Search PubMed⌕ Search

Biomedical subjects

S Mitra

Publications and source records attributed to S Mitra.

At least 253 records · Page 14Linked to original sources
PubMed1990

Isolation and structural characterization of a cDNA clone encoding the human DNA repair protein for O6-alkylguanine.

O6-Methylguanine-DNA methyltransferase (MGMT; DNA-O6-methylguanine:protein-L-cysteine S-methyltransferase, EC 2.1.1.63), a unique DNA repair protein present in most organisms, removes the carcinogenic and mutagenic adduct O6-alkylguanine from DNA by stoichiometrically accepting the alkyl group on a cysteine residue in a suicide reaction. The mammalian protein is highly regulated in both somatic and germ-line cells. In addition, the toxicity of certain alkylating drugs in tumor and normal cells is inversely related to the levels of this protein. The cDNA of the human gene, henceforth named MGMT, has been cloned in an expression vector on the basis of its rescue of a methyltransferase-deficient (ada-) Escherichia coli host. A 22-kDa active methyltransferase encoded entirely by the cDNA contains an amino acid sequence of 61 residues that bears 60-65% similarity with segments of E. coli methyltransferase (products of the ada and ogt genes), which encompass the alkyl-acceptor residues. The human cDNA has no sequence similarity with the ada and ogt genes, due in part to differences in codon usage, and shows no detectable homology with E. coli genomic DNA. However, it hybridizes with distinct restriction fragments of human, mouse, and rat DNAs. The lack of methyltransferase observed in many human cell lines is due to the absence of the MGMT gene or to lack of synthesis and/or stability of its 0.95-kilobase poly(A)+ RNA transcript.

Amino Acid Sequence↗
PubMed1990

In vivo measurement of total body carbon using 238Pu/Be neutron sources.

Total body carbon has been measured by in vivo neutron activation analysis (IVNAA) in 278 surgical gastroenterological patients and 29 normal volunteers. This is based on the inelastic scattering reaction [12C (n,n') 12C*] for neutrons with energy above 4.8 MeV, producing 4.43 MeV gamma rays. Since only part of the body is scanned, total body carbon is estimated as the ratio of the gamma ray emission from carbon to the emission from hydrogen, using hydrogen as the internal standard. The precision of the estimate is +/- 1.6 kg for a whole body dose of 0.3 mSv. There is a significant difference between the estimates of total body water from IVNAA measurements of carbon and nitrogen and measurements of body water in these subjects by tritium dilution (t = 3.1, p less than 0.005).

Adipose Tissue↗
PubMed1990

Long term demographic effect of a constant stream of immigration when the population is not reproducing itself.

The author describes the use of a model to investigate "the age composition of the stationary population resulting from a condition of below replacement fertility and a continuous stream of immigrants, the size of which for any age remain invariant over time.... We have shown that the number dying among [migrants] is the same as the number of them entering over any time interval, and that is precisely how their stationarity is maintained." The geographical focus is on developed countries that are experiencing below-replacement fertility levels. (SUMMARY IN FRE AND SPA)

Age Distribution↗
PubMed1990

A birth model with oscillating rate of growth.

"Integral equations similar to those generated by the assumption of unchanging vital rates in a closed population leading to eventual stability can be obtained by allowing the rates to vary according to some prescribed rules. In spite of these changing patterns of the vital rates, some of these models have earlier been found to approach stability where the solutions of the stable parameters can be obtained by following the usual straightforward methods. In this paper, a similar integral equation with changing vital rates has been presented which can also be solved in a similar manner. However, the resulting rate of growth does not stabilize but continues to oscillate. The period and amplitude of this oscillating rate of growth together with its central value can be determined from the specified pattern of variation of the vital rates."

Demography↗
PubMed1990

Complex roots of Lotka's integral equation for a special model of net maternity rates.

"An attempt to obtain the solutions of Lotka's integral equation has been made in this paper by assuming a Pearsonian Type III function for the age distribution of the net maternity rates. The shifting of the origin of the function from the traditional value of age zero to the most meaningful lower boundary of the reproductive interval gave rise to an analytical expression of the equation with interesting properties most of which have not yet been encountered in similar endeavors."

Age Factors↗
PubMed1990

C-reactive protein (CRP) in haemolymph of a mollusc, Achatina fulica Bowdich.

C-reactive protein (CRP) was found to be one of the major components of the haemolymph of A. fulica with a normal level of approximately 2 mg CRP/ml haemolymph. Immunological cross-reactivity was found between CRP from A. fulica, a mollusc and Limulus polyphemus, an arthropod. The present finding demonstrates the evolutionary significance of C-reactive protein.

Animals↗
PubMed1990

Immigration, below-replacement fertility, and long-term national population trends.

The long-term demographic effects of immigration on a population experiencing below-replacement fertility are studied by assuming that the size and age composition of the immigrant population do not change over time. The size of the first-generation immigrant population becomes stationary within a time period not greater than the human life span. Thereafter, the number dying equals the number entering over any given time interval. The stationarity of the native population, among which deaths exceed births, is maintained by the compensating number of births to the immigrant population. The limiting age distribution of the country's population, although stationary, may not decline monotonically with age and may look like a camel's back, with one or two humps.

Demography↗
PubMed1989

Interaction of thiocyanate with horseradish peroxidase. 1H and 15N nuclear magnetic resonance studies.

Interaction of thiocyanate with horseradish peroxidase (HRP) was investigated by relaxation rate measurements (at 50.68 MHz) of the 15N resonance of thiocyanate nitrogen and by following the hyperfine shifted ring methyl proton resonances (at 500 MHz) of the heme group of SCN-.HRP solutions. At pH 4.0, the apparent dissociation constant (KD) for thiocyanate binding to HRP was deduced to be 158 mM from the relaxation rate measurements. Chemical shift changes of 1- and 8-ring methyl proton resonances in the presence of various amounts of thiocyanate at pH 4.0 yielded KD values of 166 and 136 mM, respectively. From the pH dependence of KD and the 15N resonance line width, it was observed that thiocyanate binds to HRP only under acidic conditions (pH less than 6). The binding was found to be facilitated by protonation of an acid group on the enzyme with pKa 4.0. The pH dependence of the 15N line width as well as the apparent dissociation constant were quantitatively analyzed on the basis of a reaction scheme in which thiocyanate in deprotonated ionic form binds to the enzyme in protonated acidic form. The KD for thiocyanate binding to HRP was also evaluated in the presence of an excess of exogenous substrates such as resorcinol, cyanide, and iodide ions. It was found that the presence of cyanide (which binds to heme iron at the sixth coordination position) and resorcinol did not have any effect on the binding of thiocyanate, indicating that the binding site of the thiocyanate ion is located away from the ferric center as well as from the aromatic donor binding site. The KD in the presence of iodide, however, showed that iodide competes with thiocyanate for binding at the same site. The distance of the bound thiocyanate ion from the ferric center was deduced from the 15N relaxation time measurements and was found to be a 6.8 A. From the distance as well as the change in the chemical shifts and line width of 1- and 8-methyl proton resonances, it is suggested that the binding site of thiocyanate may be located near heme, placed symmetrically with respect to 1- and 8-methyl groups of the heme of HRP. Similarity in the modes of binding of iodide and thiocyanate suggests that the oxidation of thiocyanate ion by H2O2 may also proceed via the two-electron transfer pathway under acidic conditions, as is the case for iodide.

Horseradish Peroxidase↗
PubMed1989

Binding of aromatic donor molecules to lactoperoxidase: proton NMR and optical difference spectroscopic studies.

The interaction of aromatic donor molecules with lactoperoxidase (LPO) was studied using 1H-NMR and optical difference spectroscopy techniques. pH dependence of substrate proton resonance line-widths indicated that the binding was facilitated by protonation of an amino acid residue (with pKa of 6.1) which is presumably a distal histidine. Dissociation constants evaluated from both optical difference spectroscopy and 1H-NMR relaxation measurements were found to be an order of magnitude larger than those for binding to horse radish peroxidase (HRP), indicating relatively weak binding of the donors to LPO. The dissociation constants evaluated in presence of excess of I- and SCN- showed a considerable increase in their values, indicating that the iodide and thiocyanate ions compete for binding at the same site. The dissociation constant of the substrate binding was, however, not affected by cyanide binding to the ferric centre of LPO. All these results indicate that the organic substrates bind to LPO away from the ferric center. Comparison of the dissociation constants between the different substrates suggested that hydrogen bonding of the donors with the distal histidine amino acid, and hydrophobic interaction between the donors and the active site contribute significantly towards the associating forces. Free energy, entropy and enthalpy changes associated with the LPO-substrate equilibrium have been evaluated. These thermodynamic parameters were found to be all negative and relatively low compared to those for binding to HRP. The distances of the substrate protons from the ferric center were found to be in the range 9.4-11.1 A which are 2-3 A larger than those reported for the HRP-substrate complexes. These structural informations suggest that the heme in LPO may be more deeply buried in the heme crevice than that in the HRP.

Aniline Compounds↗
PubMed1989

Binding of thiocyanate to lactoperoxidase: 1H and 15N nuclear magnetic resonance studies.

The binding of thiocyanate to lactoperoxidase (LPO) has been investigated by 1H and 15N NMR spectroscopy. 1H NMR of LPO shows that the major broad heme methyl proton resonance at about 61 ppm is shifted upfield by addition of the thiocyanate, indicating binding of the thiocyanate to the enzyme. The pH dependence of line width of 15N resonance of SC15N- in the presence of the enzyme has revealed that the binding of the thiocyanate to the enzyme is facilitated by protonation of an ionizable group (with pKa of 6.4), which is presumably distal histidine. Dissociation constants (KD) of SC15N-/LPO, SC15N-/LPO/I-, and SC15N-/LPO/CN- equilibria have been determined by 15N T1 measurements and found to be 90 +/- 5, 173 +/- 20, and 83 +/- 6 mM, respectively. On the basis of these values of KD, it is suggested that the iodide ion inhibits the binding of the thiocyanate but cyanide ion does not. The thiocyanate is shown to bind at the same site of LPO as iodide does, but the binding is considerably weaker and is away from the ferric ion. The distance of 15N of the bound thiocyanate ion from the iron is determined to be 7.2 +/- 0.2 A from the 15N T1 measurements.

Animals↗
PubMed1989

Age-dependent modulation of tissue-specific repair activity for 3-methyladenine and O6-methylguanine in DNA in inbred mice.

3-Methyladenine-DNA N-glycosylase (MAG) and O6-methylguanine-DNA methyltransferase (MGMT) activities were assayed in liver, lungs, brain and ovaries of female mice of two inbred stocks, C3Hf and C57BL/E, as a function of age. In addition to differences in the enzyme levels between the two stocks for each organ, the suckling animals (9-day-old) have consistently lower levels of both MAG and MGMT than young adults (7- or 8-week-old). While the MGMT levels in adults did not decrease with age, the MAG levels in 15- to 17-month-old animals were, in general, significantly lower than those in young adults. These results raise the possibility that the older animals are at a higher risk than young adults following exposure to alkylating mutagens.

Age Factors↗
PubMed1989

A simple calibration of a shadow shield counter for the measurement of total body potassium in critically ill patients.

A shadow shield counter has been built for the determination of total body potassium (TBK), based on the detection of the 1.46 MeV gamma rays emitted by the naturally occurring radioisotope (0.01%) 40K. Simple calibration procedures have been adopted which do not require administration of 42K to the subject. Self-absorption effects of gamma rays have been taken into account by monitoring the activity of 40K per gram of potassium as a function of weight. All activity measurements, either during calibration or of a subject, include background in the spectral band covering the full width at half maximum (FWHM) of the 40K photopeak. The overall precision for determining the TBK was in the range of 1.8-3.0%.

Calibration↗
PubMed1989

Site-directed mutation of the Escherichia coli ada gene: effects of substitution of methyl acceptor cysteine-321 by histidine in Ada protein.

Oligodeoxynucleotide-mediated mutagenesis of the ada gene of Escherichia coli was used to produce two mutant Ada proteins. In mutant I the methyl acceptor Cys-321 for O6-methylguanine was replaced by histidine; and in mutant II the positions of Cys-321 and His-322 of the wild-type protein were inverted. Neither mutant protein had O6-methylguanine-DNA methyltransferase activity, but both retained the phosphotriester-DNA methyltransferase activity involving methyl group transfer to Cys-69. Under the control of the endogenous promoter, synthesis of mutant I protein was undetectable before or after adaptation treatment with promoter, synthesis of mutant I protein was undetectable before or after adaptation treatment with N-methyl-N'-nitro-N-nitrosoguanidine. This appeared to be due to both inhibition of transcription of the mutant gene and degradation of the synthesized protein. On the other hand, mutant II protein was inducible by N-methyl-N'-nitro-N-nitrosoguanidine, although to a smaller extent than the wild-type protein was, and the phosphotriester-DNA methyltransferase activity appeared to reside in 24- to 30-kilodalton cleavage products. Mutant I protein could be produced under lac promoter control, and its cleavage products, unlike those of mutant II protein, tended to aggregate. These results indicate that (i) Cys-321 cannot be replaced or transposed with the nucleophilic amino acid histidine for O6-methylguanine-DNA methyltransferase function, (ii) single amino acid replacement or transposition at the O6-methylguanine methyl acceptor site can have a profound effect on the in vivo stability and regulatory function of the Ada protein, and (iii) the integrity of the protein may not be absolutely needed for its transcription-activation function.

Amino Acid Sequence↗
PubMed1989

Effects of amino acids on colony phenotype of Neisseria gonorrhoeae.

The effects of amino acid supplementation of commercially available semidefined GC media (Oxoid GC medium and Difco GC base) on the stability of the virulent colony phenotype of Neisseria gonorrhoeae were studied. The -SH-containing amino acids, together with glycine, were found to cause segregation of avirulent colony types followed by growth inhibition after addition of 40 mg/100 ml. Proline and arginine were found to change the colony phenotype profile significantly without reducing the viable count. Arginine supplementation in both media caused a progressive loss of virulent colony types, whereas proline had the opposite effect.

Amino Acids↗
PubMed1989

O6-alkylguanine-DNA alkyltransferase and sensitivity to procarbazine in human brain-tumor xenografts.

The level of O6-alkylguanine-deoxyribonucleic acid (DNA) alkyltransferase (AT) was determined in 15 human brain-tumor xenografts in athymic mice. This enzyme is a primary intracellular repair mechanism for lesions produced at the O6 position of guanine by a wide range of alkylating agents, including nitrosoureas and procarbazine. Its activity ranged from undetectable in five tumor lines to 2338 fmol/mg protein in N-1941, a human glioblastoma xenograft. The sensitivity of 10 of these xenografts to procarbazine was determined and it was found that four of the five tumor lines with AT levels of more than 100 fmol/mg protein had growth delays after procarbazine treatment of less than 20 days, whereas all five lines with undetectable AT levels had growth delays of over 30 days. The primary cytotoxic DNA adduct produced by procarbazine (namely, O6-methylguanine) was found to be significantly higher in two sensitive lines with low AT levels than in a highly resistant line with a high AT level. These data suggest that the AT levels of individual brain tumors can be used as predictive indicators of their susceptibility to drugs that exert their antineoplastic effect primarily by O6-alkylation of guanine in nuclear DNA.

Animals↗
PubMed1989

Some ways of incorporating both sexes in models that become stable.

The author presents "four models of birth trajectories in which the female age-specific fertility rates have been assumed to depend on the relative age compositions of both sexes. In each case, the rates fluctuate but approach their limiting values and the age compositions become stable."

Age Distribution↗