Threonine deaminase: a novel activity stain on polyacrylamide gels

Conserved YjgF protein family deaminates reactive enamine/imine intermediates of pyridoxal 5'-phosphate (PLP)-dependent enzyme reactions

The YjgF/YER057c/UK114 family of proteins is conserved in all domains of life, suggesting that the role of these proteins arose early and was maintained throughout evolution. Metabolic consequences of lacking this protein in Salmonella enterica and other organisms have been described, but the biochemical function of YjgF remained unknown. This work provides the first description of a conserved biochemical activity for the YjgF protein family. Our data support the conclusion that YjgF proteins have enamine/imine deaminase activity and accelerate the release of ammonia from reactive enamine/imine intermediates of the pyridoxal 5'-phosphate-dependent threonine dehydratase (IlvA). Results from structure-guided mutagenesis experiments suggest that YjgF lacks a catalytic residue and that it facilitates ammonia release by positioning a critical water molecule in the active site. YjgF is renamed RidA (reactive intermediate/imine deaminase A) to reflect the conserved activity of the protein family described here. This study, combined with previous physiological studies on yjgF mutants, suggests that intermediates of pyridoxal 5'-phosphate-mediated reactions may have metabolic consequences in vivo that were previously unappreciated. The conservation of the RidA/YjgF family suggests that reactive enamine/imine metabolites are of concern to all organisms.

Purification and properties of an iron-sulfur-containing and pyridoxal-phosphate-independent L-serine dehydratase from Peptostreptococcus asaccharolyticus

L-Serine dehydratase with a specific activity of 15 nkat/mg protein was present in the anaerobic eubacterium Peptostreptococcus asaccharolyticus grown either on L-glutamate or L-serine. The enzyme was highly specific for L-serine with the lowest Km = 0.8 mM ever reported for an L-serine dehydratase. L-Threonine (Km = 22 mM) was the only other substrate. V/Km for L-serine was 500 times higher than that for L-threonine. L-Cysteine was the best inhibitor (Ki = 0.3 mM, competitive towards L-serine). The enzyme was purified 400-fold to homogeneity under anaerobic conditions (specific activity 6 mukat/mg). PAGE in the presence of SDS revealed two subunits with similar intensities (alpha, 30 kDa; beta, 25 kDa). The molecular mass of the native enzyme was estimated as 200 +/- 20 kDa (gel filtration) and 180 kDa (gradient PAGE). In the absence of oxygen the enzyme was moderately stable even in the presence of sodium borohydride or phenylhydrazine (5 mM each). However, by exposure to air the activity was lost, especially when the latter agent was added. The enzyme was reactivated by ferrous ion under anaerobic conditions. The inability of several nucleophilic agents to inactivate the enzyme indicated the absence of pyridoxal phosphate. This was confirmed by a microbiological determination of pyridoxal phosphate. However, the enzyme contained 3.8 +/- 0.2 mol Fe and 5.6 +/- 0.3 mol inorganic sulfur/mol heterodimer (55 kDa) indicating the presence of an [Fe-S] center. The enzyme was successfully applied to measure L-serine concentrations in bacterial media and in human sera.

[Characterization of L-threonine deaminase activity of guinea pig liver induced by a high protein diet]

In a foregoing paper, we demonstrated that under equilibrated diet conditions, guinea pig liver L-threonine deaminase activity should be allocated to two distinct enzymes: a specific L-threonine deaminase without activity toward L-serine and a L-serine deaminase having a secondary activity toward L-threonine. In the present work, we observed that a high protidic diet caused an elevation of total threonine deaminase activity. Thus purification of guinea pig liver L-threonine deaminase was attempted, using ultracentrifugation, salt precipitation, heat treatment, ion exchange chromatography on DEAE Sephacel, Sephadex G 200 molecular sieve, 2 amino-2 methyl-1 propanol linked CH 4B Sepharose chromatography. The weak variations of the ratios of specific activities respectively toward L-threonine and L-serine observed at each stage of the purification procedure indicated that both activities are very likely supported by a single enzyme preexisting in the liver of guinea pigs fed an equilibrated diet. No isoenzyme was evidenced by polyacrylamide gel electrophoresis or DEAE Sephacel chromatography. Moreover, our purification procedure demonstrated that not only inducible L-threonine deaminase guinea pig liver activity was due to L-serine deaminase, but also that an initially existing specific L-threonine deaminase activity paradoxically disappeared with a protein rich diet.

The identity of bovine liver "nothing dehydrogenase"

An assay system was developed which allowed the partial purification of the substrate for beef liver nothing dehydrogenase. This was also found to be a substrate for lactate dehydrogenase (LDH). Using electrophoretic separations of purified LDH and beef liver extracts, nothing dehydrogenase was determined to be primarily the H4 isoenzyme of LDH. The nothing dehydrogenase phenomena was also observed with highly-purified, commercially-obtained LDH.

Use of monoclonal antibody to increase sensitivity and specificity in quantitative immunodiffusion assays

A mouse monoclonal antibody has been produced which recognizes human liver gamma-cystathionase, Radioiodination of the monoclonal antibody facilitated its use in combination with non-specific precipitating rabbit antisera in classical immunodiffusion assays. The technique may have broad applicability in the detection and quantitation of rare antigens which are difficult to purify but easily recognizable in immunodiffusion assays. It may also be used for the initial detection of monoclonal antibodies to unique antigens of interest.

Threonine deaminase: autogenous regulator of the ilv genes in Escherichia coli K-12

In this paper we analyze the effect of mutations in three genes, ilvO, ilvA and rho, on the expression of the ilvEJGDA gene cluster of Escherichia coli K-12. The ilvO603 mutation causes a cis-dominant derepression of the ilvEJGDA genes. In particular, the ilvG gene, not expressed in the wild type, becomes expressed in the ilvO603 strain. We have introduced ilvA mutations (ilvA454 or ilvA628) in the ilvO603 strain and we show that ilvG expression requires the presence in cis of both an ilvO603 mutation and of an ilvA+ allele. The ilvG gene is not expressed when in trans is present an ilvO+, ilvA+ genotype. However, it is expressed when the chromosome in trans is ilvO603, ilvA+ (ilvG-). We suggest that ilvO603 is part of ilvA, the structural gene for threonine deaminase, and that threonine deaminase from the ilvO603 mutant binds the ilvO603 site and not the ilvO+ site. Therefore, the ilvA gene product would be a cis-acting protein. Mutations in the rho gene cause derepression of the ilvEJGDA gene cluster without a concomitant expression of the ilvG gene. We show that introduction of either a rho-218 or a rho-115 mutation into the ilvO603, ilvA454 double mutant causes expression of ilvG. We therefore suggest that the ilvA gene product, threonine deaminase, is involved in termination of transcription as an antagonist of the rho gene product. Introduction of ilvA454 into an ilvO603 strain causes also a decrease in expression of the ilvE, ilvJ and ilvD genes. This effect is maximum in the case of the ilvD gene and we studied it in detail in isogenic strains containing also the rho-218 mutation.

Biodegradative threonine dehydratase. Reduction of ferricyanide by an intermediate of the enzyme-catalyzed reaction

The threonine-dependent reduction of ferricyanide catalyzed by the purified biodegradative threonine dehydratase of Escherichia coli has been studied. The rate of production of 2-oxobutyrate in the presence of ferricyanide was lower than that found in the absence of ferricyanide. The concentrations of threonine required for half-maximal effects for the reduction of ferricyanide and, in the presence of the dye, for 2-oxobutyrate production, were 3 mM and 9mM, respectively. Reduction of ferricyanide was accompanied by evolution of CO2, and even within a very short incubation time with the enzyme, the ratio of ferricyanide reduced over CO2 evolved was approximately 7. Stopping the enzyme activity after a brief exposure to threonine at pH 9.7 resulted in the accumulation of an intermediate (with a half-life of 4 min at 25 degrees C) which formed an adduct with N-ethylmaleimide; the accumulated intermediate, in the absence of N-ethylmaleimide, reduced ferricyanide with concomitant evolution of CO2. We conclude from these results that 2-aminocrotonate is the intermediate which serves as a source of reducing equivalent for ferricyanide, and nonstoichiometric amount of ferricyanide reduction may be attributed to some secondary reactions of ferricyanide with compounds derived from the oxidation product of 2-aminocrotonate.

Dual autogenous regulatory role of threonine deaminase in Escherichia coli K-12

We describe the regulatory properties of two strains carrying either the ilvA624 or the ilvA625 mutations, located in the structural gene for threonine deaminase. Crude extracts of both these strains possess a threonine deaminase activity migrating on polyacrylamide gels, differently from the wild type enzyme. Growth studies demonstrate that these mutations do not cause a limitation of isoleucine biosynthesis, suggesting normal catalytic activity of deaminase. A regulatory consequence of the ilvA624 allele is a derepression of the isoleucine-valine biosynthetic enzymes, which is recessive to an ilvA+ allele. The ilvA625 mutation causes a derepression which is dominant in an ilvA625/ILVA+ diploid. We interpret these data assuming that threonine deaminase, previously shown to be an autogenous regulator of the ilv genes, lacks a repressor function in the ilvA624 mutant, while in the ilvA625 mutant it is a better activator than wild type threonine deaminase. The data are discussed in terms of a model requiring that threonine deaminase, or a precursor of it, is in equilibrium between two forms, one being an activator of gene expression and the other being a repressor.

Sheep liver serine-threonine dehydratase. I. Purification and evidence for covalently linked alpha-ketobutyrate as its cofactor

L-Serine-threonine dehydratase (EC 4.2.1.16) from sheep liver has been obtained as a highly purified preparation as shown by ultracentrifuge studies and analytical disc gel electrophoresis. The dehydratase has a molecular weight of 98,000 +/- 10,000 and is composed of two nonidentical subunits with molecular weights of 41,000 and 47,000. The 41,000 subunit is covalently linked to the carbonyl reagent-sensitive coenzyme which has been identified as alpha-ketobutyric acid.

Studies of homogeneous "biosynthetic" L-threonine dehydratase from Escherichia coli K-12. Some kinetic properties and molecular multiplicity

"Biosynthetic" L-threonine dehydratase (EC 4.2.1.16) was purified to a homogeneous state with 29% yield of total activity from Escherichia coli K-12. The homogeneity of the enzyme was shown by polyacrylamide gel disc electrophoresis in the presence of dodecyl sulphate. The enzyme consisted of equal subunits having a molecular weight of about 57 000. The polyacrylamide gel disc electrophoresis has shown that the native enzyme consisted of a set of oligomeric forms. The multiplicity of molecular organization of the enzyme was reflected in complicated kinetic behaviour: at pH greater than 9 on the plots of initial reaction rate (v) versus initial substrate concentration ([S]o) there were four inflexion points (two intermediate plateaux), the position and deepness of which depended on enzyme concentration. At pH 8.3 on the v versus [S]o plots appeared two inflexion points (one intermediate plateu), the position of which practically did not depend on enzyme concentration in the reaction mixture, but strongly depended on the enzyme concentration in the stock solution. Repeated polyacrylamide gel disc electrophoresis of several oligomeric forms, isolated by the first electrophoresis, has shown that the oligomeric forms underwent a slow polymerization. It was suggested that "biosynthetic" L-threonine dehydratase from E. coli K-12 is a set of multiple oligomeric forms, having different kinetic parameters. Probably, each form of the enzyme has a "simple" kinetics characterized by hyperbolic or sigmoidal shape of v versus [S]o plots. The rate of equilibrium installation between the oligomeric forms was small in comparison with the enzyme reaction velocity, that lead to the complex kinetic curves, appearing as a result of summing up of the kinetics inherent to theindividual forms.

Purification and properties of threonine deaminase from the X-1 isolate of the genus Thermus

Threonine deaminase (l-threonine dehydratase EC 4.2.1.16) has been partially purified from a new extreme thermophilic bacterium, Thermus X-1, which is similar to T. aquaticus YT-1. The threonine deaminase of strain X-1 has a maximal rate of reaction at 85 to 90 C and is more thermostable than the threonine deaminase from mesophilic bacteria. The enzyme has an apparent molecular weight of 100,000 to 115,000, a K(m) for l-threonine of 14 mM, a pH optimum of 8.0, and like other threonine deaminases also catalyzes the deamination of serine. However the Thermus X-1 threonine deaminase does not show a strong feedback inhibition by isoleucine. It is suggested that the regulation of the biosynthesis of isoleucine in this extreme theromophile may resemble that reported in Rodospirillum rubrum.

Involvement of threonine deaminase in repression of the isoleucine-valine and leucine pathways in Saccharomyces cerevisiae

l-Threonine deaminase (l-threonine dehydratase [deaminating], EC 4.2.2.16) has been shown to be involved in the regulation of three of the enzymes of isoleucine-valine biosynthesis in yeast. Mutations affecting the affinity of the enzyme for isoleucine also affected the repression of acetohydroxyacid synthase, dihydroxyacid dehydrase, and reductoisomerase. The data indicate that isoleucine must be bound for effective repression of these enzymes to take place. In a strain with a nonsense mutation midway in liv 1, the gene for threonine deaminase, starvation for isoleucine or valine did not lead to derepression of the three enzymes; starvation for leucine did. The effect of the nonsense mutation is recessive; it is tentatively concluded, therefore, that intact threonine deaminase is required for derepression by two of the effectors for multivalent repression, but not by the third. A model is presented which proposes that a regulatory species of leu tRNA(leu) is the key intermediate for repression and that threonine deaminase is a positive element, regulating the available pool of charged leu tRNA by binding it.