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Biomedical subjects

S Mitra

Publications and source records attributed to S Mitra.

At least 271 records · Page 15Linked to original sources
PubMed1988

Rapid, large-scale purification and characterization of 'Ada protein' (O6 methylguanine-DNA methyltransferase) of E. coli.

The E. coli Ada protein (O6-methylguanine-DNA methyltransferase) has been purified using a high-level expression vector with a yield of about 3 mg per liter of E. coli culture. The 39-kDa protein has an extinction coefficient (E280 nm (1%)) of 5.3. Its isoelectric point of 7.1 is lower than that predicted from the amino acid content. The homogeneous Ada protein is fully active as a methyl acceptor from O6-methylguanine in DNA. Its reaction with O6-methylguanine in a synthetic DNA has a second-order rate constant of 1.1 x 10(9) M-1 min-1 at O degree C. Both the native form and the protein methylated at Cys-69 are monomeric. The CD spectrum suggests a low alpha-helical content and the radius of gyration of 23 A indicates a compact, globular shape. The middle region of the protein is sensitive to a variety of proteases, including an endogenous activity in E. coli, suggesting that the protein is composed of N-terminal and C-terminal domains connected by a hinge region. E. coli B has a higher level of this protease than does K12.

Circular Dichroism↗
PubMed1988

High-level expression of the cloned ada gene of Escherichia coli by deletion of its regulatory sequence.

The Ada protein, a methyltransferase for repair of several alkyl adducts in DNA, was expressed in its native form at a high level in Escherichia coli from a pUC9 recombinant plasmid carrying ada gene from which the sequence controlling the Ada induction was deleted. The regulatory sequence appears to act as a terminator of transcription initiated from the lac promoter of the vector. However, deletion of the regulatory sequence resulted in elimination of ada induction by alkylating agents, providing confirmation of its role in activation of ada expression.

Alkylating Agents↗
PubMed1988

Effect of polyvalent metal ions on the reactivity of human O6-methylguanine-DNA methyltransferase.

Polyvalent metal ions are highly effective in inhibiting human O6-methylguanine-DNA methyltransferase, the repair protein responsible for the removal of the promutagenic and presumably procarcinogenic adduct, O6-alkylguanine, in DNA. The sulfhydryl group-reacting metal ions (Cd2+, Zn2+, Hg2+, Pb2+) completely inhibited the reaction at concentrations of 100-500 microM while other metal ions (Al3+, Fe3+) required concentrations of 1 mM or greater for significant reduction of the reaction rate. Inhibition by the former group of metals could be reversed by dithiothreitol but not by EDTA, while the opposite was true for the second group. Under conditions of partial inhibition of the initial reaction rate by either Hg2+ or Al3+, the extent of reaction was not significantly affected, indicating reversible binding of these ions.

Cations↗
PubMed1988

On "mortality comparisons" by Krishnamoorthy.

"This short note deals with Krishnamoorthy's interpretation of the cumulative values of the force of mortality function and his measure of improvement in life expectancy if everyone's life is saved once or more than once."

Demography↗
PubMed1988

A generalized stable model with fluctuating vital rates.

The amenability of the stable population model to certain situations where net maternity function need not be invariant over time is exemplified by relating the changes in fertility at 2 time points by the equation m(a,t) = m(a) h(a,t)/h(t) where the form of h(t) is general and can be determined based on observed birth trajectories. Using the extended form of Easterlin's hypothesis, h(t) is taken as the reciprocal of NRR at t. The model is then applied to project NRR.

Birth Rate↗
PubMed1987

Function of Lys-166 of Rhodospirillum rubrum ribulosebisphosphate carboxylase/oxygenase as examined by site-directed mutagenesis.

Affinity labeling and comparative sequence analyses have placed Lys-166 of ribulosebisphosphate carboxylase/oxygenase from Rhodospirillum rubrum at the active site. The unusual nucleophilicity and acidity of the epsilon-amino group of Lys 166 (pKa = 7.9) suggest its involvement in catalysis, perhaps as the base that enolizes ribulosebisphosphate (Hartman, F.C., Milanez, S., and Lee, E.H. (1985) J. Biol. Chem. 260, 13968-13975). In attempts to clarify the role of Lys-166 of the carboxylase, we have used site-directed mutagenesis to replace this lysyl residue with glycine, alanine, serine, glutamine, arginine, cysteine, or histidine. All seven of these mutant proteins, purified by immunoaffinity chromatography, are severely deficient in carboxylase activity; the serine mutant, which is the most active, has a kcat only 0.2% that of the wild-type enzyme. Although low, the carboxylase activity displayed by some of the mutant proteins proves that Lys-166 is not required for substrate binding and argues that the detrimental effects brought about by amino acid substitutions at position 166 do not reflect gross conformational changes. As demonstrated by their ability to tightly bind a transition-state analogue (2-carboxyarabinitol 1,5-bisphosphate) in the presence of CO2 and Mg2+, some of the mutant proteins undergo the carbamylation reaction that is required for activation of the wild-type enzyme. Since Lys-166 is required neither for activation (i.e. carbamylation by CO2) nor for substrate binding, it must be essential to catalysis. When viewed within the context of previous related studies, the results of site-directed mutagenesis are entirely consistent with Lys-166 functioning as the base that initiates catalysis by abstracting the C-3 proton from ribulosebisphosphate. An alternative possibility that Lys-166 acts to stabilize a transition state in the reaction pathway cannot be rigorously excluded.

Amino Acid Sequence↗
PubMed1987

Visual dysfunction in Parkinson's disease. Loss in spatiotemporal contrast sensitivity.

Flicker sensitivity and spatial contrast sensitivity (CS) were examined in a total of 99 patients with Parkinson's disease (PD). All patients were undergoing treatment with dopaminergic agents. Specific losses in sensitivity observed in PD were (1) a loss in flicker sensitivity which was most pronounced around the peak of the function (8 Hz) and (2) a loss near the peak of the spatial CS curve, often with no noticeable low frequency attenuation. Several PD patients affected by the 'on-off' syndrome were tested in both 'on' and 'off' phases, and the results show that the CS function switches in parallel with motor symptoms of the disease. These data suggest that not only is the visual system affected in PD, but that dopamine may have an essential role in receptive field organization in human vision.

Adult↗
PubMed1987

Models of birth trajectories with certain patterns of variations in vital rates.

This article records experiments with a modified version of Lotka's stable population model. The traditional assumption of a set of unchanging net maternity rates as one of the preconditions has been replaced by allowing the rates to vary in certain ways while approaching or varying around a set of underlying constant rates. The integral equation assumes a different form but can be simplified and solved in the same old way. As a result, the birth trajectory does not necessarily attain a stable state. For certain patterns of mathematical functions the trajectory eventually becomes stable while for certain others it shows cyclical fluctuations. The variations in net maternity rates can also be postulated to follow the patterns of some measurable empirical functions which, like the mathematical functions, should be amenable to projections in the future. This information is needed for deriving the long-term solutions in additions to the information about the intrinsic rate of growth which is obtained from the traditional integral equation.

Birth Rate↗
PubMed1987

Weighted least square estimates of the parameters of a model of survivorship probabilities.

"A weighted regression has been fitted to estimate the parameters of a model involving functions of survivorship probability and age. Earlier, the parameters were estimated by the method of ordinary least squares and the results were very encouraging. However, a multiple regression equation passing through the origin has been found appropriate for the present model from statistical consideration. Fortunately, this method, while methodologically more sophisticated, has a slight edge over the former as evidenced by the respective measures of reproducibility in the model and actual life tables selected for this study."

Age Factors↗