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Biomedical subjects

A Meister

Publications and source records attributed to A Meister.

At least 109 records · Page 6Linked to original sources
PubMed1989

Activation of glutamate by gamma-glutamate kinase: formation of gamma-cis-cycloglutamyl phosphate, an analog of gamma-glutamyl phosphate.

gamma-Glutamate kinase, the enzyme that catalyzes the first step in the pathway from glutamate to proline, has been postulated to convert glutamate to a gamma-activated form (possibly gamma-glutamyl phosphate), which is reduced by a NADPH-linked reductase to yield glutamate gamma-semialdehyde (in equilibrium with delta 1-pyrroline-5-carboxylate). In the present work we found that the kinase, in the absence or presence of the reductase (and in the absence of NADPH), catalyzes stoichiometric formation of 5-oxo-L-proline and Pi from L-glutamate and ATP, but catalyzes hydroxamate formation at only about 10% of the rate of ATP-cleavage. A new substrate of the kinase was found; thus, cis-cycloglutamate (cis-1-amino-1,3-dicarboxycyclohexane), a glutamate analog which cannot cyclize to form an analog of 5-oxoproline, interacts effectively with the kinase. The trans form of cycloglutamate does not interact with the kinase; only the cis form can assume a diequatorial conformation equivalent to the extended conformation of glutamate. cis-Cycloglutamyl phosphate formation was shown and evidence was obtained for formation of an enzyme-ADP-cycloglutamyl phosphate complex. Although cis-cycloglutamyl phosphate is not a reducible substrate of the NADPH-dependent reductase, the findings indicate that it interacts with the reductase. These studies, which elucidate several aspects of the mechanism of the utilization of glutamate for formation of delta 1-pyrroline-5-carboxylate, support the hypothesis that the kinase and reductase function as an enzyme complex. A model is suggested in which gamma-glutamyl phosphate formed on the kinase interacts with the reductase to form a gamma-glutamyl-reductase complex, which is reduced by NADPH to yield glutamate gamma-semialdehyde.

Adenosine Diphosphate↗
PubMed1989

Study of the 5-oxoprolinase reaction by 13C NMR.

5-Oxoprolinase catalyzes the ATP-dependent decyclization of 5-oxo-L-proline to L-glutamate. Previous studies provided evidence for the intermediate formation of a phosphorylated form of 5-oxoproline (Seddon, A. P., and Meister, A. (1986) J. Biol. Chem. 261, 11538-11541) and of a tetrahedral intermediate (Li, L., Seddon, A. P., and Meister, A. (1987) J. Biol. Chem. 262, 11020-11025). A new approach to the study of the reaction mechanism using the 18O isotope effect on the 13C NMR signals for 5-oxoproline and glutamate is reported here. The 13C chemical shifts induced by 18O substitution for the carbonyl group of 5-oxoproline and the gamma-carboxyl group of glutamate are about 0.03 ppm with respect to the corresponding 16O-compounds. Using 5-[18O]oxo[5-13C]proline (97 and 79.5 atom % excess, 13C and 18O, respectively), the disappearance of the 18O-labeled and unlabeled 5-oxoproline and formation of the corresponding glutamate species were followed in the reactions catalyzed by purified preparations of 5-oxoprolinase isolated from Pseudomonas putida and from rat kidney. This procedure permits simultaneous determinations of the rates of 18O exchange and of the overall decyclization reaction. The ratios of 18O exchange rates to the overall reaction rates for the bacterial and kidney enzyme catalyzed-reactions were 0.28 and 0.14, respectively. The findings support the view that the coupling of ATP hydrolysis to 5-oxoproline decyclization involves formation of a phosphorylated tetrahedal intermediate. Although the exchange phenomena are consistent with the mechanistic interpretations, they seem not to be required for catalysis.

Adenosine Diphosphate↗
PubMed1989

An appreciation of Professor Alexander E. Braunstein. The discovery and scope of enzymatic transamination.

Nonenzymatic transamination was discovered in the early 1930s. In the mid-1930s Braunstein and associates discovered the process of enzymatic transamination and established the biological significance of this reaction. Over the next 50 years, Braunstein and coworkers continued to contribute many new ideas and make important discoveries in the field of aminotransferases and other pyridoxal 5'-phosphate enzymes. This review outlines (1) the events leading to the discovery of enzymatic transamination, (2) how the discovery was made, (3) the findings that led to the recognition by the mid-1950s of the very wide scope and biological importance of aminotransferase reactions, and (4) the elucidation of the primary amino acid sequence and three-dimensional structure of aspartate aminotransferases.

Animals↗
PubMed1989

Glutathione metabolism in the lung: inhibition of its synthesis leads to lamellar body and mitochondrial defects.

Mice treated with buthionine sulfoximine, an inhibitor of glutathione synthesis, showed striking alterations of morphology of lung type 2 cell lamellar bodies (swelling and disintegration) and mitochondria (degeneration) and of lung capillary endothelial cells (mitochondrial swelling). These effects probably may be ascribed to glutathione deficiency; administration of glutathione monoester protects against them. Measurements of arteriovenous plasma glutathione levels across the lung indicate that the net uptake of glutathione by this organ is substantial. Thus, glutathione exported from the liver to the blood plasma is utilized by the lung which, like the liver, kidney, and lymphocytes (and unlike skeletal muscle), exhibits a high overall rate of glutathione turnover. Intraperitoneal injection of glutathione into buthionine sulfoximine-treated mice leads to very high levels of plasma glutathione without significant increase in the glutathione levels of liver, lung, and lymphocytes; on the other hand, administration of glutathione monoester leads to markedly increased tissue and mitochondrial levels of glutathione. Administration of glutathione monoester (in contrast to glutathione) to control mice also increases mitochondrial glutathione levels. The findings indicate that glutathione is required for mitochondrial integrity and that it probably also functions in the processing and storage of surfactant in lamellar bodies. The morphological changes observed after treatment with buthionine sulfoximine and their prevention by glutathione monoester as well as findings on glutathione metabolism indicate that this tripeptide plays an important role in the lung. The previously observed failure of buthionine sulfoximine-treated mice to gain weight is mainly due to glutathione deficiency in the intestinal mucosa.

Animals↗
PubMed1989

Mitochondrial damage in muscle occurs after marked depletion of glutathione and is prevented by giving glutathione monoester.

Skeletal muscle degeneration associated with mitochondrial damage was found after marked depletion of glutathione produced by administration to mice of buthionine sulfoximine, an irreversible inhibitor of gamma-glutamylcysteine synthetase. No mitochondrial damage was found in heart. These studies show that in the absence of applied stress (such as ischemia, drug toxicity), very marked depletion (to approximately 3% of the controls) of glutathione must occur before skeletal muscle mitochondria are affected and thus suggest that muscle has a large excess of glutathione. Depletion of glutathione followed a biphasic pattern in skeletal muscle and heart, probably reflecting, in the slow phase, loss of glutathione from mitochondria. Skeletal muscle degeneration did not occur when glutathione monoisopropyl ester was given together with buthionine sulfoximine; it did occur, however, when glutathione was given together with buthionine sulfoximine. Administration of the glutathione monoester (but not of glutathione) prevented the marked decline of mitochondrial glutathione produced by buthionine sulfoximine in skeletal muscle and increased the level of glutathione in heart mitochondria to values higher than the controls. The findings suggest that glutathione monoesters may be useful agents for protection of heart and skeletal muscle against toxicity.

Animals↗
PubMed1989

Glutathione ester prevents buthionine sulfoximine-induced cataracts and lens epithelial cell damage.

Treatment of newborn rats and mice with buthionine sulfoximine, an inhibitor of glutathione synthesis, leads to development of cataracts, which are not prevented by treatment with glutathione, but they are prevented by treatment with glutathione monoester. Cataracts are associated with glutathione deficiency in the lens epithelium, which undergoes severe degeneration. The findings indicate that glutathione normally functions in the protection of the lens and lens epithelium against oxidative injury, suggesting that procedures that increase lens glutathione levels might be useful for prevention of other types of cataracts. Relatively low doses of buthionine sulfoximine produce cataracts in newborn animals, and treatment of pregnant mice with buthionine sulfoximine during the last part of gestation leads to cataract formation in the offspring. The high sensitivity of the developing lens to the effects of glutathione deficiency suggests that this tissue may be a useful model for studies on glutathione function.

Animals↗
PubMed1989

Increased capacity for glutathione synthesis enhances resistance to radiation in Escherichia coli: a possible model for mammalian cell protection.

A strain of Escherichia coli, enriched in its content of gamma-glutamylcysteine synthetase and glutathione synthetase activities by recombinant DNA techniques, is more resistant to the lethal effects of gamma-irradiation than is the corresponding wild strain. Although the gene-enriched strain has higher glutathione levels than the wild strain, the observed radioresistance appears to be associated with the increased capacity of the gene-enriched strain to synthesize glutathione when irradiated rather than to the cellular levels of glutathione per se. Thus, resistance was abolished in the presence of buthionine sulfoximine, a selective inactivator of gamma-glutamylcysteine synthetase that decreases glutathione synthesis but that does not act directly to lower cellular glutathione levels. Conclusions drawn from studies on this E. coli model system may have relevance to protection of mammalian cells by glutathione.

Antimetabolites↗
PubMed1989

Spermatogenic cell-somatic cell interactions are required for maintenance of spermatogenic cell glutathione.

Sertoli cells play a major role in the regulation of spermatogenic cell energy metabolism and differentiation. This study demonstrates that Sertoli cells are essential for the maintenance of spermatogenic cell glutathione (GSH), an important intracellular reductant and detoxicant. Primary spermatocytes and round spermatids isolated from Xenopus laevis contained 1.5 +/- 0.1 mM GSH, but sperm lacked detectable GSH. During a 5-day culture period, isolated spermatocytes and spermatids lost 80% of the initial GSH (t 1/2 = 55 h). The levels of GSH were unaffected by L-buthionine-SR-sulfoximine (BSO), a selective inhibitor of GSH synthesis. Cultures of testicular lobules and spermatocysts (composed of germ cells and Sertoli cells) depleted of interstitial tissue lost only 30% of their initial GSH in 4.5 days; the GSH levels decreased during treatment with BSO. Spermatogenic cells in cultured testes maintained their GSH levels for 7 days by a BSO-sensitive mechanism. These results demonstrate that the intracellular GSH levels of spermatogenic cells are dependent upon germ cell-somatic cell interactions. Spermatogenic cells were shown to possess gamma-glutamyl transpeptidase, glutathione synthetase, 5-oxoprolinase, and gamma-glutamylcysteine synthetase activities. [35S] Cysteine incorporation and distribution as analyzed by high performance liquid chromatography (HPLC) showed that isolated spermatogenic cells are capable of GSH synthesis. The rate of GSH synthesis, however, was insufficient to compensate for GSH turnover. These results demonstrate that production of spermatogenic cell GSH is dependent upon Sertoli cells. To our knowledge, this is the first evidence that interactions between different cell types may be of significance in GSH metabolism.

Animals↗
PubMed1989

Glutathione metabolism at the blood-cerebrospinal fluid barrier.

Glutathione metabolism and transport in the choroid plexus were probed by determining the effects of administration to rats of several compounds (buthionine sulfoximine, L-2-oxothiazolidine-4-carboxylate, L-(alpha 5,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazole acetic acid, gamma-glutamyl alanine, and glutathione monoethyl ester) on the levels of glutathione and cysteine in the cerebrospinal fluid. The findings indicate that glutathione is actively metabolized in the choroid plexus by pathways similar to those in kidney and other tissues. The level of glutathione in the cerebrospinal fluid can be decreased or increased by giving compounds that do not, under similar conditions, appreciably alter total brain levels of glutathione. Glutathione monoethyl ester is effectively transported into the cerebrospinal fluid.

Animals↗
PubMed1989

Marked increase of cysteine levels in many regions of the brain after administration of 2-oxothiazolidine-4-carboxylate.

Although brain cysteine levels can be increased by administration of cysteine, treatment with this amino acid causes toxicity. L-2-Oxothiazolidine-4-carboxylate, a compound in which the thiol group is masked, is effectively transported into the mouse and rat brain. It is converted intracellularly by the action of 5-oxoprolinase into L-cysteine. Study of various regions of the rat brain (cerebellum, hypothalamus, cortex, brain stem, pons, caudate nucleus) showed that the levels of cysteine increased significantly after administration of L-2-oxothiazolidine-4-carboxylate. Glutathione levels were not increased or were only slightly increased under these conditions, reflecting the low rate of glutathione synthesis in many regions of the brain.

Animals↗
PubMed1989

[The effect of a high-frequency electromagnetic field (2.45 GHz) on perceptual processes, psychological performance and well-being].

In a sample of 8 male subjects the effect of an ultra-highfrequency electromagnetic field (f = 2.45 GHz) with power densities of 1,000, 500 and 100 microW/cm2 was investigated. As parameters were used the perceptional threshold in a tachistoscopic perceptional test, the total time from the numbersearch-test and the sum of acute symptoms and complaints measured by a special questionnaire. At the highest field intensity on the one side increases of the total times and of the sum of complaints occurred. The differential threshold increased distinctly under 500 microW/cm2 exposure, whereas the total times and the sum of symptoms were not affected. At the lowest power density no differences between exposure and nonexposure conditions have been found. The intraindividual variability was considerable.

Adult↗
PubMed1989

[Laboratory studies of vibration transmission in motor vehicle seats].

Operator seats (Models 050 and 500, Möve) were examined with vertical sinusoidal whole-body vibration (WBV) in the frequency range from 1 to 40 Hz (1.5 and 3.0 ms-2rms) and with simulated stochastic vibration of combines for the harvest of sugarcane. Six male subjects volunteered for the experiments. The WBV transmitted from the seat mounting base to the seat cushion and head were compared with data of a hard experimental seat. Interindividual differences cannot be explained by different body masses only. The iolation and main resonance of the two operator seats differed. Additional resonant peaks occurred above 6 Hz. They can cause unfavourable conditions at certain applications. The operator seats tested are probably suited for the reduction of WBV exposure on combines for the harvest of sugarcane.

Adult↗
PubMed1989

[Laboratory studies on the subjective evaluation of motor vehicle seats].

Operator seats (Models 050 and 500, Möve) were examined with vertical sinusoidal whole-body vibration (WBV) in the frequency range from 1 to 40 Hz (1.5 and 3.0 ms-2 rms) and with simulated stochastic vibration of combines for the harvest of sugarcane. Six male subjects volunteered for the experiments. The subjective assessments of operator seats by cross-modality matching, paired comparisons, and questionnaires were compared with data of a hard experimental seat. Transmitted WBV and subjective evaluation correlated partially only. Suggestions are derived for the improvement of the seats tested.

Adult↗
PubMed1989

[The NSI (Noise Sensitivity Index)--a method for the demonstration of acute physical symptoms in noise exposure and combination exposure].

The NSI (Noise Sensitive Index) is a method suitable for the single or repeated recording of acute, subjectively perceived bodily complaints caused by exposure against noise alone or combined with additional physical factors (e.g. physical load, high temperature) during the lapse of exposure. The paper describes the application of NSI in 6 experimental investigations. The results are indicative regarding their dependence on components of exposure and personality. If certain rules are taken into consideration, application, scoring and interpretation of NSI is easy, not very time consuming, and profitable.

Adult↗
PubMed1988

Interaction of the protein components of 5-oxoprolinase. Substrate-dependent enzyme complex formation.

5-Oxo-L-prolinase from Pseudomonas putida is composed of two reversibly dissociable proteins: Component A catalyzes 5-oxoproline-dependent cleavage of ATP, but does not catalyze the decyclization of 5-oxoproline; Component B is required for the coupling of ATP cleavage to ring-opening of 5-oxoproline to form glutamate (Seddon, A. P., Li, L., and Meister, A. (1984) J. Biol. Chem. 259, 8091-8094). We describe here the purifications of Components A and B to apparent homogeneity and the interactions between these two proteins. The cellular content of Component B activity is significantly greater than that of Component A. By gel filtration, Component A is a hexamer; but in the presence of substrates, it is a dimer. Component B can exist as an aggregate, an octamer, or a tetramer, depending upon the conditions used. Gel filtration of a mixture of Components A and B in the presence of substrates gives a unique protein species that exhibits 5-oxoprolinase activity. The Mr of this Component A-Component B complex indicates that it probably has an A2-B2 structure. The molar ratio of Component A to Component B in the complex was determined to be 1:1 by the continuous variation method (Job). Titrations of each component by the other suggest that phosphorylated 5-oxoproline-bound Component A is the entity that interacts with Component B. These studies indicate that the binding of phosphorylated 5-oxoproline-bound Component A to Component B to form a complex proceeds by a cooperative type mechanism. This is supported by the observed shifts of the intersection points of the Job curves (see Appendix).

Amidohydrolases↗
PubMed1988

Relationship between intracellular GSH levels and hypoxic cell radiosensitivity.

The present studies show the following: (a) The plating density of cells (V79 and GM3877) affects the experimentally determined OER values. (b) Fibroblasts depleted of cysteine and GSH are more sensitive to radiation under hypoxic conditions (under aerobic conditions the absence of cysteine or GSH has no detectable effect on radiation sensitivity). (c) Addition of GEE to V79 cells (previously depleted of GSH) leads to increased intracellular GSH levels and protects the cells against radiation under hypoxic conditions.

Animals↗