Interaction of octanoate with branched-chain 2-oxo acid oxidation in rat and human muscle in vitro

Octanoic acid promotes branched-chain amino acid catabolisms via the inhibition of hepatic branched-chain alpha-keto acid dehydrogenase kinase in rats

OBJECTIVE

It has been reported that administration of octanoic acid, one of medium-chain fatty acids (MCFAs), promoted leucine oxidation in vitro and in vivo, but it remained unclear how octanoic acid stimulated leucine oxidation. Therefore, the aim of this study was to elucidate the mechanism that octanoic acid facilitates branched-chain amino acid (BCAA) catabolism.

MATERIALS/METHODS

In in vivo experiments, male rats were orally administered MCFAs as free fatty acids or triacylglycerol (trioctanoin), and then activities of hepatic branched-chain α-ketoacid dehydrogenase (BCKDH) complex (BCKDC) and BCKDH kinase (BDK) and alterations in the concentration of blood components were analyzed. In in vitro experiments, purified BCKDC associated with BDK (BCKDH-BDK complex) was reacted with various concentrations of hexanoic, octanoic, and decanoic acids.

RESULTS

Oral administration of trioctanoin in rats activated hepatic BCKDC via down-regulation of BDK activity in association with a decrease in plasma BCAA concentration and an increase in serum ketone body concentration. In vitro experiments using purified BCKDH-BDK complex showed that MCFAs (hexanoic, octanoic, and decanoic acids) inhibited BDK activity and that this inhibition was higher in hexanoic and octanoic acids than in decanoic acid. Oral administration of octanoic acid, but not decanoic acid, in rats activated hepatic BCKDC via down-regulation of BDK activity by decreasing the amount of BDK bound to the complex. The serum ketone body level was elevated by both administration of octanoic acid and decanoic acid.

CONCLUSION

These results suggest that octanoic acid promotes BCAA catabolism in vivo by activation of BCKDC via decreasing the bound form of BDK.

Octanoate oxidation measured by 13C-NMR spectroscopy in rat skeletal muscle, heart, and liver

Contribution of octanoate to the oxidative metabolism of the major sites of fatty acid oxidation (heart, liver, and resting and contracting skeletal muscle) was assessed in the intact rat with 13C-NMR spectroscopy. Under inhalation anesthesia, [2,4,6,8-13C4]octanoate was infused into the jugular vein and the sciatic nerve of one limb was stimulated for 1 h. Octanoate was a principal contributor to the acetyl-CoA pool in all tissues examined, with highest oxidation occurring in heart and soleus muscle followed by predominantly red portion of gastrocnemius muscle (RG), liver, and then white portion of gastrocnemius muscle (WG). Fractional contribution of 13C-labeled octanoate to the acetyl-CoA pool (Fc2) was 0.563 +/- 0.066 for heart and 0.367 +/- 0.054 for liver. Significant differences were observed between each of the muscle types during both rest and contraction. In muscle, Fc2 was highest in soleus (0.565 +/- 0.089 rested, 0.564 +/- 0.096 contracted), followed by RG (0.470 +/- 0.092 rested, 0.438 +/- 0.072 contracted), and lowest in WG (0.340 +/- 0.081 rested, 0.272 +/- 0.065 contracted). Our findings demonstrate that the fractional contribution of octanoate to oxidative metabolism correlates with oxidative capacity of the tissue and that octanoate metabolism increases in contracted muscle in proportion to the overall increase in oxidative rate.

Leucine metabolism in preterm infants receiving parenteral nutrition with medium-chain compared with long-chain triacylglycerol emulsions

BACKGROUND

Although medium-chain triacylglycerols (MCTs) may be utilized more efficiently than long-chain triacylglycerols (LCTs), their effect on protein metabolism remains controversial.

OBJECTIVE

The aim of the study was to compare the effects of mixed MCT-LCT and pure LCT emulsions on leucine metabolism in preterm infants.

DESIGN

Fourteen preterm [gestational age: 30+/-1 wk; birth weight: 1409+/-78 g (x +/- SE)] neonates were randomly assigned to receive, from the first day of life, either a 50:50 MCT-LCT (mixed MCT group; n = 7) or an LCT (LCT group; n = 7) lipid emulsion as part of an isonitrogenous, isoenergetic total parenteral nutrition program. On the fourth day, infants received intravenous feeding providing 3 g lipid, 15 g glucose, and 3 g amino acids kg(-1) x d(-1) and underwent 1) indirect calorimetry and 2) a primed, 2-h infusion of H13CO3Na to assess the recovery of 13C in breath, immediately followed by 3) a 3-h infusion of L-[1-13C]leucine.

RESULTS

The respiratory quotient tended to be slightly but not significantly higher in the mixed MCT than in the LCT group (0.96+/-0.06 compared with 0.93+/-0.03). We did not detect a significant difference between the mixed MCT and LCT groups with regard to release of leucine from protein breakdown (B; 309+/-40 compared with 257+/-46 micromol x kg(-1) x h(-1)) and nonoxidative leucine disposal (NOLD; 296+/-36 compared with 285+/-49 micromol x kg(-1) x h(-1)). In contrast, leucine oxidation was greater in the mixed MCT than in the LCT group (113+/-10 compared with 67+/-10 micromol x kg(-1) x h(-1); P = 0.007). Net leucine balance (NOLD - B) was less positive in the mixed MCT than in the LCT group (-14+/-9 compared with 28+/-10 micromol x kg(-1) x h(-1); P = 0.011).

CONCLUSION

Mixed MCTs may not be as effective as LCT-containing emulsions in promoting protein accretion in parenterally fed preterm neonates.

Effects of D-beta-hydroxybutyrate and long- and medium-chain triglycerides on leucine metabolism in humans

Ketone bodies and/or fatty acids might play a protein-sparing role during prolonged fasting or parenteral nutrition. To assess this problem, we studied whole body leucine metabolism, using L-[1-13C]leucine in normal postabsorptive volunteers who received either long-chain triglycerides (LCT, 0.15 g.kg-1.h-1, 6 subjects), a 50-50 mixture of medium-chain triglycerides (MCT) and LCT (0.15 g.kg-1.h-1, 6 subjects), D-beta-hydroxybutyrate (540 mumol.kg-1.h-1, 6 subjects), or saline (4 subjects). Leucine concentration decreased only with MCT-LCT. Leucine flux decreased by 10-20% from basal in all groups. Leucine oxidation, which was corrected for the contribution to 13CO2 of the 13C natural abundance of the infused substrates, decreased during LCT infusion (0.31 +/- 0.02 to 0.24 +/- 0.01 mumol.kg-1.min-1, P less than 0.01), but was unaffected by MCT-LCT (despite plasma free fatty acid levels similar to those obtained with LCT), D-beta-hydroxybutyrate, or saline infusion. Therefore, 1) the effect of fatty acids on amino acid oxidation is not mediated by ketone bodies, 2) it depends on the fatty acid chain length, 3) long-chain fatty acids but not medium-chain fatty acids could play a protein-sparing role during parenteral nutrition.

Effect of pyruvate, octanoate and glucose on leucine degradation in skeletal muscle from fed and fasted chicks

1. Pyruvate at 5 mM decreased the rate of leucine oxidative decarboxylation and increased the rate of 2-oxoisocaproate production in extensor digitorum communis (EDC) muscles from both fed and 24-hr fasted chicks. Pyruvate at 5 mM increased the net rate of leucine transamination in EDC muscle from fed chicks and had no effect in EDC muscles from 24-hr fasted chicks. 2. Octanoate at 0.2 and 1 mM markedly increased the rates of net leucine transamination, leucine oxidative decarboxylation and oxidation of decarboxylated leucine carbons 2-6 in EDC muscles from fed chicks, but had no effect on these parameters of leucine degradation in muscles from 24-hr fasted chicks. 3. Glucose at 5 and 12 mM decreased the rates of leucine oxidative decarboxylation and oxidation of decarboxylated leucine carbons 2-6, and increased the net rate of 2-oxoisocaproate production as compared to control (no glucose) group in muscles from fed chicks. Glucose had no effect on these parameters of leucine degradation in muscles from 24-hr fasted chicks.

Interaction of short-chain and branched-chain fatty acids and their carnitine and CoA esters and of various metabolites and agents with branched-chain 2-oxo acid oxidation in rat muscle and liver mitochondria

Interaction of various compounds with the 14CO2 production from [1-14C]-labelled branched-chain 2-oxo acids was studied in intact rat quadriceps muscle and liver mitochrondria. In the absence of carnitine, CoA esters of short-chain and branched-chain fatty acids, CoA and acetyl-L-carnitine stimulated oxidation of 4-methyl-2-oxopentanoate and 3-methyl-2-oxobutanoate in muscle mitochondria. Octanoyl-L-carnitine inhibited oxidation of the latter, but stimulated that of the former substrate. Isovaleryl-L-carnitine was inhibitory with both substrates. Carnitine stimulates markedly 3-methyl-2-oxobutanoate oxidation in liver mitochondria at substrate concentrations higher than 0.1 mM, in contrast to 4-methyl-2-oxopentanoate oxidation. In the presence of carnitine, 3-methyl-2-oxobutanoate oxidation was inhibited in muscle and liver mitochondria by octanoate, octanoyl-L-carnitine and isovaleryl-L-carnitine. The latter ester and octanoyl-D-carnitine inhibited also 4-methyl-2-oxopentanoate oxidation in muscle mitochondria. Branched-chain 2-oxo acids inhibited mutaly their oxidation, except that 3-methyl-2-oxobutanoate did not inhibit 4-methyl-2-oxopentanoate oxidation in liver mitochondria. Their degradation products, isovalerate, 3-methylcrotonate, isobutyrate and 3-hydroxyisobutyrate inhibited to a different extent 2-oxo acid oxidation in liver mitochondria. The effect of CoA esters was studied in permeabilized and with cofactors reinforced mitochondria. Acetyl-CoA and isovaleryl-CoA inhibited only 3-methyl-2-oxobutanoate oxidation in muscle mitochondria. Octanoyl-CoA inhibited oxidation of both 2-oxo acids in muscle and 4-methyl-2-oxopentanoate oxidation in liver mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)

The metabolic fate of branched-chain amino acids and 2-oxo acids in rat muscle homogenates and diaphragms

After incubation of muscle preparations with [U-14C]branched-chain amino acids or 2-oxo acids, radioactive metabolites were separated, identified and quantified. Homogenates of rat heart and skeletal muscle incubated with 4-methyl-2-oxopentanoate accumulated isovalerate, 3-hydroxyisovalerate and the corresponding carnitine esters. Incubation with 3-methyl-2-oxobutanoate resulted in the production of isobutyrate, 3-hydroxyisobutyrate and their carnitine esters. Addition of L-carnitine increased the production of the esters. The enzymes 3-methylcrotonyl-CoA carboxylase and 3-hydroxyisobutyric acid dehydrogenase apparently are inactive during incubation of muscle homogenates. With liver homogenates the degradation of both 2-oxo acids was more complete. Rat hemidiaphragms incubated with leucine, valine and isoleucine accumulated the corresponding branched-chain 2-oxo acids, fatty acids and hydroxylated fatty acids. The degradation of valine was markedly limited by the release of these metabolites. Considerable amounts (relatively smaller for valine) of radioactivity were also recovered in CO2 and glutamine and glutamate. Incubations with branched-chain 2-oxo acids gave the same radioactive products, except for glutamine and glutamate. Radioactivity was never found in lactate, pyruvate or alanine. These data indicate that the carbon-chains of amino acids entering the citric acid cycle in muscle, are not used for oxidation or for alanine synthesis, but are converted exclusively to glutamine.

Increase of the activity state and loss of total activity of the branched-chain 2-oxo acid dehydrogenase in rat diaphragm during incubation

Actual and total branched-chain 2-oxo acid dehydrogenase activities were determined in homogenates of incubated diaphragms from fed and starved rats. Incubation in Krebs-Ringer buffer increased the activity state, but caused considerable loss of total activity. Palmitate oxidation rates and citrate synthase activities did not significantly change on incubation. Starved muscles showed a higher extent of activation after 15 min of incubation (not after 30 and 60 min) and a smaller loss of total activity. Experiments with the transaminase inhibitor amino-oxyacetate confirm that the contribution of endogenous amino acids to the oxidation precursor pool is also smaller in diaphragms from starved rats on incubation in vitro. These phenomena together cause the higher 14CO2 production from 14C-labelled branched-chain amino acids and 2-oxo acids in muscles from starved than from fed rats. High concentrations of branched-chain 2-oxo acids, and the presence of 2-chloro-4-methyl-pentanoate, octanoate or ketone bodies, increase the extent of activation of the dehydrogenase complex; glucose and pyruvate had no effect. The observed changes of the activity state by these metabolites are discussed in relation to their interaction with branched-chain 2-oxo acid oxidation in incubated hemidiaphragms.