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C Majumdar

Publications and source records attributed to C Majumdar.

15 recordsLinked to original sources
PubMed1997

Specific binding of inorganic mercury to Na(+)-K(+)-ATPase in rat liver plasma membrane and signal transduction.

Specific binding of Hg2+ to ouabain-sensitive Na(+)-K(+)-ATPase of rat liver plasma membrane was demonstrated with a Ka of 2.64 x 10(9) and Bmax of 1.6 nmole mg-1 protein. The binding of mercury to the enzyme also causes significant inhibition of the enzyme, which is greater than its ouabain sensitivity. In the cytosol Hg2+ binding to reduced glutathione (GSH) is stimulated by GSH-S-transferase (GST), the activity of which was found to be significantly enhanced by 15 mM Na+ and 10 mM Hg2+. It is proposed that the transport of Hg2+ inside the cell takes place by increased dissociation of Hg2+ from the membrane due to greater avidity of Hg2+ towards cytosolar GSH binding. The GSH-Hg complex enters the nucleus where it dissociates to bind the metal response element (MRE) of the metallothionein (MT) gene to induce MT transcription.

Animals↗
PubMed1989

Stepwise mechanism of HIV reverse transcriptase: primer function of phosphorothioate oligodeoxynucleotide.

Primer recognition by purified HIV reverse transcriptase has been investigated. Earlier we found that the reaction pathway for DNA synthesis is ordered, with template-primer and free enzyme combining to form the first complex in the reaction sequence (Majumdar et al., 1988). We now find that d(C)28 is a linear competitive inhibitor of DNA synthesis against poly[r(A)].oligo[d(T)] as template.primer, indicating that d(C)28 and the template.primer combine with the same form of the enzyme in the reaction scheme, i.e., the free enzyme. The phosphorothioate oligodeoxynucleotide Sd(C)28 also is a linear competitive inhibitor against template.primer. However, the Ki for inhibition (approximately 2.8 nM) is approximately 200-fold lower than the Ki for inhibition by d(C)28. Since the inhibition is linear competitive, the dissociation constant is equal to the Ki for inhibition. Filter binding assays confirmed high-affinity binding between Sd(C)28 and the enzyme and yielded a KD similar to the Ki for inhibition. Substrate kinetic studies of DNA synthesis using Sd(C)28 as primer, and poly[r(I)] as template, revealed that the Km for Sd(C)28 is 24 nM. The Km for this primer is, therefore, 8-fold higher than the KD for enzyme-primer binding (2.8 nM). These results enable calculation of real time rate values for the enzyme-primer association (kon = 5.7 x 10(8) M-1 s-1) and dissociation (koff = 1.6 s-1).

Binding, Competitive↗
PubMed1988

Studies on the mechanism of human immunodeficiency virus reverse transcriptase. Steady-state kinetics, processivity, and polynucleotide inhibition.

A study of steady-state kinetics of polymerization by purified human immunodeficiency virus DNA polymerase (reverse transcriptase) has been conducted. DNA synthesis was examined using a system of poly(rA) as template, oligo(dT) as primer, and dTTP as nucleotide substrate. The substrate initial velocity patterns point to an ordered mechanism with template-primer adding first. Product inhibition kinetics with either pyrophosphate or phosphonoformate are consistent with this mechanism. The human immunodeficiency virus reverse transcriptase acts processively in this replication system, but exhibits some probability of terminating after each dTMP addition to the nascent chain. The probability of terminating was approximately 20-fold higher after the first dTMP addition than after subsequent additions. With this information on the mode of polymerization, appropriate kinetic models and steady-state rate equations are discussed. In further studies, we found that a heterologous polynucleotide, poly(rC), is a potent inhibitor of the enzyme. The pattern of this inhibition is uncompetitive against template-primer, suggesting that interaction with free enzyme is not the mechanism of the inhibition.

Algorithms↗
PubMed1986

Recombinant murine retroviruses containing avian v-myc induce a wide spectrum of neoplasms in newborn mice.

NFS/N mice were infected within 48 hr of birth with pseudotypes of recombinant murine leukemia viruses containing avian v-myc developed T-cell, pre-B-cell, and B-cell lymphomas and epithelial tumors including pancreatic and mammary adenocarcinomas. Primary hematopoietic and epithelial tumors and continuous in vitro cell lines derived from some of these tumors, established in the absence of added growth factors, exhibited clonal integrations of v-myc and expressed v-myc RNA. These results show that deregulated expression of the myc oncogene in mammalian cells can initiate a wide variety of neoplasms.

Adenocarcinoma↗
PubMed1976

Altered properties of deoxyribonucleic acid polymerase I isolated from the membrane of bacteriophage T4-infected Escherichia coli.

Previous studies have shown that a large fraction of the host cell deoxyribonucleic acid (DNA) polymerase I (EC 2.7.7.7) becomes associated with the cell membrane shortly after infection with bacteriophages T4 and T7. The present investigation of the bound enzyme revealed that the polymerase activity can be eluted from the membrane with chelating agents, and that the material thus obtained shows many properties that distinguish it from purified DNA polymerase I. These include its chromatographic behavior, sedimentation rate, sensitivity to anti-DNA polymerase I antiserum, and activity with synthetic and natural DNA primers. Several of these physical and biological parameters were shown to revert slowly during storage to those exhibited by the purified enzyme. Efforts to determine whether the unusual properties of the membrane enzyme resulted from its association with DNA failed to support that possibility. These observations suggest that either the cause or the result of membrane binding of DNA polymerase I is a transient change in conformation or structure of the enzyme, with a resultant change in its enzymatic activity.

Cell Membrane↗
PubMedDate not supplied

Studies on the binding of the estradiol-receptor complex to rat DNA fragments.

The activation by estradiol of a series of reactions in hormone target tissues provides a model for the study of gene regulation in eukaryotic cells. We have examined the interaction of the estradiol--receptor complex with the various frequency classes of rat DNA to determine whether specificity can be detected at this level of chromosome organization. Interaction of the 8S estradiol--receptor complex with DNA was assayed by a change in the sedimentation rate of the complex. The receptor was found to bind to high molecular weight rat DNA as well as to 450 nucleotide long fragments of rat DNA. When the sheared DNA was separated by denaturation and reassociation into three frequency classes, the unique and moderately repeated sequences bound the receptor almost as effectively as the total sheared DNA. The resulting DNA--receptor complexes were sensitive to pancreatic DNAase. The highly repeated class of sequences bound the receptor less effectively than the other fractions. This did not result from the presence in this fraction of an inhibitor of binding, since addition of unique DNA fragments resulted in formation of a normal DNA--receptor complex. Nor was nuclease activity responsible since intact DNA could be isolated from the incubation mixture. An increase in length of the highly repeated DNA to 800 nucleotide long fragments caused them to bind the receptor almost as effectively as the other DNA fractions. These studies suggest that there may be sequences in eukaryotic DNA that preferentially bind the estradiol--receptor complex.

Animals↗