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Biomedical subjects

A Saha

Publications and source records attributed to A Saha.

At least 109 records · Page 6Linked to original sources
PubMed1988

Modification of NAD-dependent isocitrate dehydrogenase by the 2',3'-dialdehyde derivatives of NAD, NADH, NADP, and NADPH.

The 2',3'-dialdehyde nicotinamide ribose derivatives of NAD (oNAD) and NADH (oNADH) have been prepared enzymatically from the corresponding 2',3'-dialdehyde analogs of NADP and NADPH. Pig heart NAD-dependent isocitrate dehydrogenase requires NAD as coenzyme but binds NADPH, as well as NADH, ADP, and ATP, at regulatory sites. Incubation of 1-3 mM oNAD or oNADH with this isocitrate dehydrogenase causes a time-dependent decrease in activity to a limiting value 40% that of the initial enzyme, suggesting that reaction does not occur at the catalytic coenzyme site. Upon varying the concentration of oNAD or oNADH from 0.2 to 3 mM, the inactivation rate constants increase in a nonlinear manner, consistent with reversible binding of oNAD and oNADH to the enzyme prior to covalent reaction. Inactivation is accompanied by incorporation of radioactive reagent with extrapolation to 0.54 mol [14C]oNAD or 0.45 mol [14C]oNADH/mol average enzyme subunit (or about 2 mol reagent/mol enzyme tetramer) when the enzyme is maximally inactivated; this value corresponds to the number of reversible binding sites for each of the natural ligands of isocitrate dehydrogenase. The protection against oNAD or oNADH inactivation by NADH, NADPH, and ADP (but not by isocitrate, NAD, or NADP) indicates that reaction occurs in the region of a nucleotide regulatory site. In contrast to the effects of oNAD and oNADH, oNADP and oNADPH cause total inactivation of the NAD-dependent isocitrate dehydrogenase, concomitant with incorporation, respectively, of about 3.5 mol [14C]oNADP or 1.3 mol [14C]oNADPH/mol average subunit. Reaction rates exhibit a linear dependence on [oNADP] or [oNADPH] and protection by natural ligands against inactivation is not striking. These results imply that oNADP and oNADPH are acting in this case as general chemical modifiers and indicate the importance of the free adenosine 2'-OH of oNAD and oNADH for specific labeling of the NAD-dependent isocitrate dehydrogenase. The new availability of 2',3'-dialdehyde nicotinamide ribose derivatives of NAD, NADH, NADP, and NADPH may allow selection of the appropriate reactive coenzyme analog for affinity labeling of a variety of dehydrogenases.

Adenosine Diphosphate↗
PubMed1982

Attempts to detect by physicochemical methods plasmid DNA in mycoplasmas of human origin before and after transformation to tetracycline resistance.

Physicochemical methods have been used to compare mycoplasma DNA capable of the genetic transformation of tetracycline resistance with DNA from tetracycline-sensitive mycoplasmas and their transformants. These mycoplasmas were isolated from human patients. The DNA extracted from Mycoplasma hominis tetr resistant to 100 microgram/mL tetracycline transforms tetracycline resistance to sensitive strains of Mycoplasma salivarium tets and Mycoplasma hominis tets but not Mycoplasma fermentans tets. Bulk DNA and DNA extracted by methods which increase the yield of circular DNA moieties were analyzed by cesium chloride and cesium chloride--ethidium bromide buoyant density ultracentrifugation and by horizontal and vertical agarose gel electrophoresis. Extrachromosomal DNA was not detected, which suggests that transformation was mediated by the recombination of chromosomal genes for tetracycline resistance and not by R factors. Moreover, no significant differences were detected in the DNA from the resistant and sensitive species or from their transformants and Mycoplasma fermentans tets which could not be transformed to resistance to 10 micrograms tetracycline/mL medium.

Centrifugation, Density Gradient↗
PubMed1982

Influence of heat on the conformational stability of a human IgG cryoglobulin.

Temperature-induced conformational changes of a human immunoglobulin G cryoglobulin (cryoIgG) (IgG) (gamma 1:lambda, Gm4) was investigated and compared with a human myeloma IgG (gamma 1:lambda, Gm4) employing spectrofluorimetric and immunochemical methods. Fluorescence measurements revealed the major changes in protein conformation of both proteins at a temperature of 62 degrees C and above, the measurements being carried out with excitation wavelengths at 278 and 295 nm, respectively. Studies on both cryoIgG and myeloma IgG, which were heat denatured at high temperatures and subsequently cooled at 25 degress C, indicated that both proteins underwent progressively irreversible conformational changes beyond 65 degress C and cryoIgG appeared to be more temperature sensitive than myeloma IgG. Evaluation of the changes on specific antigenic determinant sites using antigen-antibody interaction revealed that the Fc determinant sites of both the proteins were disorganized to a greater extent at a temperture of 68 degress C than Fd or lambda-chain antigenic determinant sites. The Fd determinant sites of myeloma IgG were, however, found to be more heat labile than those of cryoIgG.

Cryoglobulins↗
PubMed1979

Conformational stability of a human cryoglobulin.

Studies on a single component human cryoimmunoglobulin (cryo-IgG) (gamma 1 : lambda, Gm 4) were undertaken to gain a better understanding of the conformational stability of macromolecular interfaces essential for self-association of cryo-IgG leading to the formation of visible gel mass. Changes in the gross and localized conformation of cryo-IgG and a monoclonal IgG (gamma 1 : lambda, Gm 4) isolated from a myeloma patient (Hy) (Hy IgG) (gamma 1 : lambda, Gm 4) in alkaline media were determined by analytical ultracentrifugation, fluorescence characteristics, tyrosine ionization and H+ titration. Ultracentrifugal studies revealed that major transition in gross conformation took place at pH 11.4 for cryo-IgG and pH 11.7 for Hy IgG, whereby the number of charges and tyrosine residues exposed to aqueous environment was 110 and 26 for cryo-IgG, and 111 and 48 for Hy IgG, respectively. Beyond this transition pH fragmentation of both the proteins occurred and cryo-IgG lost its capacity for gel formation. Self-association of cryo-IgG was observed upto pH 11.4 in decreasing order with increase in denaturation pH. Cryo-IgG renatured from exposure to higher alkaline pH upto pH 11.4, showed the capability for forming gel, in spite of the irreversible local conformational changes as established by direct and reverse fluorimetric titration and tyrosine ionization studies. Cryo-IgG could be maintained in the optically clear sol phase at pH 10.5, at which pH 12 out of 62 tyrosine residues became exposed to aqueous media. There are distinct differences in the accessibility of tyrosine residues of cryo-IgG and Hy IgG as reflected in their tyrosine ionization profiles.

Cryoglobulins↗
PubMed1978

Study of "spontaneous" abortion in Thailand.

This analysis includes data on 3 530 patients with septic or incomplete, inevitable or threatened abortions who were treated at the Siriraj Hospital in Bangkok between January 1972 and December 1973. Data on the patients' socio-demographic characteristics; their reproductive, abortion, and medical histories; their pre- and postabortion contraceptive acceptance; and a clinical profile of their abortion treatment, including procedure time, length of hospitalization, and immediate and delayed postoperative complications are analyzed.

Abortion, Incomplete↗