Anthranilate synthase-anthranilate 5-phosphoribosyl 1-pyrophosphate phosphoribosyltransferase from Aerobacter aerogenes

1. Anthranilate synthase and phosphoribosyltransferase from Aerobacter aerogenes purify simultaneously and sediment together on sucrose gradients, showing that they occur as an enzyme aggregate. Both activities of the intact aggregate are subject to inhibition by tryptophan. 2. By using appropriate auxotrophic mutants it was shown that an intact active enzyme aggregate is formed when the components come from separate mutant strains. An intact active aggregate can also be formed when one component is from Escherichia coli and the other from A. aerogenes. 3. Phosphoribosyltransferase of A. aerogenes is active when not in an aggregate with anthranilate synthase, but is not subject to tryptophan inhibition, indicating that the inhibitor site is on the anthranilate synthase component. 4. Anthranilate synthase can be active and sensitive to tryptophan inhibition when complexed with an inactive phosphoribosyltransferase. 5. Kinetic studies on the anthranilate synthase activity show that tryptophan is a competitive inhibitor with respect to chorismate and a non-competitive inhibitor with respect to either glutamine or NH(4) (+) ions. This is consistent with a sequential mechanism of the ordered type in which chorismate is the first reactant.

The kinetics of reactions catalyzed by alkaline phosphatase: the effects of added nucleophiles

An experimental study was made of the hydrolyses of phenyl phosphate and p-nitrophenyl phosphate catalyzed by chicken intestinal alkaline phosphatase. The work was done at pH 8.0 and 10.0, 25.0 °C, and an ionic strength of 0.1 M, and particular attention was paid to the kinetics of formation of the products in the presence of Tris and ethanolamine. It was found that the rates of formation of phenol or p-nitrophenol (P1) and of the phosphorylated nucleophile (P3) were dependent on the concentration of added nucleophile; on the other hand the rate of formation of phosphate (P2) and the Michaelis constant were independent of nucleophile concentration. This result cannot be reconciled with any of the mechanisms discussed in the preceding paper with the exception of mechanism VI, which is an elaboration of one proposed by Trentham and Gutfreund; mechanism VI is[Formula: see text]where W is water and N the alternative nucleophile. ES and E*S are two conformers of the enzyme–substrate complex, and E*S′ and ES′ two forms of the phosphorylated enzyme; only the latter can react with water and only the former with nucleophile.

Steady-state kinetics of enzyme reactions in the presence of added nucleophiles

Many enzyme-catalyzed reactions, such as hydrolyses, give rise to two products P1 and P2 which are formed in different reaction steps. The second product P2 is frequently formed by hydrolysis of an intermediate such as an acyl-enzyme or a phosphoryl-enzyme. An alternative nucleophile N introduced into the system forms an additional product P3. The present paper is concerned with the kinetics of formation of P1, P2, and P3 in the presence of added nucleophiles. A number of alternative mechanisms are considered, and equations are derived for the rates of formation of the three products, and the Michaelis constant, as functions of nucleophile concentration. Graphs are presented showing the variations of these parameters with the concentration of N, for a variety of special cases. Special attention is given to the possibility of specific binding sites for the water and the nucleophile molecules.The data for a number of enzyme systems are discussed with reference to the treatment. For reactions catalyzed by alkaline phosphatase it is concluded that only one mechanism (mechanism VI) is consistent with the results.

Ultrastructural cytochemical demonstration of tartrate-resistant acid phosphatase isoenzyme activity in "hairy cells" of leukemic reticuloendotheliosis

The pathognomonic "hairy cell" of leukemic reticuloendotheliosis was studied for ultrastructural localization of tartrate-resistant acid phosphatase activity. The enzyme activity could not be demonstrated by Gomori's method, but it was readily demonstrated in the cytoplasmic vesicles and Golgi saccules of the hairy cell by substitution of naphthol AS-BI phosphoric acid for Na-beta-glycerophosphate of Gomori's medium. This apparent substrate specificity may be used as a marker for electron microscopic identification of the hairy cell, and may facilitate the study of histogenesis of leukemic reticuloendotheliosis.

Development of the activities of enzymes of the isoprenoid pathway during early stages of pea-seed germination

The activities of individual enzymes of the isoprenoid pathway from mevalonate kinase to squalene synthetase in homogenates of seeds germinated up to 32h were assayed. Changes in the activity of each enzyme were observed and compared with the activity at the 2h germination stage. Activities of alkaline phosphatase and fructose 1,6-diphosphate aldolase were similarly measured to provide a reference for changes in the general metabolic activity of seeds during imbibition of water. Water uptake reached a plateau after 12h. The reference enzymes almost doubled in activity between 2 and 8h and thereafter their activities steadily declined. All of the enzymes of the isoprenoid pathway increased in activity between 2 and 6h and, thereafter, with the exception of the prenyltransferase, their activities remained relatively constant. With the prenyltransferase activity the initial increase was followed by a short plateau between 6 and 9h and then a second increase to a maximum between 14 and 16h. After 16h the activity declined. The relative activities of the isoprenoid enzymes at 16h of germination were mevalonate kinase>phosphomevalonate kinase>pyrophosphomevalonate decarboxylase approximately isopentenyl pyrophosphate isomerase>squalene synthetase>isopentenyl pyrophosphate/dimethylallyl pyrophosphate prenyltransferase. The finding that the prenyltransferase may be the rate-limiting enzyme in squalene synthesis from mevalonate is discussed in relation to regulation of isoprenoid synthesis during pea-seed germination.

8-Azaguanine resistance in mammalian cells. I. Hypoxanthine-guanine phosphoribosyltransferase

Chinese hamster cells were treated with ethyl methanesulfonate or N-methyl-N'-nitro-N-nitrosoguanidine, and mutants resistant to 8-azaguanine were selected and characterized. Hypoxanthine-guanine phosphoribosyltransferase activity of sixteen mutants is extremely negative, making them suitable for reversion to HGPRTase(+). Ten of the extremely negative mutants revert at a frequency higher than 10(-7) suggesting their point mutational character. The remaining mutants have demonstrable HGPRTase activity and are not useful for reversion analysis. Five of these mutants have < 2% HGPRTase and are presumably also HGPRTase point mutants. The remaining 14 mutants utilize exogenous hypoxanthine for nucleic acid synthesis poorly, and possess 20-150% of wild-type HGPRTase activity in in vitro. Their mechanism of 8-azaguanine resistance is not yet defined.