Biomedical subjects
Publications and source records attributed to T Leisinger.
Evidence is presented that the increase in specific activity of N-acetylglutamate synthase observed upon heat induction of Escherichia coli (lambdadargA) is primarily due to a gene dosage effect.
Procedures that have been developed for the purification of acetylornithine delta-transaminase from Escherichia coli W also lead to the simultaneous purification of ornithine delta-transaminase. These two enzymatic activities have the same electrophoretic mobility and are identical immunochemically. Studies of inhibition kinetics demonstrate that the two substrates, acetylornithine and ornithine, compete for the same active site of acetylornithine delta-transaminase; thus, the ornithine delta-transaminase activity in E coli is due to acetylornithine delta-transaminase and not to a separate specific ornithine delta-transaminase.
4-Aminobutyrate aminotransferase (GABAT) from Pseudomonas aeruginosa was purified 64-fold to apparent electrophoretic homogeneity from cells grown with 4-aminobutyrate as the only source of carbon and nitrogen. Purified GABAT catalyzed the transamination of 4-aminobutyrate, N2-acetyl-L-ornithine, L-ornithine, putrescine, L-lysine, and cadaverine with 2-oxoglutarate (listed in order of decreasing activity). The enzyme is induced in cells grown on 4-guanidinobutyrate, 4-aminobutyrate, or putrescine as the only carbon and nitrogen source. Cells grown on arginine or on glutamate contained low levels of the enzyme. The regulation of the synthesis of GABAT as well as the properties of the mutant with an inactive N2-acetyl-L-ornithin 5-aminotransferase suggest that GABAT functions in the biosynthesis of arginine by convertine N2-acetyl-L-glutamate 5-semialdehyde to N2-acetyl-Lornithine as well as in catabolic reactions during growth on putrescine or 4-guanidinobutyrate but not during growth on arginine.
Mutants with a feedback resistant N-acetylglutamate synthase have been isolated from a proA/B, argD, argR strain by screening for proline excretion on minimal medium with arginine. The feedback resistant character of three mutants was transduced into an argA (N-acetylglutamate synthase negative) strain. It was cotransducible with argA at a frequency of greater than 99%. N-acetylglutamate synthase extracted from the three mutants was approximately one hundred times less sensitive to L-arginine than the enzyme from the feedback sensitive parent strain.
N-Acetylglutamate 5-phosphotransferase (ATP: N-acetyl-L-glutamate 5-phosphotransferase EC 2.7.2.8), the second enzyme of arginine biosynthesis, was purified over 2000-fold from Pseudomonas aeruginosa. The purification procedure involved a heat treatment, ammonium sulfate precipitation, and chromatography on DEAE-cellulose, Sephadex G-150, and hydroxyapatite. The purified enzyme was greater than 90% pure as judged by analytical polyacrylamide gel electrophoresis. A molecular weight of approximately 230000 was obtained by gel filtration. Electrophoresis in sodium dodecyl sulfate gels gave a single band corresponding to a molecular weight of 29000. Due to the capacity for self-association, the enzyme can exist in different states of aggregation depending on the nature of ligands and the concentrations of phosphate buffer. As estimated by gel filtration, the molecular weight was about 230000 in the presence of N-acetyl-L-glutamate. With L-arginine, the feedback inhibitor, and MgATP forms of smaller molecular weight (minimum of approximately 65000) were found. A concurrent change in the sedimentation coefficient as a function of ligands was demonstrated by sucrose gradient centrifugation. The synthesis of N-acetylglutamate 5-phosphotransferase was not repressed by exogenous L-arginine or its precursors.
Some kinetic properties of N-acetylglutamate 5-phosphotransferase (ATP: N-acetyl-L-glutamate 5-phosphotransferase EC 2.7.2.8) purified approx. 2000-fold from Pseudomonas aeruginosa have been studied. The enzyme required Mg2+ for activity. Mn2+, Zn2+, Co2+, and Ca2+, in this order, could replace Mg2+ partially. The substrate specificity was narrow: N-carbamoyl-L-glutamate and N-formyl-L-glutamate were phosphorylated, but at a lower rate than N-acetyl-L-glutamate; N-propionyl-L-glutamate was almost inactive as a substrate. dATP, but neither GTP nor ITP, could be used instead of ATP. The enzyme had a broad pH optimum from pH 6.5 to 9. Feedback inhibition by L-arginine was markedly dependent on pH. Above pH 9 no inhibition was observed. L-Citrulline was three times less potent an inhibitor than L-arginine. The enzyme showed Michaelis-Menten kinetics, even at low concentration of the second substrate. The apparent Km was 2 mM for N-acetyl-L-glutamate (at 10 mM ATP) and approx. 3 mM for ATP (at 40 mM N-acetyl-L-glutamate). In the presence of L-arginine the rate-concentration curves for N-acetyl-L-glutamate became signoidal, while no cooperativity was detected for ATP. A method was developed allowing the determination of N-acetyl-L-glutamate in the nanomolar range by means of purified enzyme.
N-Acetylglutamate synthase, the first enzyme of arginine biosynthesis, was stabilized in crude extracts from Escherichia coli. At 4 degrees the enzyme lost less than 5% of activity per day. L-Arginine repressed the formation of N-acetylglutamate synthase. Under conditions of genetic or physiological derepression, a specific activity of approximately 50 nmol per min per mg of protein was measured. No activity (i.e. less than 0.2 nmol per min per mg of protein) could be detected in extracts from cells grown under conditions of repression, whereas an intermediate level was found in cell cultivated on minimal medium. In a 6-fold purified preparation L-arginine inhibited the enzyme. Of 11 precursors and analogues of arginine tested only O-[L-norvalyl-5]-isourea inhibited N-acetylglutamate synthase as strongly as L-agrinine.
In Pseudomonas aeruginosa N-2-acetylornithine 5-aminotransferase (ACOAT), the fourth enzyme of arginine biosynthesis is induced about 15-fold by cultivating the organism on a medium with L-arginine as the sole carbon and nitrogen source. Synthesis of the enzyme is subject to catabolite repression and nitrogen source. Synthesis of the enzyme is subject to catabolite repression by a variety of carbon sources. ACOAT from strain PAO 1 was purified over 40-fold to electrophoretic homogeneity. A molecular weight of approximately 110,000 was obtained by thin-layer gel filtration. Electrophoresis in sodium dodecyl sulfate gels gave a single band corresponding to a molecular weight of 55,000. Purified ACOAT catalyzes the transamination of N-2-acetyl-L-ornithine as well as of L-ornithine with 2-oxoglutarate (Km values of 1.1, 10.0, and 0.7 mM, respectively). With N-2-acetyl-L-ornithine as amino donor, the pH-optimum of the enzymatic reaction is 8.5; with L-ornithine as amino donor, 9.5. The catalytic properties of ACOAT as well as the regulation of its synthesis indicate that in P. aeruginosa this enzyme functions in the biosynthesis as well as in the catabolism of L-arginine.
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Treatment of susceptible Escherichia coli K12 derivatives with 0.4 M Mg(++) at 37 degrees , potentiated by L-arginine or L-canavanine, leads to alteration of acetylornithine delta-transaminase. The alteration, obtained in the absence of protein synthesis and reversible at 0 or 37 degrees , is manifested in extracts by lowered activity and modified substrate affinity behavior of the enzyme without gross changes in sedimentation properties. Cells grown under arginine repression are susceptible to the treatment; cells grown under genetic or steady-state physiological derepression are not. Transaminase synthesized during early derepression can be altered, although to progressively diminishing extents. Enzyme formed under steady-state derepression becomes alterable following transition to repression. The Mg(++) -dependent alteration can be thought to arise while the enzyme, arginine (or canavanine), and aporepressor are in contact, and to reflect a physiological process such as the participation of the enzyme in the repressive complex.
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