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Biomedical subjects

A Meister

Publications and source records attributed to A Meister.

At least 289 records · Page 16Linked to original sources
PubMed1975

Enhancement of the glutaminase activity of carbamyl phosphate synthetase by alterations in the interaction between the heavy and light subunits.

Glutamine-dependent carbamyl phosphate synthetase (from Escherichia coli) was previously shown to be composed of a light subunit (molecular weight similar to 42,000) which has the binding site for glutamine and a heavy subunit (molecular weight similar to 130,000) which has binding sites for the other reactants and allosteric effectors. The subunits may be separated with retention of catalytic activities; only the separated light subunit exhibits glutaminase activity. The previous finding that storage of the native enzyme at pH 9 at 0 degrees increased its glutaminase activity by about 25-fold was further investigated; such storage markedly decreased the glutamine- and ammonia-dependent synthetase activities of the enzyme. Treatment of the enzyme with p-hydroxymercuribenzoate led to transient increase of glutaminase activity followed by inhibition. When the enzyme was treated with N-ethylmaleimide or with 5,5'-dithiobis-(2-nitrobenzoate), the glutaminase activity was increased by about 250-fold with concomitant loss of synthetase activities. The enhancement of glutaminase produced by storage of the enzyme at pH 9 was associated with intermolecular disulfide bond formation and aggregation of the enzyme. Aggregation also was observed after extensive treatment of the enzyme with 5,5'-dithiobis-(2-nitrobenzoate) or N-ethylmaleimide. However, a moderate increase of glutaminase activity (about 30-fold) was observed without aggregation under conditions in which one sulfhydryl group on the light subunit reacted with either reagent. The findings suggest that the increased glutaminase activities observed here are associated with structural changes in the enzyme in which the intersubunit relationship is altered so as to uncouple the catalytic functions of the enzyme and to facilitate access of water to the glutamine binding site on the light subunit.

Carbamoyl-Phosphate Synthase (Glutamine-Hydrolyzin↗
PubMed1975

Regulation of gamma-glutamyl-cysteine synthetase by nonallosteric feedback inhibition by glutathione.

Gamma-Glutamyl-cysteine synthetase is inhibited by glutathione under conditions similar to those which prevail in vivo, thus strongly suggesting a physiologically significant feedback mechanism. Inhibition by glutathione, which is not allosteric, appears to involve the binding of glutathione to the glutamate site of the enzyme as well as to another enzyme site; the latter binding appears to require a sulfhydryl group since ophthalmic acid (gamma-glutamyl-alpha-aminobutyryl-glycine) is only a weak inhibitor. The finding that glutathione regulates its own synthesis by inhibiting synthesis of gamma-glutamyl-cysteine appears to explain observations on patients with 5-oxoprolinuria, who were shown to have a block in the gamma-glutamyl cycle consisting of a marked deficiency of glutathione synthetase and consequently of glutathione. These patients produce greater than normal amounts of gamma-glutamyl-cysteine, which is converted by the action of gamma-glutamyl cyclotransferase to 5-oxoproline; production of the latter compound exceeds the capacity of 5-oxoprolinase to convert it to glutamate. The apparent Km value for L-cysteine for gamma-glutamyl-cysteine synthetase (0.35 mM) is not far from intracellular concentrations of L-cysteine suggesting that the availability of L-cysteine may also play a role in the regulation of glutathione synthesis.

Allosteric Regulation↗
PubMed1975

Utilization of L-cystine by the gamma-glutamyl transpeptidase-gamma-glutamyl cyclotransferase pathway.

Cystine is a good acceptor of the gamma-glutamyl group of gamma-glutamyl donors in the reaction catalyzed by gamma-glutamyl transpeptidase. The product of the enzymatic reaction and an authentic sample of gamma-glutamylcystine were shown to exhibit identical chromatographic and electrophoretic behaviors; acid hydrolysis gave equimolar amounts of cystine and glutamate. In studies with two gamma-glutamyl donors, apparent Km values in the neighborhood of 0.3 mM were found for L-cystine; these values are not far from the concentrations of L-cystine in mammalian blood plasma. At an amino-acid acceptor concentration of about 0.5 mM, L-cystine is somewhat more active than L-glutamine, and much more active than L-cystein. L-gamma-Glutamyl-L-cystine was found to be a good substrate of gamma-glutamyl cyclotransferase. These observations thus indicate that L-cystine is a very active substrate of the gamma-glutamyl transpeptidase-gamma-glutamyl cyclotransferase pathway. In relation to the hypothesis that the gamma-glutamyl cycle functions in animo-acid transport, it may be significant that glutathione (which is the most abundant intracellular form) is a much better gamma-glutamyl donor than glutathione disulfide, while the predominant extracellular form-cystine-is a much better gamma-glutamyl acceptor substrate than cystein.

Acyltransferases↗