Assay of total complex and activity state of branched-chain alpha-keto acid dehydrogenase complex and of activator protein in mitochondria, cells, and tissues

Selection by UV Mutagenesis and Physiological Characterization of Mutant Strains of the Yeast Saprochaete suaveolens (Former Geotrichum fragrans) with Higher Capacity to Produce Flavor Compounds

Yeast volatile organic compounds (VOCs), i.e. low molecular weight organic acids, alcohols and esters, are considered as potential and sustainable sources of natural aromas that can replace commonly used artificial flavors in food and other industrial sectors. Although research generally focuses on the yeast Saccharomyces cerevisiae, other so-called unconventional yeasts (NCY) are beginning to attract the attention of researchers, particularly for their ability to produce alternative panels of VOCs. With this respect, a Saprochaete suaveolens strain isolated from dragon fruit in Reunion Island was shown to produce α-unsaturated esters from branched-chain amino acids (BCAAs) such as isobutyl, isoamyl or ethyl tiglate, which are rarely found in other yeasts strains. Given that β-oxidation allows the growth of S. suaveolens on BCAAs as sole carbon source, we developped a method based on UV mutagenesis to generate mutants that can no longer grow on BCAAs, while redirecting the carbon flow towards esterification of α-unsaturated esters. Among the 15,000 clones generated through UV irradiation, we identified nine clones unable to grow on BCAAs with one of them able to produce eight times more VOCs as compared to the wild-type strain. This higher production of α-unsaturated esters in this mutant strain coincided with an almost complete loss of enoyl-CoA hydratase activity of the β-oxidation pathways and with a twofold increase of acyl-CoA hydrolase with not significant changes in the enzymes of the Ehrlich pathway. Moreover, from our knowledge, it constituted the first example of VOCs enhancement in a microbial strain by UV mutagenesis.

Twice-told tales of metabolic acidosis, glucocorticoids, and protein wasting: what do results from rats tell us about patients with kidney disease?

Much has been learned from animal studies in chronic renal failure that is germane to clinical studies because animal models parallel human responses. Such studies have affirmed that correction of metabolic acidosis has a favorable effect on protein metabolism, nitrogen balance and growth. In the presence of metabolic acidosis, catabolism is increased in uremia. Glucocorticoids are involved in accelerating protein degradation in muscle, which results in loss of lean body mass, while a low insulin level appears to play a permissive role in accelerating increased catabolism. Cellular mechanisms mediating these changes include upregulation of the ubiquitin-proteasome pathway and branched-chain ketoacid dehydrogenase enzyme activity in muscle. Many of these findings from rat studies have been confirmed in human studies and have important clinical implications because correction of metabolic acidosis improves nutritional status and blunts the associated increase in protein catabolism.

Stimulation of rat-liver branched-chain alpha-keto acid dehydrogenase activity by chronic metabolic acidosis

During chronic metabolic acidosis, the degradation of protein and amino acids reportedly increases. Branched-chain alpha-keto acid dehydrogenase complex (BCKDH) relates amino-acid catabolism and mitochondrial-energy metabolism. This study was designed to evaluate the effect of acidosis on the activity of liver BCKDH, the key regulatory enzyme in the catabolism of branched-chain amino acids. Experimental acidosis was induced in rats by ingestion of 0.28 M ammonium chloride solution for 10 days. We made two different liver-mitochondrial extracts to assay independently the active form of BCKDH and the total BCKDH activity. Acidosis significantly increased both active and total BCKDH specific activities (P < 0.05). The mean value for the active form of the BCKDH complex was 9.27 +/- 1.10 (S.E.M., n = 7) mU/mg of mitochondrial protein in acidotic rats and 5.18 +/- 0.84 (n = 7) for the control rats. The value of the total complex was 16.10 +/- 1.22 (n = 7) for the acidosis and 11.51 +/- 0.58 (n = 7) for the control. No significant changes were found in the activity state of the complex. Citrate synthase activity did not show significant variations between treatments. The stimulation of liver BCKDH activities by the acidosis may contribute to maintaining the level of intermediates of the tricarboxylic-acid cycle in this metabolic situation in which the net release of glutamine are produced.

Transamination of 2-oxo-4-[methylthio]butanoic acid in chicken tissues

The keto acid 2-oxo-4[methylthio]butanoic acid (OMTB) is an intermediate in the conversion of synthetic feed grade methionine sources to L-methionine in vivo in poultry and other animals. Because methionine sources are utilized by the chick with considerably less than 100% efficiency as sources of L-methionine, it is important to determine what metabolic process may limit the utilization of these sources. Because OMTB is converted to L-methionine by transamination, a study was conducted to determine which amino acids might serve as nitrogen donors in the conversion of OMTB to L-methionine in the chicken. Dialyzed tissue homogenates, mitochondria, and cytosol from liver, kidney, intestine, and skeletal muscle were incubated with OMTB and individual L-amino acids (isoleucine, leucine, valine, glutamic acid, aspartic acid, alanine, glutamine, asparagine, and phenylalanine) and the methionine that accumulated was determined by ion exchange chromatography. Tissues differed in the conversion of OMTB to methionine: kidney was most active, liver and intestinal mucosa were intermediate, and skeletal muscle had lowest activity. All amino acids supported methionine synthesis. Branched-chain amino acids and glutamic acid were the most effective substrates in tissue cytosols except in intestinal mucosa, in which asparagine was also effective. The preferred substrates in mitochondria were glutamate in liver mitochondria, isoleucine and alanine in kidney mitochondria, and branched-chain amino acids and glutamic acid in skeletal muscle mitochondria. All amino acids except alanine supported methionine synthesis from OMTB in mitochondria of intestinal mucosa. We conclude that a wide variety of amino acids can serve as substrates for transamination of OMTB in the chicken, and that the availability of nitrogen donors is unlikely to be a limiting factor in the conversion of OMTB to methionine.

Gender difference in regulation of branched-chain amino acid catabolism

Regulation of the activity state of the hepatic branched-chain 2-oxo acid dehydrogenase (BCODH) complex during the light-dark cycle differs markedly in male and female rats. Female rats exhibit a profound diurnal rhythm in the activity state of the complex that is not observed in male rats. Regardless of gender, most of the complex was dephosphorylated and active in the middle of the dark period and early in the light period, and this form of the complex predominated in male rats at the end of the light period. In contrast, most of the complex in female rats became phosphorylated and inactive by the end of the light period. Gonadectomy prevented the diurnal rhythm in females but was without effect in males, indicating that female sex hormones are required for this gender difference in regulation of the BCODH complex. Changes in levels of branched-chain 2-oxo acids, known regulators of BCODH kinase, do not seem to be involved; rather, an increase in BCODH kinase activity occurring between morning and evening is responsible for inactivation of the BCODH complex in female rats. The increase in kinase activity is due to an increase in the amount of kinase protein associated with the BCODH complex. Thus a marked diurnal variation in the amount of BCODH kinase and therefore its activity results in large swings in the activity state of the liver BCODH complex in female rats. This study provides the first evidence for a gender-specific difference in the regulation of branched-chain amino acid catabolism.

Regulation of branched-chain ketoacid dehydrogenase flux by extracellular pH and glucocorticoids

In muscles of rats with metabolic acidosis, branched-chain alpha-ketoacid dehydrogenase (BCKAD) activity is increased. Potential stimulatory signals include acidemia and/or glucocorticoids. It is unclear whether the signal(s) increases BCKAD activity by changing the activation state of the enzyme or by increasing the amount of enzyme. To separate the influences of extracellular pH and glucocorticoids on leucine catabolism, maximal BCKAD flux and the activation state (the ratio of basal to total flux) were measured in two cell types: 1) cells that do not express glucocorticoid receptors and 2) cells stably transfected to express glucocorticoid receptors. Acidification (pH 6.95) increased 1) the activation state from 67.2% at pH 7.4 to 82.8% at pH 6.95, 2) maximal BCKAD flux by 50%, and 3) the BCKAD subunit contents in both cell types (57, 410, and 270% for E2, E1 alpha, and E1 beta, respectively). Dexamethasone increased the BCKAD activation state from 67.2 to 82.3% in cells expressing glucocorticoid receptors, whereas dexamethasone plus acidification increased the activation state to 98%. The time course of stimulation by dexamethasone was slower than that by acidification. These results demonstrate that BCKAD is differentially regulated by extracellular pH and glucocorticoids.

Molecular cloning and analysis of the expression of the E1 beta subunit of branched chain alpha-ketoacid dehydrogenase in mice

The cDNA sequence encoding the murine liver E1 beta subunit of the branched-chain alpha-ketoacid dehydrogenase complex (BCKAD) was determined. In the region encoding the mature E1 beta subunit protein, both the nucleotide composition and predicted amino acid sequence are highly conserved between murine, rat, human, and bovine species. In contrast, they are less well conserved in the 5' sequence encoding the amino terminal preprotein sequence and 3' untranslated region. The pattern of tissue-specific expression of three BCKAD subunit RNAs was determined to be similar in both rat and murine tissues except for that observed in murine liver, where a higher than expected level of E1 beta subunit RNA was observed. Variation in the levels of this subunit might play a major role in the regulation of the overall ability of the murine liver to modulate BCKAD content in response to changing physiologic needs.

Cyclic AMP and free fatty acids in the longer-term regulation of pyruvate dehydrogenase kinase in rat soleus muscle

Starvation increased pyruvate dehydrogenase (PDH) kinase activity in extracts of freshly excised rat soleus 2.2-fold (from 0.6 min-1 in fed rats to 1.31 min-1 in 48-h-starved rats). In fed rats, activities were unchanged following 24 h of culture in medium 199, but increased 2.1-fold on 24 h of culture with 50 microM dibutyryl cAMP plus 1 mM n-octanoate and 1.6-1.7-fold with either agent alone. Approx. 70% of the increase in PDH kinase induced by starvation was lost following 24 h of culture in medium 199; the loss was prevented by 50 microM dibutyryl cAMP plus 1 mM n-octanoate. cAMP concentrations in fresh soleus muscle were 1 nmol/g (fed rats) and 1.6 nmol/g (starved rats). After 20-60 min of culture the fed-starved difference disappeared and [cAMP] fell to 0.4 nmol/g. Calcitonin-gene-related peptide (CGRP) increased cAMP 3-fold; the increase was maintained throughout 24 h of culture, but was readily reversed at 30 min or 24 h of culture by 60-min incubation with CGRP-free medium. Starvation of the rat (48 h) had no effect on the sensitivity of soleus towards the [cAMP]-increasing effect of CGRP. It is concluded that culture may reverse effects of starvation on PDH kinase activity by lowering cAMP and by removal from the in vivo effects of circulating free fatty acids; and that starvation and CGRP had no detectable long-term effects on the cAMP system in soleus muscle.