Search PubMed⌕ Search

Biomedical subjects

M Benziman

Publications and source records attributed to M Benziman.

50 records · Page 3Linked to original sources
PubMed1970

Characterization and properties of the pyruvate phosphorylation system of Acetobacter xylinum.

The enzyme responsible for the direct phosphorylation of pyruvate during gluconeogenesis in Acetobacter xylinum has been purified 46-fold from ultrasonic extracts and freed from interfering enzyme activities. The enzyme was shown to catalyze the reversible Mg(2+) ion-dependent conversion of equimolar amounts of pyruvate, adenosine triphosphate (ATP), and orthophosphate (P(i)) into phosphoenolpyruvate (PEP), adenosine monophosphate (AMP), and pyrophosphate (PP). The optimal pH for PEP synthesis was pH 8.2; for the reversal it was pH 6.5. The ratio between the initial rates of the reaction in the forward and reverse directions was 5.1 at pH 8.2 and 0.45 at pH 6.5. The apparent K(m) values of the components of the system in the forward reaction were: pyruvate, 0.2 mm; ATP, 0.4 mm; P(i), 0.8 mm; Mg(2+), 2.2 mm; and for the reverse reaction: PEP, 0.1 mm; AMP, 1.6 mum; PP, 0.067 mm; Mg(2+), 0.87 mm. PEP formation was inhibited by AMP and PP. The inhibition by AMP was competitive with regard to ATP (K(i) = 0.2 mm). The reverse reaction was inhibited competitively by ATP and noncompetitively by pyruvate. The enzyme was strongly inhibited by p-hydroxymercuribenzoate. The inhibition was reversed by dithiothreitol and glutathione. The properties of the enzyme are discussed in relation to the regulation of the opposing enzymatic activities involved in the interconversion of PEP and pyruvate in A. xylinum.

Adenosine Triphosphate↗
PubMed1969

Factors afecting the activity of pyruvate kinase of Acetobacter xylinum.

1. Extracts of Acetobacter xylinum were found to contain the glycolytic enzymes involved in the conversion of triose phosphate into pyruvate. Pyruvate kinase had the lowest relative activity. Phosphofructokinase activity was not detected in the extracts. 2. Only slight differences in the activity of pyruvate kinase were observed between cells grown on glucose and those grown on intermediates of the tricarboxylic acid cycle. 3. Pyruvate kinase, partially purified from ultrasonic extracts by ammonium sulphate fractionation, required Mg(2+) ions for activity. It was not activated by K(+) or NH(4) (+) ions. 4. The plots representing the relationship between initial velocity and phosphoenolpyruvate concentration were sigmoidal, suggesting a co-operative effect for phosphoenolpyruvate. The Hill coefficient (n) for phosphoenolpyruvate was 2. The rate of the reaction changed with increasing ADP concentrations according to normal Michaelis-Menten kinetics. 5. The enzyme was inhibited by ATP (K(i)0.9x10(-3)m). The inhibition was competitive with regard to ADP but not with regard to phosphoenolpyruvate. It was not relieved by excess of Mg(2+) ions. 6. The possible relationship of the properties of pyruvate kinase to regulatory mechanisms for controlling gluconeogenesis and carbohydrate oxidation in A. xylinum is discussed.

Adenine Nucleotides↗
PubMed1969

Role of phosphoenolpyruvate carboxylation in Acetobacter xylinum.

Glucose-grown cells of Acetobacter xylinum oxidized acetate only when the reaction mixture was supplemented with catalytic quantities of glucose or intermediates of the citrate cycle. Extracts, prepared by sonic treatment, catalyzed the formation of oxalacetate when incubated with phosphoenolpyruvate (PEP) and bicarbonate. Oxalacetate was not formed in the presence of pyruvate plus adenosine triphosphate. The ability to promote carboxylation of PEP was lower in succinate-grown cells than in glucose-grown cells. PEP carboxylase, partially purified from extracts by ammonium sulfate fractionation, catalyzed the stoichiometric formation of oxalacetate and inorganic phosphate from PEP and bicarbonate. The enzyme was not affected by acetyl-coenzyme A or inorganic phosphate. It was inhibited by adenosine diphosphate in a manner competitive with PEP (K(1) = 1.3 mm) and by dicarboxylic acids of the citrate cycle; of these, succinate was the most potent inhibitor. It is suggested that the physiological role of PEP carboxylase in A. xylinum is to affect the net formation of C(4) acids from C(3) precursors, which are essential for the maintainance of the citrate cycle during growth on glucose. The relationship of PEP carboxylase to other enzyme systems metabolizing PEP and oxalacetate in A. xylinum is discussed.

Acetates↗
PubMed1968

The role of ubiquinone in the respiratory chain of Acetobacter xylinum.

1. Whole cells of Acetobacter xylinum were found to contain a quinone of the ubiquinone (coenzyme Q) group. The quinone was isolated from the cells and crystallized. It was identified by its physical, chemical and spectroscopic properties as a ubiquinone with 10 isoprene units (ubiquinone-10). No naphthaquinone was detected in the cells. 2. Cell-free extracts prepared by means of a French pressure cell were separated into three fractions by differential centrifugation. The ubiquinone was located predominantly in the particulate fraction sedimenting at 33000g, which also contained most of the NADH oxidase and malate oxidase activities. The concentration of ubiquinone-10 in extracts was similar to that of the flavoproteins and about three times the concentration of the individual cytochromes. 3. Aerobic incubations of crude extracts with either NADH or malate resulted in reduction of the endogenous ubiquinone-10 to steady-state concentrations of 55 and 40% of the total quinone respectively. In the presence of cyanide more than 95% of the endogenous ubiquinone-10 was reduced by either NADH or malate. 4. The initial rate of reduction of endogenous ubiquinone-10 by malate and the rate of ubiquinol oxidation, in A. xylinum extracts, were found to be compatible with the overall rate of malate oxidation with oxygen. 5. The effects of various respiratory inhibitors on the oxidation-reduction reactions of the endogenous quinone indicate that its position on the respiratory chain is between the malate flavoprotein dehydrogenase and the cytochrome chain.

Alcohol Oxidoreductases↗
PubMed1967

The formation of ornithine from proline in animal tissues.

1. Homogenates of liver or kidney from rat, mouse, dog and guinea pig formed ornithine from proline but not from glutamate. Rat kidney was most active in this reaction and was used for further studies. 2. The overall reaction was found to be catalysed by proline oxidase to yield glutamic gamma-semialdehyde, followed by transamination of this product with glutamate as catalysed by ornithine-keto acid aminotransferase. 3. The unfavourable equilibrium of the ornithine-keto acid aminotransferase reaction was overcome chiefly by glutamate dehydrogenase in the tissue, which removed the alpha-oxoglutarate produced, by reduction with endogenous ammonia and NADH. 4. Aspartate aminotransferase in these preparations also aided in the removal of alpha-oxoglutarate. In this case the overall reaction was driven also by the rapid decarboxylation of oxaloacetate. 5. No evidence could be found for a pathway of ornithine synthesis involving acylated intermediates as has been observed in some micro-organisms. 6. The rate of ornithine synthesis in homogenates of several rat tissues paralleled the activity of ornithine-keto acid aminotransferase in these tissues, indicating that this enzyme was rate-determining for the synthesis. 7. The possible influence of these reactions on urea synthesis is discussed.

Alanine Transaminase↗
PubMed1961

Acetate formation in Clostridium acidi-urici: acetokinase.

Sagers, Richard D. (University of Illinois, Urbana), Moshe Benziman, and I. C. Gunsalus. Acetate formation in Clostridium acidiurici: Acetokinase. J. Bacteriol. 82:233-238. 1961.-To define the energy-yielding reaction(s) during fermentation of purines by Clostridium acidi-urici, an eightfold purified acetokinase was obtained by protamine and ammonium sulfate fractionation of crude extracts. Enzyme activity was determined by measuring the disappearance of acetyl phosphate using adenosine diphosphate (ADP) as phosphate acceptor or by following the generation of acetyl phosphate from adenosine triphosphate (ATP) + acetate. The optimal pH for the reaction was observed to be 7.4. K(m) values for acetyl phosphate, ADP, and magnesium ions were shown to be 2.1 x 10(-3)m, 3.2 x 10(-3)m, and 2.4 x 10(-3)m, respectively. Acetyl phosphate disappearance is dependent upon ADP and is stoichiometric with addition of the latter. The acetokinase reaction probably represents the major energy-yielding reaction during purine fermentation by this organism. A pathway for acetate generation from formiminoglycine is proposed, and evidence for a number of enzyme activities intermediate between formiminoglycine degradation and acetate formation is given. The enzyme activities demonstrated are compatible with the over-all purine fermentation rate by whole cells.

Acetate Kinase↗