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Biomedical subjects

A Datta

Publications and source records attributed to A Datta.

At least 163 records · Page 9Linked to original sources
PubMed1981

Removal of beta subunit of the eukaryotic polypeptide chain initiation factor 2 by limited proteolysis.

It is generally considered that the eukaryotic polypeptide chain initiation factor 2 (eIF-2) from rabbit reticulocytes consists of three nonidentical subunits termed alpha, beta, and gamma, in order of increasing molecular weight. However, a recent report [Stringer, E. A., Chaudhuri, A., Valenzuela, D. & Maitra, U. (1980) Proc. Natl. Acad. Sci. USA 77, 3356-3359] suggested that this factor is made up of only two subunits. In this paper we show that limited proteolysis of rabbit reticulocyte eIF-2 leads to loss of the beta subunit. This modified eIF-2 has the same activity as the native factor in promoting ternary (eIF-2.GTP.Met-tRNAi) and 40S (eIF-2.GTP.Met-tRNAi.40S ribosome) initiation complex formation. Like native eIF-2, the protease-treated factor can restore translation in heme-deficient lysates. On the other hand, the treated factor is less stable than the native protein.

Animals↗
PubMed1980

Inhibitory effect of glucose and adenosine 3',5'-monophosphate on the synthesis of inducible N-acetylglucosamine catabolic enzymes in yeast.

Glucose can block the utilization of N-acetylglucosamine in Saccharomyces cerevisiae, a facultative aerobe, but not in Candida albicans, an obligatory aerobe. Furthermore, glucose represses the synthesis of the enzymes of the N-acetylglucosamine catabolic pathway in S. cerevisiae, but not in C. albicans. The results suggest that catabolite repression is present in S. cerevisiae, but not in C. albicans. Cyclic AMP added to S. cerevisiae cells maintained in a glucose medium cannot bring about their release from catabolite repression. On the contrary, the synthesis of inducible enzymes of N-acetylglucosamine pathway was inhibited by cyclic AMP in both the yeasts. This seems to indicate that cyclic AMP can penetrate into the yeast cells. Furthermore, cyclic AMP inhibits protein synthesis, suggesting that protein synthesis in yeast is under cyclic AMP control.

Acetylglucosamine↗
PubMed1980

Purification and some properties of inducible N-acetylglucosamine kinase from Candida albicans.

N-Acetylglucosamine kinase (ATP:2-acetamido-2-deoxy-D-glucose 6-phosphotransferase, EC 2.7.1.59) catalyzes the first reaction in the inducible N-acetylglucosamine catabolic pathway of Candida albicans, an obligatory aerobic yeast. As a part of continuing biochemical studies concerning the regulation of gene expression in a simple eukaryote, N-acetylglucosamine kinase has been purified and characterized biochemically. The enzyme has been purified about 300-fold from the crude extract and its molecular weight of 75 000 has been determined by Sephadex G-100 gel filtration. Isolation and analysis procedures are described. The kinase reaction is optimal within a pH range of 7--8. The enzyme is strictly specific for GlcNAc as phosphate acceptor; ATP is the phosphoryl group donor for the kinase reaction and to a lesser extent dATP and CTP. Km values for GlcNAc and ATP are 1.33 mM and 1.82 mM, respectively. The enzyme required Mg2+, which may be replaced by other bivalent metal ions such as Mn2+, Ca2+, Ba2+ and Co2+ for a lesser degree of effectiveness. The purified enzyme is extremely sensitive to thermal denaturation and becomes completely inactive by heating at 65% C for 2 min. The enzyme is also inactivated by sulphydryl reagents such as p-chloromercuribenzene sulfonic acid and N-ethylmaleimide.

Candida albicans↗
PubMed1980

Inducible N-acetyglucosamine-binding protein in yeasts.

Addition of N-acetylglucosamine (GlcNAc) to the medium elicits an immediate synthesis of a specific GlcNAc-binding protein in yeasts. Synthesis of this protein requires the continuous presence of GlcNAc as the inducer and is inhibited completely by the inhibitors of ribonucleic acid and protein syntheses. Furthermore, this protein has been partially purified from GlcNAc-grown Candida albicans cells and is quite distinct from the other induced enzymes of the GlcNAc catabolic pathway. A good correlation between the level of GlcNAc-binding protein and GlcNAc uptake capacity of the cells during induction was observed. Some of the sugars, e.g., N-acetylmannosamine, N-acetylgalactosamine, and glucose, had a similar competitive effect on the binding of GlcNAc as well as on its uptake. Furthermore, both the binding and uptake activities were sensitive to sulfhydryl reagents.

Acetylglucosamine↗
PubMed1979

Regulation of N-acetylglucosamine uptake in yeast.

Various yeasts have been investigated for their ability to grow on N-acetylglucosamine as the sole carbon source and only those which are associated with the disease, candidiasis, gave positive results. The yeasts unable to grow on N-acetylglucosamine lacked the capacity to transport the aminosugar across the cell membrane. In pathogenic yeasts, two systems of different affinity for substrate were found to operate in the uptake of N-acetylglucosamine. In glucose-grown cells a constitutive, low affinity uptake system was present, but upon addition of inducer, a specific high affinity uptake system was synthesized. Experiments with the inhibitors of macromolecule synthesis suggested that the synthesis of RNA and protein is necessary for induction whereas the synthesis of DNA is not. In glucose-grown Candida albicans cells which are devoid of N-acetylglucosamine enters into the cells as phosphorylated form using a constitutive uptake system. Uranyl acetate (0.01 mM) which binds to cell membrane-associated polyphosphates, inhibited completely the inducible uptake of N-acetylglucosamine. Labelling experiments, designed to determine the temporal sequence of appearance of N-acetylglucosamine in intracellular free sugar and sugar-phosphate pools, indicated that N-acetylglucosamine first appeared in the cells as pohosphorylated form. Similar results were obtained with Saccharomyces phosphorylated form. Similar results were obtained with Saccharomyces cerevisiae 3059 and some other yeasts which are devoid of N-acetylglucosamine kinase in both uninduced and induced conditions. These results are consistent with the model of van Steveninck that involves phosphorylation during transpost. Furthermore, inhibitors of energy metabolism (arsenate, azide and cyanide), proton conductor (m-chlorocarbonylcyanide phenylhydrazine) and dibenzyl diammonium ion (membrane permeable cation) inhibited the inducible N-acetylglucosamine uptake in C. albicans.

Acetylglucosamine↗
PubMed1979

Induction of N-acetylmannosamine catabolic pathway in yeast.

N-Acetylmannosamine kinase activity is absent from yeast cells grown on N-acetylmannosamine. However, other enzymes of the catabolic pathway, namely, N-acetylmannosamine-2-epimerase, N-acetylglucosamine kinase and glucosamine-6-phosphate deaminase are induced. In addition, a high affinity uptake system (permease) for the uptake of N-acetylglucosamine is synthesized under these conditions. The presence of either N-acetylmannosamine or N-acetylglucosamine as inducer is essential for the induced synthesis of these enzymes. The enzyme synthesis stops and their concentration in the cells declines rapidly as soon as inducer is removed from the medium. N-Acetyl-D-galactosamine can also induce all these enzymes except for N-acetylmannosamine-2-epimerase, suggesting the convergence of catabolic pathways for both the aminosugars at N-acetyl-D-glycosamine. Experiments with inhibitors of macromolecule synthesis suggest that the snythesis of RNA and protein is necessary for the induction of these cyzymes whereas the synthesis of DNA is not.

Acetylgalactosamine↗
PubMed1979

Regulation of glucosamine-6-phosphate deaminase synthesis in yeast.

A basal level of glucosamine-6-phosphate deaminase is detected in yeast cells grown on glucose. However, a burst of enzyme production occurs in the presence of N-acetylglucosamine in pathogenic Candida albicans and non-pathogenic Saccharomyces cervisiae. The enzyme synthesis stops and its concentration in the cells declines rapidly as soon as N-acetylglucosamine is removed from the medium. Experiments with RNA- and protein-synthesis inhibitors indicate that the appearance of new enzyme activity is dependent on concomitant new protein synthesis and the inducer operates at a transcriptional level. However, inhibition of DNA synthesis either by hydroxyurea or by mitomycin-C does not impair the synthesis of glucosamine-6-phosphate deaminase.

Acetylglucosamine↗
PubMed1979

Induction of N-acetylglucosamine-catabolic pathway in spheroplasts of Candida albicans.

Synthesis of N-acetylglucosamine-catabolic enzymes, namely permease (high-affinity uptake system), kinase and deaminase was studied in the spheroplasts of the yeast Candida albicans. The presence of N-acetylglucosamine as inducer is essential for the induced synthesis of these enzymes in the spheroplasts, which were active for at least 8--9 h. However, some of the newly synthesized kinase and deaminase leaked out from the spheroplasts into the medium during induction. Experiments with inhibitors of RNA and protein synthesis indicate that the appearance of new enzyme activities is dependent on concomitant new protein synthesis and the inducer operates at a transcriptional level. However, inhibitors of DNA synthesis, e.g. mitomycin-C and hydroxyurea, had no effect on the synthesis of these enzymes.

Acetylglucosamine↗
PubMed1978

Mode of action of the hemin-controlled inhibitor of protein synthesis.

Despite the finding that the hemin-controlled translational inhibitor in reticulocyte lysates is a cyclic AMP-independent protein kinase that phosphorylates the small subunit of the initiation factor eIF-2, the mechanism of inhibition of translation remained unexplained. Whereas treatment of hemin-containing lysates with inhibitor in the presence of ATP inhibited translation, the same treatment of highly purified eIF-2 did not affect its ability to form a ternary complex with initiator Met-tRNA and GTP or a 40S initiation complex. We have isolated from ribosomal salt washes a protein (eIF-2 stimulating protein) that enhances the capacity of unphosphorylated eIF-2 to form ternary or 40S initiation complexes but has no effect on the phosphorylated factor. At low concentrations, eIF-2 is virtually inactive without this stimulating protein. Therefore, the translational inhibitor acts by converting eIF-2 to a form that is not stimulated by the stimulating protein.

Heme↗
PubMed1978

Translational control by hemin is due to binding to cyclic AMP-dependent protein kinase.

Our previous work [Proc. Natl, Acad. Sci. USA (1977) 74, 1463-1467, 3326-3329] is consistent with the view that (a) the hemin-controlled inhibitor of protein synthesis in reticulocyte lysates (active eIF-2 kinase) is formed by phosphorylation of proinhibitor (inactive eIF-2 kinase) catalyzed by cyclic AMP-dependent protein kinase (ATP-protein phosphotransferase; EC 2.7.1.37), and (b) hemin prevents this conversion by blocking the interaction of cyclic AMP with the kinase's regulation subunit, thereby rendering the enzyme inactive. We now show that hemin blocks cyclic AMP binding because it itself binds specifically to the regulatory subunit. This binding is noncompetitive with respect to cyclic AMP. Whereas unlabeled hemin can displace bound [3H]hemin as well as cyclic [3H]AMP, unlabeled cyclic AMP can displace bound cyclic [3H]AMP but not [3H]hemin. This suggests that cyclic AMP and hemin bind to different sites on the protein and that hemin binding affects cyclic AMP binding in an allosteric manner.

Allosteric Regulation↗