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At least 73 records · Page 4Linked to original sources
PubMed1982

On the chemistry and biochemistry of 3-mercaptopyruvic acid, the alpha-keto acid analog of cysteine.

3-Mercaptopyruvate, generally believed to be formed by enzymatic transamination of L-cysteine and by the oxidative deamination of L-cysteine, has been found to exist in solution, in equilibrium with the cyclic dimer, 2,5-dihydroxy-1,4-dithiane-2,5-dicarboxylic acid; the equilibrium markedly favors the cyclic dimer in aqueous solution at pH 5-7. However, in neutral solutions, and especially in alkaline solution, 3-mercaptopyruvate undergoes very rapid and irreversible conversion to an acyclic aldol dimer. The rate of aldol dimerization is about 100 times greater than that of pyruvate dimerization under comparable conditions. Thus, 10 mM solutions of 3-mercaptopyruvate at pH 7.2 lose about 75% of their lactate dehydrogenase reactivity in 90 min at 25 degrees C. The present findings explain the previous observation that 3-mercaptopyruvate exhibits anomalous behavior in transamination with glutamine. Oxidative deamination of L-cysteine by L-amino acid oxidase yields the highly reactive imine, 2-imino-3-mercaptopropionic acid, which can be quantitatively trapped with semicarbazide. In the absence of semicarbazide, almost quantitative formation of ammonia from L-cysteine occurs with only a 10-20% yield of alpha-keto acid, none of which is 3-mercaptopyruvate; however, 3-mercaptopyruvate may be trapped as 3-mercaptolactate when the oxidase reaction is carried out in the presence of lactate dehydrogenase and NADH. One of the physiological functions of glutamine transaminase may be to convert 3-mercaptopyruvate to L-cysteine, thus preventing its loss through further transformations. In the course of this work it was found that hemithioketals (formed by reaction of cysteine with alpha-keto acids) are substrates of L-amino acid oxidase. Evidence for the cyclic dimer in the solid and vapor states of 3-mercaptopyruvate has been found through infrared and mass spectroscopic observations.

Amino Acid Oxidoreductases↗
PubMed1982

A controlled study of supplementation with essential amino acids and alpha-keto acids in the conservative management of patients with chronic renal failure.

Oral therapy with essential amino acids (EAA) or alpha-keto acids (alpha-KA) has been recommended in patients with renal failure, but quality and quantity of optimal protein intake are still controversial. This study compares sequentially the effect of supplementation with EAA, and with alpha-KA versus placebo in 15 ambulatory patients with chronic renal failure (average creatinine clearance 10.8 ml/min), maintained on a protein diet of 0.57 g/kg body weight (40 g for a 70-kg patient). The actual dietary intake averaged 0.55 g protein/kg and 27 kcal/kg according to repeated 7-day dietary recordings. After a 6-week baseline period on this diet, all patients received additionally 0.112 g EAA/kg for 6 weeks followed by a double-blind crossover study of 0.105 g alpha-KA/kg versus placebo supplementation for 6 weeks each. Fasting blood samples for multiple parameters, including 15 indicators for protein deficiency, as well as anthropometric and clinical data were evaluated every 3 weeks. Laboratory data revealed no indications of protein deficiency. Therapy with alpha-KA diminished serum phosphate concentration (p less than 0.05), however no other significant beneficial effects could be demonstrated during supplementation with either EAA or alpha-KA. Therefore, such supplementation to a 0.55-g/kg-protein diet appears superfluous in stable ambulatory patients with renal insufficiency.

Adolescent↗
PubMed2004

Cysteine and keto acids modulate mosquito kynurenine aminotransferase catalyzed kynurenic acid production.

Kynurenine aminotransferase (KAT) catalyzes the formation of kynurenic acid (KYNA), the natural antagonist of ionotropic glutamate receptors. This study tests potential substrates and assesses the effects of amino acids and keto acids on the activity of mosquito KAT. Various keto acids, when simultaneously present in the same reaction mixture, display a combined effect on KAT catalyzed KYNA production. Moreover, methionine and glutamine show inhibitory effects on KAT activity, while cysteine functions as either an antagonist or an inhibitor depending on the concentration. Therefore, the overall level of keto acids and cysteine might modulate the KYNA synthesis. Results from this study will be useful in the study of KAT regulation in other animals.

Animals↗
PubMed1986

Respective contribution of plasma branched-chain amino acids and 2-keto acids to the hepatic metabolism of the carbon moiety of branched-chain amino acids in fed rats.

The splanchnic balances of branched-chain amino acids (BCAA) and ketoacids (BCKA) concentrations were compared in rats fed a 15% (basal) or a 70% (HP) casein diet. Arterial BCAA concentrations were four- to fivefold and BCKA concentrations were twofold higher in rats fed HP diet than in rats fed basal diet. In rats fed basal diet, a higher proportion (45-58%) of BCKA was removed by the liver than of BCAA (7-12%). Thus, plasma BCKA were probably the major source of substrates for hepatic BCKA dehydrogenase. In rats fed HP diet, hepatic uptake of BCAA was 10 times greater than in rats fed basal diet, owing to their higher extraction; BCKA uptake was only twice as great; thus BCKA were a minor source of substrates for hepatic metabolism. In both diet groups, uptake of ketomethylvalerate (KMV) was less efficient than ketoisovalerate (KIV) or ketoisocaproate (KIC) uptake. Studies of metabolism of intraportal loads of BCKA showed that capacities for KIV and KIC uptake were high, KMV uptake plateaued at lower portal concentrations. With HP diets, the liver may be the major site of BCAA utilization; the interorgan transfer of the carbon moiety of BCAA to the liver as ketoacids does not keep pace with an increased availability of BCAA.

Amino Acids, Branched-Chain↗
PubMed1996

Is a clinical application of hybrid liver support systems limited by an initial disorder in cellular amino acid and alpha-keto acid metabolism, rather than by later gradual loss of primary hepatocyte function?

The in-vitro amino acid (AA) and alpha-keto acid (KA) metabolism of bioreactors initially seeded with 2.5 x 10(9) pig hepatocytes was investigated with a perfusion technique. Considerable changes in the culture medium concentrations of AA and KA were measured during the first days in culture. This is indicative of dynamic cellular metabolism in the initial phase. While the concentration of pyruvate decreased starting on the first day, alpha-ketoglutarate, alpha-ketoisocaproate, alpha-ketoisovalerate, and alpha-keto-beta-methyl-n-valerate were synthesized. The long term use of hepatocyte cultures in bioreactors and thus a desirable clinical hybrid liver support therapy appears to be possible since the hepatocytes switched, after 15 days in culture, to steady-state conditions with a stable amino acid turnover featuring general AA uptake accompanied by KA release. The release of branched chain KA, in particular that of alpha-ketoisocaproate, reflects an effective transamination activity in the bioreactor system. Primary pig hepatocytes cultivated in hybrid liver support systems for therapy of acute liver failure or as devices for bridging to liver transplantation can sustain amino acid metabolism for at least 30 days in vitro. However, an initial disorder following the cell isolation that is demonstrated may limit immediate utilization of the systems prior to the reorganisation of the cells to tissue-like structures in bioreactors.

Amino Acids↗
PubMed1978

Correlations between branched-chain amino acids and branched-chain alpha-keto acids in blood in maple syrup urine disease.

In 62 blood samples from 3 patients with classical maple syrup urine disease and from one patient with a variant form, a close linear correlation was found between levels of branched chain amino acids and their corresponding alpha-keto acids. Keto acids were determined as O-trimethylsilyl quinoxalinols by gas chromatography with a nitrogen-selective detector.

Adult↗
PubMed2004

Lipoic acid-dependent oxidative catabolism of alpha-keto acids in mitochondria provides evidence for branched-chain amino acid catabolism in Arabidopsis.

Lipoic acid-dependent pathways of alpha-keto acid oxidation by mitochondria were investigated in pea (Pisum sativum), rice (Oryza sativa), and Arabidopsis. Proteins containing covalently bound lipoic acid were identified on isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis separations of mitochondrial proteins by the use of antibodies raised to this cofactor. All these proteins were identified by tandem mass spectrometry. Lipoic acid-containing acyltransferases from pyruvate dehydrogenase complex and alpha-ketoglutarate dehydrogenase complex were identified from all three species. In addition, acyltransferases from the branched-chain dehydrogenase complex were identified in both Arabidopsis and rice mitochondria. The substrate-dependent reduction of NAD(+) was analyzed by spectrophotometry using specific alpha-keto acids. Pyruvate- and alpha-ketoglutarate-dependent reactions were measured in all three species. Activity of the branched-chain dehydrogenase complex was only measurable in Arabidopsis mitochondria using substrates that represented the alpha-keto acids derived by deamination of branched-chain amino acids (Val [valine], leucine, and isoleucine). The rate of branched-chain amino acid- and alpha-keto acid-dependent oxygen consumption by intact Arabidopsis mitochondria was highest with Val and the Val-derived alpha-keto acid, alpha-ketoisovaleric acid. Sequencing of peptides derived from trypsination of Arabidopsis mitochondrial proteins revealed the presence of many of the enzymes required for the oxidation of all three branched-chain amino acids. The potential role of branched-chain amino acid catabolism as an oxidative phosphorylation energy source or as a detoxification pathway during plant stress is discussed.

3-Methyl-2-Oxobutanoate Dehydrogenase (Lipoamide)↗