Inhibition of milk xanthine oxidase by fluorodinitrobenzene

Milk xanthine oxidase reacted with fluorodinitrobenzene resulting in the modification of two lysine residues with a 6-fold decrease in catalytic activity. Continued reaction with fluorodinitrobenzene up to a total of 11 dinitrophenyl residues/equivalent of enzyme-bound FAD resulted in no further decrease in activity. Stopped flow studies revealed that the modification perturbed the reduction of the enzyme by xanthine; this was 6-fold lower with modified than with native enzyme. The reaction of the reduced modified enzyme with oxygen was qualitatively and quantitatively the same as with native enzyme. One nitro group of each dinitrophenyl lysine residue is slowly reduced by xanthine; reduction of both nitro groups is achieved by dithionite. The two dinitrophenyl lysine reduces can be distinguished on the basis of their kinetics of reduction. One appears to be located on the protein surface and is reduced in an intermolecular reaction, while the other appears to be located in a pocket of the enzyme and is reduced in a slow intramolecular reaction.

Adenine nucleotide metabolism in Azotobacter vinelandii. Two metabolic pathways of AMP degradation

AMP-degrading pathways in Azotobacter vinelandii cells were investigated. AMP nucleosidase (EC 3.2.2.4) was rapidly synthesized and reached a maximum at 24 h, while the activity of 5'-nucleotidase (EC 3.1.3.5) specific for AMP, which was negligible during the logarithmic phase of the growth, first appeared in 24 h-cultures, and reached a maximum after complete exhaustion of sucrose from the growth medium (70 h). Cell-free extracts of A. vinelandii of 48 h-cultures hydrolyzed AMP to ribose 5-phosphate and adenine in the presence of ATP, and adenine was deaminated to hypoxanthine. When ATP was excluded, AMP was dephosphorylated to adenosine, which was further metabolized to inosine, and finally to hypoxanthine. Hypoxanthine thus formed was reutilized for the salvage synthesis of IMP under the conditions where 5-phosphoribosyl 1-pyrophosphate was able to be supplied. These results suggest that the levels of ATP can determine the rate of AMP degradation by the AMP nucleosidase- and 5-'nucleotidase-pathways. The role of ATP in the AMP degradation was discussed in relation to the regulatory properties of AMP nucleosidase, inosine nucleosidase (EC. 3.2.2.2) and adenosine deaminase (EC 3.5.4.4).

Chicken liver amidophosphoribosyltransferase. Ligand-induced alterations in molecular properties

A homogeneous amidophosphoribosyltransferase (EC 2.4.2.14) preparation, which was sensitive to purine nucleotide inhibitors, was obtained from chicken liver. From the result of sodium dodecyl sulfate polyacrylamide gel electrophoresis, the subunit weight was estimated to be approximately 58 000. In Tris-HCl buffer, the predominant form of the enzyme had an S20,w of 6.5, Strokes radius of 40 A, and estimated molecular weight of 110 000. Incubation with 5-phosphoribosyl 1-pyrophosphate or Pi resulted in an increase in the S20,w to 9.1--9.5, Strokes radius 50 A, and estimated molecular weight to 200 000. Incubation of the large form with AMP led to a decrease in the molecular wight of the enzyme. It is concluded that chicken liver amidophosphoribosyltransferase is an allosteric protein whose activity is regulated by a series of conformational changes induced by a number of ligands.

AMP deaminase from baker's yeast. Purification and some regulatory properties

AMP deaminase (AMP aminohydrolase, EC 3.5.4.6) was found in extract of baker's yeast (Saccharomyces cerevisiae), and was purified to electrophoretic homogeneity using phosphocellulose adsorption chromatography and affinity elution by ATP. The enzyme shows cooperative binding of AMP (Hill coefficient, nH, 1.7) with an s0.5 value of 2.6 mM in the absence or presence of alkali metals. ATP acts as a positive effector, lowering nH to 1.0 and s0.5 to 0.02 mM. P1 inhibits the enzyme in an allosteric manner: s0.5 and nH values increase with increase in Pi concentration. In the physiological range of adenylate energy charge in yeast cells (0.5 to 0.9), the AMP deaminase activity increases sharply with decreasing energy charge, and the decrease in the size of adenylate pool causes a marked decrease in the rate of the deaminase reaction. AMP deaminase may act as a part of the system that protects against wide excursions of energy charge and adenylate pool size in yeast cells. These suggestions, based on the properties of the enzyme observed in vitro, are consistent with the results of experiments on baker's yeast in vivo reported by other workers.

Effects of monovalent cations on AMP nucleosidase from Azotobacter vinelandii

The effect of monovalent cations on the purified AMP nucleosidase (AMP phosphoribohydrolase, EC 3.2.2.4) from Azotobacter vinelandii was investigated. All the monovalent cations were activators of the enzyme: Rb+ and Cs+ were the most effective, followed by K+, Na+, NH4+ and Li+ in that order. The apparent Ka for MgATP and nH values (Hill's interaction coefficient) decreased from 0.9 to 0.1 mM, and from 4 to 1, respectively, with the increase in K+ concentration, suggesting that the cation effects are on MgATP binding rather than catalysis. Gel filtration studies have revealed that the enzyme forms a non-dissociable enzyme species with a Stokes radius of 6.0--6.2 nm in the presence of saturating concentrations of monovalent cations, which can be distinguished from the 5.5-nm enzyme species showing temperature-dependent dissociation of the molecule in sulfate or phosphate. These results suggest that these ligands affect the association of the subunits through changes in the environment of the hydrophobic side chains of the enzyme molecules.

Polyamines as activators of AMP nucleosidase from Azotobacter vinelandii

Polyamines at physiological concentrations activate AMP nucleosidase from Azotobacter vinelandii. Biological significance of the activation is discussed in relation to the control of adenylate energy charge and the purine nucleotide synthesis in prokaryotes.

Inosine nucleosidase from Azotobacter vinelandii. Purification and properties

An enzyme catalyzing the hydrolysis of purine nucleosides was found to occur in the extract of Azotobacter vinelandii, strain O, and was highly purified by ammonium sulfate fractionation, DEAE-cellulose chromatography, hydroxylapatite chromatography and gel filtration on Sephadex G-150. A strict substrate specificity of the purified enzyme was shown with respect to the base components. The enzyme specifically attacked the nucleosides without amino groups in the purine moiety: inosine gave the maximum rate of hydrolysis and xanthosine was hydrolyzed to a lesser extent. The pH optimum of inosine hydrolysis was observed from pH 7 to 9, while xanthosine was hydrolyzed maximally at pH 7. The Km values of the enzyme for inosine were 0.65 and 0.85 mM at pH 7.1 and 9.0, respectively, and the value for xanthosine was 1.2 mM at pH 7.1. Several nucleotides inhibited the enzyme: the phosphate portions of the nucleotides were suggested to be responsible for the inhibition by nucleotides. Although the inhibition of the enzyme by nucleotides was apparently non-competitive type with respect to inosine, allosteric (cooperative) binding of the substrate was suggested in the presence of the inhibitor. The physiological significance of the enzyme was discussed in connection with the degradation and salvage pathways of purine nucleotides.

The role of polyamines in the regulation of AMP deaminase isozymes

The differential effects of polyamines on the activity of AMP deaminase isozyme A (from rat muscle) and isozyme B (from rat liver) are reported. Polyamines activate isozyme B but inhibit isozyme A.

Purine nucleoside phosphorylase from bovine liver

1. Purine nucleoside phosphorylase (purine nucleoside:orthophosphate ribosyltransferase, E.C. 2.4.2.1) from liver of cattle, Bos taurus, was purified to homogeneity. Some properties of the enzymes from three different bovine tissues were compared and discussed. 2. The enzyme has a molecular weight of 83,000, a sedimentation coefficient of 5.3 S, a Stokes' radius of 3.71 nm, a frictional ratio of 1.30 and a subunit molecular weight of 30,000. 3. Optimal pH for xanthosine degradation is around 5.5, whereas a broad pH activity profile for inosine degradation was observed between 5.0 and 7.5. Lineweaver-Burk plots curved downward at high concentrations of substrates, inosine, phosphate and arsenate.

Molecular properties and a nonidentical trimeric structure of purine nucleoside phosphorylase from chicken liver

Some molecular properties of crystalline purine nucleoside phosphorylase (purine nucleoside: orthophosphate ribosyltransferase, EC 2.4.2.1) from chicken liver were investigated and discussed. The molecular weight of the native enzyme was determined to be 89 000 by gel filtration and sedimentation coefficient, and 90 000 by sedimentation equilibrium, respectively. The enzyme was assumed to be a trimer consisting of one large subunit and two identical small subunits. The molecular weights of two different sized subunits were determined to be 32 000 and 28 000 by sodium dodecyl sulfate gel electrophoresis, and 30 000 and 27 000 by 6 M guanidine hydrochloride gel filtration. The amino acid composition was determined and the partial specific volume was estimated to be 0.735 ml/g.

Flavodoxin: an allosteric inhibitor of AMP nucleosidase from Azotobacter vinelandii

Flavodoxin, which participates in nitrogen fixation, was found to be a potent allosteric inhibitor of AMP nucleosidase [EC 3.2.2.4] from Azotobacter vinelandii. It inhibited the enzyme by decreasing its affinity for ATP without affecting the maximum velocity. The inhibition constant for flavodoxin was estimated to be 10 muM, which is within the range of physiological concentration in the cells. The concentration of flavodoxin able to alter the activity in vitro suggests that this phenomenon could be of significance in the regulation of flavin biosynthesis in vivo. Flavin mononucleotide (FMN), a prosthetic group of flavodoxin, was also found to act as an allosteric inhibitor. Since no inhibitory action of apo-flavodoxin was observed, it was concluded that the FMN chromophore of the flavodoxin is responsible for the inhibition of the enzyme by this protein.

Cytosol 5'-nucleotidase from chicken liver. Purification and some properties

1. 5'-Nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5) from the cytosol of chicken liver has been purified 1860-fold with an overall yield of 20% by a combination of precipitation at pH 5.3, (NH4)2SO4 fractionation, calcium phosphate gel adsorption, phosphocellulose chromatography and gel filtration with Sephadex G-200. The enzyme has been shown to be highly purified, as judged by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. This is the first time it has been possible to obtain a purified 5'-nucleotidase from the cytosol of animal tissue. 2. An S20, W of 9.7 S for 5'-nucleotidase was obtained by the use of sucrose density gradient centrifugation and a Stokes radius of 5.1 nm was estimated by gel filtration techniques. From these values and the assumed partial specific volume of 0.725 cm3/g, the molecular weight of the enzyme was calculated to be 205 000. One major band, corresponding to a molecular weight of 51 000, was detected after sodium dodecyl sulphate-polyacrylamide gel electrophoresis indicating that the native enzyme was composed of four identical subunits. 3. Some properties of the purified enzyme, including pH optimum Mg2+ dependency and substrate specificity, resembled closely those of the partially purified enzyme from chicken liver acetone powder as reported by Itoh, R., Mitsui, A. and Tsushima, K. (1967) Biochim. Biophys. Acta 146, 151-159.

Studies on chicken liver xanthine dehydrogenase with reference to the problem of non-equivalence of FAD moieties

1. Reduction of chicken liver xanthine dehydrogenase (xanthine: NAD+ oxidoreductase, EC 1.2.1.37) by xanthine under anaerobic condition proceeded in two phases. This biphasicity may be due to functional and non-functional enzymes in the enzyme preparation. 2. Cyanolysis of a persulfide group of chicken liver enzyme resulted in an inactivation of the enzyme. The non-functional enzyme in the standard enzyme preparation was found to lack persulfide groups at the active sites. 3. The remaining NADH-Methylene Blue oxidoreductase activity, after KI treatment of the xanthine-reduced enzyme of a high flavin activity ratio, is not at the level of 50% of the initial activity, differing from the report suggesting non-equivalence of FAD chromophores. 4. The findings in the present report indicate that FAD chromophores of chicken liver enzyme are essentially equivalent.

Crystallization and some properties of purine nucleoside phosphorylase from chicken liver

Purine nucleoside phosphorylase (purine nucleoside:orthophosphate ribosyltransferase, EC 2.4.2.1) from chicken liver has been purified about 650 fold and crystallized. The crystalline enzyme was cube shaped and showed a specific activity of 46 units per mg of protein. The homogeneity of the crystalline enzyme was shown by polyacrylamide gel-disc electrophoresis. The sedimentation coefficient (s-degrees 2o,w) was 5.4 S. The crystalline enzyme was activated by the substrate inosine. The Hill coefficient was estimated to be 0.76, suggesting negative cooperativity with regard to the substrate inosine. The results of the kinetic analysis are consistent with the mechanism being a "rapid equilibrium random Bi-Bi reaction". The apparent equilibrium constant for phosphorolysis was 0.048.