[182] l-serine dehydratase (rat liver)

Characterization of aminoacrylate intermediates of pyridoxal-5'-phosphate dependent enzymes

Pyridoxal-5'-phosphate (PLP) Schiff's bases of 2-aminoacrylate are intermediates in β-elimination and β-substitution reaction of PLP-dependent enzymes. These enzymes are found in two major families, the α-, or aminotransferase, superfamily, and the β-family. While the α-family enzymes primarily catalyze β-eliminations, the β-family enzymes catalyze both β-elimination and β-substitution reactions. Tyrosine phenol-lyase (TPL), which catalyzes the reversible elimination of phenol from l-tyrosine, is an example of an α-family enzyme. Tryptophan synthase catalyzes the irreversible formation of l-tryptophan from l-serine and indole, and is an example of a β-family enzyme. The identification and characterization of aminoacrylate intermediates in the reactions of both of these enzymes is discussed. The use of UV-visible absorption and fluorescence spectroscopy, X-ray and neutron crystallography, and NMR spectroscopy to identify aminoacrylate intermediates in these and other PLP enzymes is presented.

Dynamics of amino acid utilization in Phaeobacter inhibens DSM 17395

Time-resolved utilization of multiple amino acids by Phaeobacter inhibens DSM 17395 was studied during growth with casamino acids. The 15 detected amino acids could be grouped according to depletion rate into four different categories, i.e. from rapid (category I) to nondepletion (category IV). Upon entry into stationary growth phase, amino acids of category I (e.g. glutamate) were (almost) completely depleted, while those of categories II (e.g. leucine) and III (e.g. serine) were further consumed at varying rates and to different extents. Thus, cultures entered stationary growth phase despite the ample presence of organic nutrients, i.e. under nonlimiting conditions. Integrated proteomic and metabolomic analysis identified 1747 proteins and 94 intracellular metabolites. Of these, 180 proteins and 86 metabolites displayed altered abundance levels during growth. Most strikingly, abundance and activity profiles of alanine dehydrogenase concomitantly increased with the onset of enhanced alanine utilization during transition into stationary growth phase. Most enzymes of amino acid and central metabolism, however, displayed unaltered abundances across exponential and stationary growth phases. In contrast, metabolites of the Entner-Doudoroff pathway and gluconeogenesis as well as cellular fatty acids increased markedly in abundance in early stationary growth phase.

Cobalamin deficiency results in severe metabolic disorder of serine and threonine in rats

Dietary cobalamin (vitamin B12; Cbl) deficiency caused significant increases in plasma serine, threonine, glycine, alanine, tyrosine, lysine and histidine levels in rats. In particular, the serine and threonine levels were over five and eight times, respectively, higher in the Cbl-deficient rats than those in the sufficient controls. In addition, some amino acids, including serine and threonine, were excreted into urine at significantly higher levels in the deficient rats. When Cbl was supplemented into the deficient rats for 2 weeks, in coincidence with the disappearance of the urinary excretion of methylmalonic acid (an index of Cbl deficiency), the plasma serine and threonine levels were normalized. These results indicate that Cbl deficiency results in metabolic disorder of certain amino acids, including serine and threonine. The expression level of hepatic serine dehydratase (SDH), which catalyzes the conversion of serine and threonine to pyruvate and 2-oxobutyrate, respectively, was significantly lowered by Cbl deficiency, even though Cbl does not participate directly in the enzyme reaction. The SDH activity in the deficient rats was less than 20% of that in the sufficient controls, and was normalized 2 weeks after the Cbl supplementation. It is thus suggested that the decrease of the SDH expression relates closely with the abnormalities in the plasma and urinary levels of serine and threonine in the Cbl-deficient rats.

Influence of glucogenic amino acids on the hepatic metabolism of threonine

The supplementation of a low-protein diet with L-threonine leads to a marked accumulation of threonine in plasma and liver, whereas increasing dietary protein generally leads to an induction of threonine dehydratase in the liver, hence depressed availability for extrasplanchnic tissues. The aim of the present study was, thus, to further investigate the factors which control the utilization of threonine by the liver. Increasing the dietary supply of threonine led to parallel increases in the afferent and hepatic concentrations and in the rate of utilization by the liver; however, the fractional extraction tended to decrease. It appears that the addition of a mixture of glucogenic amino acids to the diet prevented the accumulation of threonine in plasma induced by exogenous threonine. The glucogenic amino acids increased the fractional hepatic uptake of threonine, and counteracted its accumulation in the liver. These effects reflect the fact that the glucogenic amino acids elicited a potent induction of the threonine dehydratase, whereas threonine alone was uneffective. Our results suggest that, besides the well-established effect of glucogenic conditions, the availability of some glucogenic amino acids is an important factor in the control of threonine catabolism.

A novel L-glutamate oxidase from Streptomyces endus. Purification and properties

A new flavoenzyme using molecular oxygen to oxidize L-glutamic acid has been purified to homogeneity, as judged by polyacrylamide gel electrophoresis, from the culture medium of Streptomyces endus. Hydrogen peroxide, 2-oxoglutaric acid and ammonia are formed as products. Among 25 amino acids tested including D-glutamic acid, L-glutamine and L-aspartic acid, only L-glutamic acid is converted. The molecular mass of the enzyme was estimated to be about 90 kDa by gel chromatography and 50 kDa by SDS/PAGE. The subunit contains 1 molecule noncovalently bound FAD. The absorption spectrum shows maxima at 273, 355 and 457 nm and the isoelectric point is at pH 6.2. The Km value for L-glutamic acid in air-saturated phosphate pH 7.0 was estimated to be 1.1 mM, the Km for oxygen was calculated to be 1.86 mM at saturating concentration of L-glutamic acid. The enzymic reaction is inhibited by Ag+ and Hg2+ ions. The enzyme described here distinctly differs from two microbial L-glutamate oxidases purified hitherto, with regard to extremely high substrate specificity and to the subunit structure.

In vitro maintenance of terminal-differentiated state in hepatocytes entrapped within calcium alginate

In cultured hepatocytes entrapped within Ca-alginate, liver-specific functions such as induction of tyrosine aminotransferase and serine dehydratase were stimulated by increasing the cell density. In contrast, a growth-related function such as induction of glucose-6-phosphate dehydrogenase was strongly stimulated by decreasing the cell density. This reciprocal regulation was mimicked by the addition of plasma membranes purified from adult rat liver to entrapment cultures at low cell density. Also, gluconeogenesis from lactate was stimulated by the addition of epinephrine (alpha,beta-agonist) with propranolol (beta-blocker). These results suggest that entrapped hepatocytes maintain not only terminal-differentiated state but also alpha-adrenergic response as shown in vivo.

Interaction of adrenal and pancreatic hormones in the control of hepatic enzymes during development

In the liver of suckling rats, the synthesis of hepatic tyrosine aminotransferase, serine dehydratase, and phosphofructokinase 2 as well as of renal beta-glucosidase is controlled by the circulating concentrations of adrenal and pancreatic hormones. Glucagon is capable of stimulating enzyme synthesis only in the presence of a steroid hormone. Dexamethasone and estradiol have been found to exert a permissive function on the inducibility of the studied enzymes by glucagon. Between the hormones of the adrenal medulla and glucagon antagonistic effects in enzyme induction were observed. Obviously, this antagonism is mediated by the alpha 1-adrenergic signal transferring system. A characteristic age dependence of enzyme induction by dexamethasone has been established. This might be correlated to alterations in the degree of methylation of the respective promoters. The methylation inhibitor 5-azacytidine influences significantly the enzyme induction by glucocorticoid hormones.

Comparative biochemistry of the guinea-pig: a partial checklist

A great deal is known about guinea-pig biochemistry, but the information is scattered and difficult to assemble. The guinea-pig also possesses a number of unusual biochemical features which add to its interest. For these reasons we have compiled a list of biochemical characteristics of the guinea-pig, organized in a series of tables, with brief discussions of some of the entries.

The free amino acid pool in Leishmania tropica promastigotes

The principal constituents of the free amino acid pool in Leishmania tropica promastigotes include alanine, glycine, serine, threonine, alpha-aminobutyric acid and ethanolamine. Changes in levels of the pool constituents during starvation, nutrient supplementation and treatment with a metabolic inhibitor (sodium fluoride) were followed. Several enzymes capable of converting amino acids to Krebs cycle intermediates were identified. It is suggested that during prolonged starvation which is accompanied by enhanced proteolysis, these enzymes enable the cells to utilize endogenous amino acids as carbon and energy sources. This is compatible with previous reports that Leishmania promastigotes in culture could grow on amino acids as growth substrates when carbohydrates are totally lacking.

Occurrence of a catabolic L-serine (L-threonine) deaminase in Saccharomyces cerevisiae

Saccharomyces cerevisiae mutants lacking the anabolic L-threonine deaminase, the ilv1- mutants, have been found to exhibit a normal ability to grow, without auxotrophy towards isoleucine, on L-threonine of L-serine as only nitrogen nutrient. Starting from a strain carrying a ilv1- mutation, a new mutation affecting the ability to utilize L-threonine as nitrogen source was selected. This mutation, which also impairs the ability to utilize L-serine, has been denominated cha-, for 'catabolism of hydroxyamino acids' and was found to result in the lack of a catabolic L-serine (L-threonine) deaminase. This enzyme which, unlike the anabolic threonine deaminase, is more active towards serine than towards threonine, differs from the latter enzyme by a number of biochemical and regulatory properties. Whereas the anabolic enzyme is an allosteric enzyme sensitive to feedback inhibition by isoleucine, the catabolic enzyme exhibits Michaelian kinetics: no control of its activity has been detected. Its synthesis is induced by L-serine and L-threonine. These two enzymes, which thus can be easily differentiated by means of their regulations, display a limited ability to compensate for one another's absence and appear to play clearly distinct roles under normal physiological conditions.

A novel pathway for the synthesis of solanidine in the isolated chloroplast from greening potatoes

Synthesis of chlorophyll always preceded the synthesis of solanine in cold-stored potatoes in the presence of dim light (30--40 lux). Isolated chloroplasts from green peelings were able to fix CO2. The evidence obtained showed a direct reduction of CO2 to formate, which was the primary product of fixation. Apart from this the chloroplasts were capable of incorporating NaH14CO3, [14C]formate, [2-14/C]glycine, [2-14C]pyruvate, [2-14C]acetate, [2-14C]mevalonate and [U-14C]serine into the solanidine moiety of the alkaloid. The intermediates in the pathway between CO2 and acetate have been identified as formate, glycine, serine and pyruvate. The localization of the enzymes involved in the reaction sequence viz. serine hydroxymethyl-transferase, and pyruvic dehydrogenase complex, in the isolated chloroplast has been established.