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Biomedical subjects

A Goswami

Publications and source records attributed to A Goswami.

At least 73 records · Page 4Linked to original sources
PubMed1988

Effects of glutathione on iodothyronine 5'-deiodinase activity.

At nanomolar substrate levels, physiological concentrations (less than or equal to 5 mM) of glutathione (GSH) activate a low Km iodothyronine 5'-deiodinase (I-5'D) activity in renal and hepatic microsomes, but not the low Km (type II) I-5'D in the pituitary, cerebral cortex, or brown adipose tissue. The latter enzyme as well as the type I enzyme activity at micromolar substrate concentrations required higher (greater than 10 mM) concentrations of GSH. However, GSH appeared to interact with the type I and type II enzymes even at subactivation levels, since it inhibited the activation of these enzymes by dithiothreitol (DTT). Activation of the renal and hepatic low Km I-5'D by GSH resembled that by DTT in 1) the similarity of Km values for both T4 (20 nM) and rT3 (2 nM), 2) the catalytic mechanism (ordered sequential with the iodothyronine as the second substrate), 3) the values for activation energies, and 4) sensitivity to inhibition by propylthiouracil. However, the low Km I-5'D activated by GSH was about 10-fold more sensitive to inhibition by iopanoate than when activated by DTT. The responsiveness of the low Km I-5'D's in renal and hepatic microsomes to physiological concentrations of GSH suggests their participation in the metabolism of iodothyronines in vivo.

Animals↗
PubMed1988

Iodothyronine 5'-deiodination in rat kidney microsomes: sensitivity to propylthiouracil.

When activated by dithiothreitol, iodothyronine 5'-deiodinase (I-5'D) activity in kidney microsomes is less sensitive to inhibition by propylthiouracil (PTU) and iopanoate (IOP) at nanomolar, compared to micromolar, substrate concentrations. The enzymatic activities at nanomolar substrate concentrations are, however, completely eliminated in the presence of a combination of 10 microM IOP and 100 microM PTU. In this report we present evidence that 1) the relative PTU insensitivity results from the residual activities of the high Km enzyme which, while being very sensitive to PTU inhibition at micromolar substrate concentrations, becomes progressively less PTU sensitive as substrate concentrations decline relative to its Km; and 2) the relative IOP insensitivity is due to the presence in kidney microsomes of a low Km enzyme which is relatively insensitive to IOP, but highly sensitive to inhibition by PTU. Classifying the deiodinases on the basis of PTU sensitivity, therefore, requires that not only the thiol concentrations, but, as in the case of the type I enzyme, also the substrate concentrations be specified. The PTU resistance of the type I enzyme at nanomolar substrate concentrations suggests a role of this enzyme in T3 neogenesis in PTU-treated animals.

Animals↗
PubMed1987

Microbial hydroxylation of 1,4-cineole.

Microorganisms were examined for their potential to hydroxylate the oxygenated monoterpene 1,4-cineole. Using gas chromatography and thin-layer chromatography, screening experiments revealed that hydroxylation at position 2 was the most commonly observed microbial transformation reaction. In most microorganisms, the predominant alcohol metabolite was the 2-endo-alcohol isomer. Preparative-scale incubations were conducted in order to isolate and characterize microbial transformation products by comparison of proton nuclear magnetic resonance, mass spectrometry, and chromatography profiles with those of cineole standards. Streptomyces griseus yielded 8-hydroxy-1,4-cineole as the major hydroxylation product together with 2-exo- and 2-endo-hydroxy-1,4-cineoles.

Journal Article↗
PubMed1987

Thioredoxin stimulates enzymatic outer ring monodeiodination of reverse triiodothronine.

Thioredoxin (Thd) and NADPH-Thd reductase, purified to near homogeneity from rat liver cytosol, stimulated, in the presence of NADPH, the 5'-monodeiodination of rT3 by renal and hepatic microsomes at nanomolar, but not micromolar, substrate concentrations. T4 was not deiodinated at either concentration. Reduced Thd was effective at physiological concentrations in stimulating microsomal rT3 deiodination (EC50, approximately 15 microM); Thd-supported microsomal deiodination showed a maximum velocity approximately one third that in the presence of dithiothreitol (DTT), and Thd-supported deiodination, compared to that with DTT, was 10- and 2000-fold more sensitive to inhibition by propylthiouracil and iopanoate, respectively, than was the DTT-supported reaction. The Michaelis constants for rT3 (2.5 nM) were identical for the Thd- and DTT-activated reactions, suggesting that these thiols stimulated deiodination by the same enzyme. Arrhenius plots also revealed comparable activation energies for Thd- and DTT-mediated low Km rT3 monodeiodination; these activation energies were, moreover, distinct from those observed with the low Km T4 deiodination in the presence of DTT. The data suggest that renal and hepatic microsomes contain separate low Km rT3-specific and T4-specific 5'-monodeiodinases and that the rT3-specific monodeiodinase can use the reducing potential of NADPH, via the Thd system. Such an enzyme could mediate the disposal of rT3, the noncalorigenic metabolite of T4, independently of the conversion of T4 to T3.

Animals↗
PubMed1986

Anthropometric analysis of tricycle designs.

A comparison of the designs of two types of tricycles available in India for the use of persons with severe both-lower-limb disability was made from the viewpoint of the anthropometric characteristics of the users It was noted that neither design was compatible with the disabled persons' anthropometric dimensions in a number of ways. The seats did not provide for comfortable sitting. Although the hand crank was relatively well positioned in one, the distance of the crank from the back rest obliged the users to bend forward while driving their tricycles. The study concluded that suitable modifications in the dimensions of the tricycles are required to provide greater comfort, safety and ease of operation.

Journal Article↗
PubMed1986

Iodothyronine 5'-deiodinase in brown adipose tissue: thiol activation and propylthiouracil inhibition.

Brown adipose tissue (BAT) of hypothyroid rats contains a low Km (type II) iodothyronine 5'-deiodinase (I-5'D) that has been characterized as being insensitive to inhibition by propylthiouracil (PTU), based mainly on observations with homogenates prepared in a medium containing 10 mM dithiothreitol (DTT) and enzymatic assays in the presence of 20 mM DTT in vitro. In the studies reported herein, BAT homogenates from hypothyroid rats prepared in a DTT-free medium were found to contain I-5'D activity at 20 mM DTT, comparable to that in homogenates prepared in a DTT-containing medium, and were activated by submillimolar concentrations of DTT with an EC50 of approximately 0.5 mM. Almost all of the homogenate activities could be accounted for in microsomal preparations. The activity was substantially inhibited by 1 mM PTU. The PTU inhibition was progressively alleviated with increasing concentrations of added DTT and was not seen at DTT concentrations higher than 10 mM. At 250 microM DTT, the Km and maximum velocity values for rT3 and T4 were 2.9 and 1 nM and 70 and 200 fmol/mg protein X h, respectively, with a Ki for PTU of approximately 200 microM. On administration of PTU in vivo (2 mg/100 g BW; 1 h before killing) and subsequent assay at 250 microM DTT, the I-5'D in the homogenates was about 50% inhibited, and the microsomes showed a state of persistent inhibition, with activity levels about 70% of the control value. The data show that BAT type II I-5'D can be substantially activated at submillimolar concentrations of DTT, and this activation is sensitive to inhibition by PTU administered both in vitro and in vivo.

Adipose Tissue, Brown↗
PubMed1985

Purification and characterization of a cytosolic protein enhancing GSH-dependent microsomal iodothyronine 5'-monodeiodination.

A protein has been purified from rat liver cytosol which promoted GSH-responsive iodothyronine 5'-deiodinase activities in rat kidney microsomes. The factor behaved as a basic protein with an Mr of 11,000. It was active as a GSH-disulfide transhydrogenase with beta-hydroxyethyl disulfide as an acceptor and was also active in stimulating calf thymus ribonucleotide reductase with one-third the potency of native calf thymus glutaredoxin. Another basic protein, which degraded iodothyronines oxidatively, was also identified in the cytosolic preparations; this co-purified with soluble protein factor in the earlier purification stages and was partially separated from this factor by CM-cellulose chromatography. The glutaredoxin-like protein present in rat liver and kidney cytosol could provide a physiologic regulatory mechanism for GSH-dependent 5'-monodeiodination of iodothyronines.

Animals↗
PubMed1985

Effects of thyroid status on membrane-bound low Km cyclic nucleotide phosphodiesterase activities in rat adipocytes.

Adipocyte membranes from hypothyroid rats showed increased low Km cAMP phosphodiesterase activity compared to normals, provided that the subcellular fractionations were done in isotonic, as opposed to hypotonic, buffers. The enhanced cAMP phosphodiesterase activity in hypothyroid membranes was nearly normalized by incubation with a 10-fold excess of cGMP. Preincubation of hypothyroid adipocytes with cGMP also restored to normal the blunted lipolytic response to micromolar concentrations of epinephrine. DEAE-Sephacel chromatography of detergent-solubilized membrane-bound cAMP phosphodiesterase showed a 2.5-fold enhancement in hypothyroid membranes of a form of the enzyme that was completely inhibited by cGMP; the enzymatic elution profiles of the soluble fractions showed no difference between normal and hypothyroid fat pads. The results suggest a possible regulatory role of cGMP in adipocytes in the hypothyroid state.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1984

Iodothyronine 5'-deiodinase in rat kidney microsomes. Kinetic behavior at low substrate concentrations.

The thiol-activated enzymatic outer-ring monodeiodination of iodothyronines by rat kidney microsomes at low (nanomolar) substrate concentrations shows an apparently sequential reaction mechanism and is further characterized by insensitivity to inhibition by dicoumarol, a moderate sensitivity to inhibition by propylthiouracil (Ki = 100 microM) and iopanoic acid (Ki = 0.9 mM), responsiveness to 5 mM glutathione (GSH), and a thermal activation profile that is concave downward with a Td of approximately 20 degrees C. In contrast, the activity at high (micromolar) substrate concentrations shows a ping-pong reaction mechanism, is inhibited by micromolar concentrations of propylthiouracil, iopanoic acid and dicoumarol, is unresponsive to 5 mM GSH, and shows a concave upward thermal activation profile. Analysis of the microsomal deiodinase reaction over a wide range of 3,3',5'-triiodothyronine (rT3) concentrations (0.1 nM to 10 microM) suggested the presence of two enzymatic activities, with apparent Michaelis constants (Km) of 0.5 microM and 2.5 nM. Lineweaver-Burk plots of reaction velocities at nanomolar substrate concentrations in presence of 100 microM propylthiouracil also revealed an operationally distinct enzymatic activity with Km's of 2.5 and 0.63 nM and maximum velocities (Vmax's) of 16 and 0.58 pmol/mg protein per h for rT3 and thyroxine (T4), respectively. These findings are consistent with the presence of a low Km iodothyronine 5'-deiodinase in rat kidney microsomes distinct from the well characterized high Km enzyme and suggest that at circulating levels of free T4 the postulated low Km enzyme could be physiologically important.

Animals↗