Mammalian folylpoly-gamma-glutamate synthetase. 2. Substrate specificity and kinetic properties

The specificity of hog liver folylpolyglutamate synthetase for folate substrates and for nucleotide and glutamate substrates and analogues has been investigated. The kinetic mechanism, determined by using aminopterin as the folate substrate, is ordered Ter-Ter with MgATP binding first, folate second, and glutamate last. This mechanism precludes the sequential addition of glutamate moieties to enzyme-bound folate. Folate, dihydrofolate, and tetrahydrofolate possess the optimal configurations for catalysis (kcat = 2.5 s-1) while 5- and 10-position substitutions of the folate molecule impair catalysis. kcat values decrease with increasing glutamate chain length, and the rate of decrease varies depending on the state of reduction and substitution of the folate molecule. Folate binding, as assessed by on rates, is slow. Dihydrofolate exhibits the fastest rate, and the rates are slightly reduced for tetrahydrofolate and 10-formyltetrahydrofolate and greatly reduced for 5-methyltetrahydrofolate and folic acid. The on rates for most pteroyldiglutamates are similar to the rates for their respective monoglutamate derivatives, but further extension of the glutamate chain results in a progressive decrease in on rates. Tetrahydrofolate polyglutamates are the only long glutamate chain length folates with detectable substrate activity. The specificity of the L-glutamate binding site is very narrow. L-Homocysteate and 4-threo-fluoroglutamate are alternate substrates and act as chain termination inhibitors in that their addition to the folate molecule prevents or severely retards the further addition of glutamate moieties. The Km for glutamate is dependent on the folate substrate used. MgATP is the preferred nucleotide substrate, and beta,gamma-methylene-ATP, beta,gamma-imido-ATP, adenosine 5'-O-(3-thiotriphosphate), P1,P5-di(adenosine-5') pentaphosphate, and free ATP4- are potent inhibitors of the reaction.     

Mammalian folylpoly-gamma-glutamate synthetase. 3. Specificity for folate analogues

A variety of folate analogues were synthesized to explore the specificity of the folate binding site of hog liver folylpolyglutamate synthetase and the requirements for catalysis. Modifications of the internal and terminal glutamate moieties of folate cause large drops in on rates and/or affinity for the protein. The only exceptions are glutamine, homocysteate, and ornithine analogues, indicating a less stringent specificity around the delta-carbon of glutamate. It is proposed that initial folate binding to the enzyme involves low-affinity interactions at a pterin and a glutamate site and that the first glutamate bound is the internal residue adjacent to the benzoyl group. Processive movement of the polyglutamate chain through the glutamate site and a possible conformational change in the protein when the terminal residue is bound would result in tight binding and would position the gamma-carboxyl of the terminal glutamate in the correct position for catalysis. Steric limitations imposed on the internal glutamate residues that loop out and additional steric constraints imposed by binding of different pterin moieties would be expected to effect slight conformational changes in the protein and/or the terminal glutamate and would explain the decrease in on rate and catalytic rate with increased polyglutamate chain length, and the differential effect of one-carbon substitution on the catalytic rate with polyglutamate derivatives. The 4-amino substitution of folate increases the on rate for monoglutamate derivatives but severely impairs catalysis with diglutamate derivatives. Pteroylornithine derivatives are the first potent and specific inhibitors of folylpolyglutamate synthetase to be identified and may act as analogues of reaction intermediates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mammalian folyl polyglutamate synthetase: partial purification and properties of the mouse liver enzyme

Folyl polyglutamate synthetase has been partially purified from mouse liver, and the general features of this enzyme have been characterized. The purification procedure utilized fractionation with ammonium sulfate, gel filtration, and affinity chromatography on ATP-agarose and resulted in a 350-fold increase in specific activity with 8-20% recovery of enzyme activity. Enzyme could be stabilized by glycerol or by ATP, but stability was not appreciably enhanced by folate. The enzymatic reaction was completely dependent on folate, ATP, and Mg2+ while partial reaction rates were observed in the absence of KCl or beta-mercaptoethanol. Highest reaction rates were observed at pH 8.2-9.5 at 37 degrees C. Chromatography of purified enzyme on calibrated gel filtration columns suggested a molecular weight of 65 000. Mouse liver folyl polyglutamate synthetase coupled [3H]glutamic acid to all of the naturally occurring folates studied. Analysis of the reaction products by high-performance liquid chromatography demonstrated that several folyl oligoglutamates were formed at low substrate concentrations but that only folyl diglutamate was formed at substrate concentrations approaching saturation. Dihydrofolate, tetrahydrofolate, 5,10-methylenetetrahydrofolate, 10-formyltetrahydrofolate, and 5-formyltetrahydrofolate were the best substrates. Folic acid and 5-methyltetrahydrofolate were also substrates for this reaction, but much higher concentrations of these compounds were required to saturate the enzyme. These data suggest that all of the tetrahydrofolyl compounds (except 5-methyltetrahydrofolate) are the monoglutamyl substrates for polyglutamation in vivo and that 5-methyltetrahydrofolate is not likely to be a direct precursor for folate polyglutamates in mouse liver.     

Purification and properties of hog liver 4-hydroxyphenylpyruvate dioxygenase

The purification of hog liver 4-hydroxyphenylpyruvate dioxygenase (EC 1.13.11.27), and the determination of some of its characteristics, are reported. The enzyme was purified 330-fold in 22% yield from an acetone powder extract by ammonium sulfate fractionation, chromatography twice using sulfopropyl Sephadex under carefully controlled pH conditions (once at pH 5.36 and a second time with a pH gradient from 5.25 to 5.80), and a final chromatography on DEAE-cellulose. The purified enzyme was found to be homogeneous by several standard criteria, but activity measurements indicated that a small amount (less than 5%) of a carboxylesterase (EC 3.1.1.1) isoenzyme is present as a minor impurity. On long-term storage at ?20 °C the enzyme forms polymers but this can be reversed with thiols. The molecular weight of the freshly prepared or depolymerized enzyme was estimated to be 89,000 ± 2000 by equilibrium ultracentrifugation, and 50,000 to 54,000 by gel filtration. Sodium dodecyl sulfate-gel electrophoresis experiments, performed in the presence and absence of mercaptoethanol, indicate that the enzyme is composed of two nonidentical subunits with similar molecular weights (44,000 ± 2000). The enzyme gives a typical protein ultraviolet absorption spectrum with no noticeable peaks above 300 nm, it has no detectable carbohydrate content, and it contains 0.9 atom iron and 0.4 atom copper/89,000 daltons. Added iron and copper salts activate the enzyme to some extent but by less than a factor of 2. The enzymatic reaction has a large temperature coefficient (the rate increases ca. fivefold for each 10 °C rise) and is markedly stimulated (up to sixfold) by the presence of some organic solvents in concentrations up to 10% of the medium. These results suggest that a protein conformation change, possibly aided by binding of the organic solvent, is involved in the rate-determining step of the reaction. The similarities and differences of this 4-hydroxyphenylpyruvate dioxygenase to those from other sources, and to prolyl hydroxylase, are discussed.     

Purification and properties of asparagine synthetase from rat liver.

Asparagine synthetase (L-aspartate:ammonia ligase (AMP-forming, EC 6.3.1.1) activity in rat liver increased when the animals were put on a low casein diet. The enzyme was purified about 280-fold from the supernatant of rat liver homogenate by a procedure comprising ammonium sulfate fractionation. DEAE-Sepharose column chromatography, and Sephadex G-100 gel filtration. The optimal pH of the enzyme was in the range 7.4-7.6 with glutamine as an amide donor. The molecular weight was estimated to be approximately 110,000 by gel filtration. Chloride ion was required for the enzyme activity. The apparent Km values for L-aspartate, L-glutamine, ammonium chloride, ATP, and Cl- were calculated to be 0.76, 4.3, 10, 0.14, and 1.7 mM, respectively. The activity was inhibited by L-asparagine, nucleoside triphosphates except ATP, and sulfhydryl reagents. It has been observed that the properties of asparagine synthetase from rat liver are not so different from those of tumors such as Novikoff hepatoma and RADA 1.     

Purification and some properties of rat liver tyrosyl-tRNA synthetase.

Rat liver cytoplasmic tyrosine:tRNA ligase (tyrosine:tRNA ligase, EC 6.1.1.1) was purified by ultracentrifugation, DEAE-cellulose chromatography and repeated phosphocellulose chromatography by more than 1500-fold. The molecular weight of the enzyme was approx. 150 000 as determined by Sephadex G-200 gel filtration. On the basis of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the enzyme consisted of two subunits, each of 68 000 daltons. We found the following Km values for the enzyme: 13 micrometer for tyrosine and 1.7 mM for ATP in the ATP:PPi exchange reaction and 13 micrometer for tyrosine, 210 micrometer for ATP and 0.14 micrometer for tRNATyr in the aminoacylation reaction. The rate of tyrosyl-tRNA synthesis was 50-fold lower than that of ATP:PPi exchange. Addition of a saturating amount of tRNA did not affect the rate of ATP:PPi exchange.     

Purification and properties of tyrosyl tRNA synthetase of rat liver.

Purification of rat liver tyrosine tRNA synthetase yields two protein fractions A and B and both fractions are required for charging of tyrosine to tRNAtyr. Fraction B catalyzes the activation of tyrosine. Fractions A and B have been purified to near homogeneity and they are composed of single polypeptide chains of 62,000 daltons each. Gel filtration studies suggest a molecular weight of 120,000 for the synthetase.     

Purification and general characterization of FAD synthetase from rat liver

Flavin adenine dinucleotide synthetase (ATP:FMN adenylyltransferase, EC 2.7.7.2) has been enriched more extensively than previously from fresh rat liver. For this, 10% homogenates in sucrose-phosphate buffer were treated with 0.1% Tween-20 prior to high-speed centrifugation to obtain soluble proteins. Those precipitated by 40% saturation with ammonium sulfate were subjected to stepwise addition of calcium phosphate gel to remove pyrophosphatase, and the remaining synthetase was further enriched by passage through a tricalcium phosphate column. An apparent yield of greater than 70% and purification over 70-fold was achieved from the high-speed supernatant fraction. The synthetase activity in solution at 4 degrees was largely lost within a week unless protected by thiols which could partly restore inactivated enzyme. The pH optimum for synthetase activity is near 7.7 when assayed with suitable concentrations of FMN, ATP, and Mg2+. Purified enzyme could be separated into lower (140,000) and higher (325,000) molecular weight components when subjected to molecular sieving on a Sephadex G-200 column.     

Mammalian tRNA sulfurtransferase: properties of the enzyme in rat liver.

Transfer RNA sulfurtransferase activity was detected in 105,000 x g supernatant preparations from rat liver and several other rat tissues. Sulfur is transferred from [35S] cysteine to tRNA in a reaction which also requires ATP, Mg2+, and supernatant protein. While [35S] beta-mercaptopyruvate appeared to be a substrate for this enzyme, the reaction product was sensitive to deacylation and the reaction was inhibited by [32S] cysteine. Of the various nucleic acids tested, only tRNAs were effective sulfur acceptors, with rat liver tRNA being the poorest substrate. The [35S] reaction product was sensitive to ribonuclease, cochromatographed with tRNA on methylated-albumin kieselguhr columns, and was converted to nucleotide material after alkaline hydrolysis. DEAE-cellulose chromatography of the neutralized [35S] nucleotide digest revealed a single thionucleotide peak. These studies demonstrate that tRNA sulfurtransferase is present in various rat tissues, and that the requirements of the liver enzyme are similar to those of bacterial enzymes.     

delta-Aminolevulinic acid synthetase from rat liver mitochondria. Purification and properties.

delta-Aminolevulinic acid synthetase has been purified from liver mitochondria of young, uninduced rats. After nonionic detergent solubilization of mitochondrial inner membrane-matrix fractions, the enzyme was purified to a specific activity of approximately 2,000 nmol of delta-aminolevulinic acid formed/h/mg of protein at 30 degrees C, by means of ammonium sulfate precipitation, diethylaminoethyl cellulose chromatography, Sephacryl chromatography, and preparative gel electrophoresis. The purified enzyme preparation thus obtained was apparently homogeneous as judged by its migration as a single band with a molecular weight of 58,000 +/- 6,000 upon electrophoresis in sodium dodecyl sulfate polyacrylamide gels. The native enzyme probably exists as a dimer with a molecular weight of approximately 120,000. A pH optimum of 7.5 and an isoelectric point of 4.5 were also determined. Both monovalent cations and hemin strongly inhibited the activity of the purified enzyme.