Search PubMed⌕ Search

Biomedical subjects

A Meister

Publications and source records attributed to A Meister.

At least 325 records · Page 18Linked to original sources
PubMed1973

Observations on the pH dependence of the glutaminase activity of a glutamine amidotransferase, carbamylphosphate synthetase.

Glutamine-dependent carbamylphosphate synthetase (Escherichia coli) is composed of a heavy subunit (molecular weight about 130,000) and a light subunit (molecular weight about 42,000), which can be separated with retention of catalytic activities. The separated heavy subunit can catalyze activation of CO(2) by ATP and synthesis of carbamylphosphate from ammonia (but not from glutamine). The only catalytic activity exhibited by the separated light subunit is the ability to hydrolyze glutamine; the separated heavy subunit does not exhibit glutaminase activity. The pH-activity curve of the glutaminase activity of native carbamylphosphate synthetase exhibits maxima at about pH 4.2 and 9.5, while the glutaminase activity of the separated light subunit exhibits only a single optimum at about pH 6.7. When the light and heavy subunits are recombined, the two pH optima characteristic of native enzyme are restored. Glutaminase activities of native enzyme at both pH optima are similarly inhibited by the glutamine analog, L-2-amino-4-oxo-5-chloropentanoic acid, and also by dithiothreitol. Storage of native enzyme at pH 9 abolishes the glutaminase optimum at acid pH, but greatly increases the activity at alkaline pH. Treatment of native enzyme with N-ethylmaleimide increases the glutaminase activity dramatically and converts the pH profile to one that closely resembles that of the isolated light subunit. The data indicate that the same active site is involved in hydrolysis of glutamine at both acid and alkaline values of pH, and that this property of the enzyme depends upon interactions between the heavy and light subunits of native enzyme. The double-optima behavior of native enzyme seems to be related to participation of different catalytic groups of the enzyme which affect the maximum velocity rather than the binding of substrate. The findings offer additional evidence for occurrence of significant interactions between the subunits of carbamylphosphate synthetase, and may have significance in relation to other glutamine amidotransferases, including glutaminases.

Catalysis↗
PubMed1973

Inhibition of 5-oxoprolinase by 2-imidazolidone-4-carboxylic acid.

L-2-Imidazolidone-4-carboxylic acid is an effective competitive inhibitor of the reaction catalyzed by 5-oxoprolinase, in which 5-oxo-L-proline (L-pyroglutamic acid, L-2-pyrrolidone-5-carboxylic acid, L-5-oxopyrrolidine-2-carboxylic acid) is converted to L-glutamate, with concomitant cleavage of ATP to ADP and orthophosphate. L-2-Imidazolidone-4-carboxylate decreased the rate of metabolism of 5-oxo-L-[(14)C]proline to (14)CO(2) by rat-kidney slices but had no effect on the metabolism of [(14)C]glutamate. Mice injected with L-2-imidazolidone-4-carboxylate exhibited greatly reduced ability to metabolize 5-oxo-L-proline, but metabolized glutamate at an essentially normal rate. The findings provide an approach to an animal model for the human condition 5-oxoprolinuria, in which there is apparently a deficiency of renal 5-oxoprolinase activity. The evidence indicates that 5-oxoproline is a normal metabolite.

Amidohydrolases↗
PubMed1973

The -glutamyl cycle in the choroid plexus: its possible function in amino acid transport.

Various anatomic regions of rabbit brain have been examined for activities of the enzymes of the gamma-glutamyl cycle. While these enzyme activities were widely distributed in the brain, they are present in much higher concentrations in the choroid plexus than in other parts of the brain. The activities observed are of about the same order of magnitude as found in the kidney. These observations and other considerations suggest that the gamma-glutamyl cycle may play a significant role in the transport of amino acids between blood and cerebrospinal fluid.

Acyltransferases↗
PubMed1973

Enzymes of the gamma-glutamyl cycle in the ciliary body and lens.

The enzymes of the gamma-glutamyl cycle have been found in rabbit ciliary body and, except for 5-oxoprolinase, also in the ocular lens. Histochemical studies show that gamma-glutamyl transpeptidase is localized mainly in the basal portions of the epithelial cells of the ciliary body; the findings are similar to those observed in the chloroid plexuses. The histochemical staining reaction in the ciliary epithelium is more intense than in the chloroid plexus, intestine, and kidney. gamma-Glutamyl transpeptidase staining activity in the epithelium of the intestinal and renal proximal convoluted tubules is confined to the microvillus border. Moderate transpeptidase activity was found in the cytoplasm of nonpigmented epithelial cells of the iris at the posterior pupillary margin. The histochemical and enzyme activity studies are consistent with the thesis that the gamma-glutamyl cycle functions in transport of amino acids across the blood-aqueous humor barrier.

Acyltransferases↗
PubMed1972

Decarboxylation of 3,4-dihydroxyphenylalanine (DOPA) by erythrocytes: a reaction promoted by methemoglobin and other ferriheme proteins.

The decarboxylation of DOPA by erythrocyte hemolysates differs from DOPA decarboxylation catalyzed by aromatic aminoacid decarboxylases that contain vitamin B(6) in several significant respects. The ability of erythrocyte hemolysates to decarboxylate DOPA is associated with interaction between DOPA and methemoglobin; the ferriheme protein is reduced and DOPA is decarboxylated, probably after oxidation to a quinone intermediate. An analogous reaction takes place between DOPA and other ferriheme proteins, such as metmyoglobin and cytochrome c. This phenomenon may be of significance in relation to the side effects observed in patients with Parkinson's disease who are treated with very large doses of DOPA.

Animals↗