidation of [8-¹⁴C]octanoyl-CoA was terminated by addition of neutral ethanolic hydroxylamine and the resulting hydroxamates were separated chromatographically. Hydroxamate derivatives of 3-hydroxyoctanoyl-, hexanoyl-, butyryl- and acetyl-CoA were obtained. 4. These and other observations suggest that oxidation of octanoyl-CoA by extracts involves participation of free intermediates rather than uninterrupted complete degradation of individual molecules to acetyl-CoA by a multienzyme complex. 5. Intact liver mitochondria studied by the hydroxamate technique were also shown to form intermediates during oxidation of labelled octanoates. In addition to octanoylhydroxamate, [8-¹⁴C]octanoate gave rise to small amounts of hexanoyl-, butyryl- and 3-hydroxyoctanoyl-hydroxamate. In contrast with extracts, however, where the quantity of intermediates found was a significant fraction of the precursors, mitochondria oxidizing octanoate contained much larger quantities of octanoyl-CoA than of any other intermediate.     

Isolation and characterization of 1 alpha-hydroxy-23-carboxytetranorvitamin D: a major metabolite of 1,25-dihydroxyvitamin D3.

The in vivo side-chain oxidation of 1 alpha,25-dihydroxyvitamin D3 was investigated by using a double-label radiotracer technique. Rats dosed with 1 alpha,25-dihydroxy-[3 alpha-3H]vitamin D3 and 1 alpha,25-dihydroxy[26,27-14C]vitamin D3 produced compounds with a high 3H/14C ratio. These compounds were found in sizable quantities in intestine and liver within 3 h after dosing. The major side-chain oxidized metabolite migrated as an acid on DEAE-Sephadex chromatography and contained no 14C. Methyl esterification of this compound with diazomethane proceeded in good yield and rendered the compound more amenable to chromatographic purification. The metabolite was isolated in several steps from rats dosed with 1 microgram of 1 alpha,25-dihydroxy[3 alpha-3H]vitamin D3. The metabolite was obtained in pure form as the methyl ester and was positively identified as 1 alpha,3 beta-dihydroxy-24-nor-9,10-seco-5,7,10(19)cholatrien-23-oic acid. The trivial name calcitroic acid is proposed for this major side-chain oxidized metabolite of 1,25-dihydroxyvitamin D3.     

Oxidation of 2-oxoisocaproate and 2-oxoisovalerate by the perfused rat heart. Interactions with fatty acid oxidation

The interactions between fatty acid oxidation and the oxidation of the 2-oxo acids of the branched chain amino acids were studied in the isolated Langendorff-perfused heart. 2-Oxoisocaproate inhibited the oxidation of oleate, but 2-oxoisovalerate and 2-oxo-3-methylvalerate did not. This difference was not attributable to the magnitude of the flux through the branched chain 2-oxo acid dehydrogenase, which was slightly higher with 2-oxoisovalerate than with 2-oxoisocaproate. Oxidation of 2-oxoisocaproate in the perfused heart was virtually complete, since more than 80% of the isovaleryl-CoA formed from 2-oxo[1-14C]isocaproate was further metabolized to CO2, as determined by comparing 14CO2 production from 2-oxo[14C(U)]isocaproate with that from the 1-14C-labelled compound. Only twice as much 14CO2 was produced from 2-oxo[14C(U)]isovalerate as from the 1-14C-labelled compound, indicating incomplete oxidation. This was confirmed by the accumulation in the perfusion medium of substantial quantities of labelled 3-hydroxyisobutyrate (an intermediate in the pathway of valine catabolism), when hearts were perfused with 2-oxo[14C(U)]isovalerate. The failure of 2-oxoisovalerate to inhibit fatty acid oxidation, then, can be attributed to the fact that its partial metabolism in the heart produces little ATP. We have previously shown that 3-hydroxyisobutyrate is a good gluconeogenic substrate in liver and kidney, and postulate that 3-hydroxyisobutyrate serves as an interorgan metabolite such that valine can serve as a glucogenic amino acid, even when its catabolism proceeds beyond the irreversible 2-oxo acid dehydrogenase in muscle.     

Accumulation and release of triglycerides by rat liver following partial hepatectomy

Regenerating liver accumulates lipid for about 20 hr following partial hepatectomy. During this time incorporation of intravenously administered palmitate-9, 10-(3)H into beta-lipoprotein increased. 13 hr after partial hepatectomy, there was no change in the level of serum beta-lipoproteins, but the specific activities of the triglycerides in the liver and beta-lipoproteins were significantly diminished. Extension of these studies to the isolated perfused liver system demonstrated that 13 hr after partial hepatectomy the regenerating liver is capable of secreting greater quantities of the lipid, but not the protein, moiety of the beta-lipoproteins in comparison with liver taken immediately from a partially hepatectomized animal, although there was no difference between the weights of the livers. However following addition of palmitate-(3)H and (14)C-labeled amino acids to the perfusate, the specific activity of the hepatic and beta-lipoprotein triglycerides of the liver excised 13 hr after partial hepatectomy was diminished, but that of the protein was not affected. Prelabeling of the accumulated triglyceride with palmitate-1-(14)C in vivo revealed that the proportions of the accumulated triglyceride secreted as beta-lipoproteins by perfused livers excised immediately and 13 hr after partial hepatectomy were identical. It is concluded that regenerating liver rapidly acquires the ability to mobilize triglycerides at a rate equal to that of the much larger normal liver, so that it can handle all free fatty acids presented to it.     

Mechanism of action of beta-oxoacyl-CoA thiolase from rat liver cytosol. Direct evidence for the order of addition of the two acetyl-CoA molecules during the formation of acetoacetyl-CoA.

Single-turnover enzyme reactions were employed with beta-oxoacyl-CoA thiolase purified from rat liver cytosol to determine the order of binding of the two acetyl-CoA molecules to the enzyme during the formation of acetoacetyl-CoA. Equimolar quantities of [1-14C]acetyl-CoA and enzyme were mixed initially in a rapid mixing device and the reaction was quenched by addition of an excess of unlabelled acetyl-CoA. Degradation of the resulting acetoacetyl-CoA revealed that the larger proportion of the radioactivity was in C-3. In the converse experiment, in which unlabelled acetyl-CoA was mixed with enzyme and the reaction was quenched with [1-14C]acetyl-CoA, radioactivity was incorporated preferentially into C-1. Similar results were obtained when [14C]acetyl-enzyme complex isolated by gel filtration was reacted with unlabelled acetyl-CoA, the radioactivity appearing largely in C-3. These findings lead to the conclusion that of the two molecules of acetyl-CoA that are bound by the enzyme and converted into acetoacetyl-CoA, it is the one giving rise to C-3 and -4 that is bound initially to the enzyme in the form of the acetyl-enzyme intermediate complex.     

N6-Trimethyl-lysine metabolism. 3-Hydroxy-N6-trimethyl-lysine and carnitine biosynthesis.

Rats injected with N6-[Me-3H]trimethyl-lysine excrete in the urine five radioactively labelled metabolites. Two of these identified metabolites are carnitine and 4-trimethylammoniobutyrate. A third metabolite, identified as 5-trimethylammoniopentanoate, is not an intermediate in the biosynthesis of carnitine; the fourth and major metabolite, N2-acetyl-N6-trimethyl-lysine, is not a precursor of carnitine. The remaining metabolite (3-hydroxy-N6-trimethyl-lysine) is converted into trimethylammoniobutyrate and carnitine by rat liver slices and into trimethylammoniobutyrate by rat kidney slices. In rat liver and kidney-slice experiments, radioactivity from DL-N6-trimethyl-[1-14C]lysine and DL-N6-trimethyl-[2-14C]lysine was incorporated into N2-acetyl-N6-trimethyl-lysine and 3-hydroxy-N6-trimethyl-lysine, but not into trimethylammoniobutyrate or carnitine. A procedure was devised to purify milligram quantities of 3-hydroxy-N6-trimethyl-lysine from the urine of rats injected chronically with N6-trimethyl-lysine (100 mg/kg body wt. per day). The structure of 3-hydroxy-N6-trimethyl-lysine was confirmed chemically and by nuclear-magnetic-resonance spectrometry [Novak, Swift & Hoppel (1980) Biochem. J. 188, 521--527]. The sequence for carnitine biosynthesis in liver is: N6-trimethyl-lysine leads to 3-hydryxy-N6-trimethyl-lysine leads to leads to 4-trimethylammoniobutyrate leads to carnitine.     

THE INFLUENCE OF BILIRUBIN ON OXIDATIVE PHOSPHORYLATION AND RELATED REACTIONS IN BRAIN AND LIVER MITOCHONDRIA: EFFECTS OF PROTEIN-BINDING

1. The uncoupling of oxidative phosphorylation of liver mitochondria by bilirubin does not occur in the presence of equimolar quantities of human serum albumin. With brain mitochondria, however, albumin was not protective. 2. A similar protective effect of albumin for liver, but not for brain, mitochondria was observed in studies of the effects of bilirubin on the ³²Pi-ATP exchange reaction. 3. The latent ATPase of fresh brain mitochondria is activated by Mg²⁺ but only slightly by DNP. Bilirubin increased the Mg²⁺ stimulated ATPase activity in liver mitochondria but depressed this activity in brain mitochondria. These effects were uninfluenced by protein binding. 4. Isotope studies with [¹⁴C]bilirubin demonstrated that the affinity of brain mitochondria for albumin-bound bilirubin is not greater than that of liver mitochondria. 5. The greater toxicity of protein-bound bilirubin for brain mitochondria than for liver mitochondria might be related to the greater lipid content of brain mitochondria.     

Enzymic studies of glycol and glycerol lipids containing O-alkyl bonds in liver and tumor tissues

The utilization of [1-¹⁴C]hexadecyl-[2-³H]ethyleneglycol and [1-¹⁴C]hexadecyl-[2-³H]glycerol as substrates for acyltransferase, phosphotransferase, phosphorylcholine, and phosphorylethanolamine transferase, and O-alkyl cleavage activities in cell-free preparations from normal rat liver and preputial gland tumors of mice was investigated. Our studies demonstrate that alkylethyleneglycols, like alkylglycerols, can serve as substrates for acyltransferases in both the liver and tumor microsomes; the product alkylacylethyleneglycerol can be readily deacylated by pancreatic lipase. A polar lipid was formed from the alkylethyleneglycol by the tumor homogenates in the presence of ATP and Mg²⁺; although the small quantities formed precluded absolute identification, its thin-layer Chromatographic behavior in acidic and basic solvent systems indicated that a free phosphate group was present. As expected, phosphorylbase transferases in these preparations did not utilize either the alkylethyleneglycol or alkylglycerol as substrates. The O-alkyl moiety of hexadecyl-ethyleneglycol was oxidized to hexadecanal by a tetrahydropteridine-dependent cleavage enzyme in rat liver microsomes, whereas in the tumor microsomes this activity was not present. We conclude that alkylethyleneglycols are metabolized in a manner similar to alkylglycerols and perhaps by identical enzymes.     

The nature of the pentose pathway in liver

[2-14C]Glucose, [3,4-14C]glucose, [5-14C]glucose, [4,5,6-14C]glucose, and [1-14C]ribose were perfused through livers of rats. The rats were fed or fasted and refed. In one experiment the liver perfused was regenerating and in another phenazine methosulfate was in the perfusate. Perfusion was for 30 or 90 min. Glucose from each perfusate and liver glucose-6-P and glycogen were isolated, purified, and degraded. The distributions of 14C in the carbons of the glucoses from the glycogens are similar to the distributions from the glucose 6-phosphates. The distributions of 14C are in accord with metabolism of glucose by the classical pentose pathway and not by the L-type pathway that has been proposed to function in liver.     

Expression of mRNAs for alpha-fetoprotein (AFP) and albumin and incorporation of AFP and docosahexaenoic acid in baboon fetuses.

alpha-Fetoprotein and albumin, two members of a multigene family, reversibly bind fatty acids with high affinity. The origin of alpha-fetoprotein (AFP) and albumin present in fetal tissues other than the liver and yolk sac is a subject of controversy. In this work, we have searched for the presence of the albumin and AFP mRNA molecules in different fetal organs of the baboon (Papio cinocephalus), using a highly sensitive gel-blot hybridization assay with human albumin and AFP cDNA probes. Large amounts of albumin and AFP mRNA molecules were found in the fetal liver; significant quantities were also present in the gastrointestinal tract and in the kidney. No detectable levels were found in the other tissues examined (brain, skin, spleen, pancreas, muscle, heart, thymus, placenta, and amnion). After injection of radiolabeled AFP into pregnant baboons, all fetal tissues took up the protein. White adipose tissue, kidney, intestine, lung, liver, and cerebral cortex showed a great uptake of exogenous AFP. [14C]Docosahexaenoic acid (22:6, n-3), injected at the same time, was actively transferred from the maternal compartment across the placenta and incorporated into cellular lipids by all fetal tissues and particularly by liver (around 70% of total incorporation). The levels of [14C]docosahexaenoic acid per gram of tissue increased in the order: maternal blood less than placenta less than fetal liver, indicating a selective accumulation of this fatty acid by the fetus. These results indicate that intracellular AFP in non-hepatic tissues of the developing baboon is, for the most part, of plasma origin.     

Comparative utilization of glycerol and alanine as liver gluconeogenic substrates in the fed late pregnant rat

The appearance of plasma [14C]glucose in the inferior cava vein after a pulse of 0.2 mmol of [U-14C]L-alanine or [U-14C]glycerol/200 g body wt given through the portal vein was studied in fed 21 day pregnant rats and virgin controls under pentobarbital anesthesia. In both groups values were much higher when [U-14C]glycerol was the administered tracer than when [U-14C]L-alanine, and they were augmented in pregnant versus virgin animals at 1 min when receiving [U-14C]glycerol and at 2 min when receiving [U-14C]L-alanine. 20 min after the tracers rats receiving [U-14C]glycerol showed much higher liver [14C]glycogen and [14C]glyceride glycerol than those receiving [U-14C]L-alanine. Radioactivity present in liver as [14C]glyceride glycerol was greater in pregnant than in virgin rats receiving [U-14C]glycerol whereas radioactivity corresponding to [14C]fatty acids was lower in the former group receiving either tracer. At 20 min after maternal treatments fetuses showed lower plasma [14C]glycerol than [14C]alanine values but plasma [14C]glucose and liver [14C]glycogen values were much greater in fetuses from mothers receiving [U-14C]glycerol than [U-14C]L-amine. Besides showing the higher gluconeogenic efficiency in pregnant than in virgin rats, results indicate that at late gestation glycerol is used as a preferential substrate for both glucose and glyceride glycerol synthesis in liver.     

Biosynthesis of dolichyl pyrophosphate N-acetylglucosaminyl intermediates in liver microsomes from hibernating ground squirrels (Citellus citellus L.)

Dolichyl pyrophosphate N-acetyl[14C]glucosamine was synthesized after incubation of liver microsomes from hibernating ground squirrels with UDP-N-acetyl[14C )glucosamine. The radioactivity of glycolipid formed by liver microsomes from hibernating ground squirrels was about 2-fold greater than by liver microsomes from active animals. Addition of exogenous dolichyl phosphate to the incubation mixture increased the formation of dolichyl pyrophosphate N-acetyl[14C]glucosamine by microsomes from both active and hibernating ground squirrels about 6 times. Liver microsomes from hibernating ground squirrels converted dolichyl pyrophosphate N-acetyl[14C]glucosamine into dolichyl pyrophosphate N,N'-diacetyl[14C]chitobiose in the presence of unlabelled UDP-N-acetylglucosamine. This conversion was maximal at 1.0 M concentration of unlabelled UDP-N-acetylglucosamine. The level of dolichyl phosphate assessed by the level of dolichyl pyrophosphate N-acetylglucosamine formation was nearly 2 times greater in liver microsomes from hibernating ground squirrels than from active animals.     

Absorption and metabolism of butyric acid in rabbit hind gut

Butyrate absorption in the large intestine of the rabbit was evaluated by the variation of concentrations in the bowel, the arterio-venous plasma and the intestinal loops. The metabolic transformations were studied with (3-4 C14) butyrate. The caeco-colonic epithelium oxidized negligible quantities of butyrate to ketone bodies and other metabolic pathways were found. These pathways were of different intensity according to the region of the gut and both phases of the excretory cycle. A portion, which may be large, was metabolized in the caeco-colonic wall and in the liver where radioactivity was found in free amino acids, carboxylic acids and sugars. The oxidation to CO2 in TCA cycle yields energy for metabolic activities. This study of metabolism takes account of the endoflora participation.