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PubMed1992

The cDNA for the type I iodothyronine 5'-deiodinase encodes an enzyme manifesting both high Km and low Km activity. Evidence that rat liver and kidney contain a single enzyme which converts thyroxine to 3,5,3'-triiodothyronine.

The enzymatic conversion of thyroxine (T4) to 3,5,3-triiodothyronine (T3) by iodothyronine 5'-deiodinase(s) is an obligate step in the physiologic action of thyroid hormones in most extrathyroidal tissues. In the rat liver and kidney, 5'-deiodinase processes having either high Km (micromolar range) or low Km (nanomolar range) values for thyroid hormone substrates have been described. The number of enzymes mediating these reactions, however, remains uncertain and controversial. To examine this question we have compared the 5'-deiodinase activity expressed in membrane preparations of Xenopus laevis oocytes after the injection of either rat liver poly(A)+ RNA or in vitro prepared RNA transcribed using the G21 full-length type I 5'-deiodinase cDNA. In oocytes injected with rat liver poly(A)+ RNA, high Km (i.e. type I) activity was observed when 20 mM dithiothreitol was used as the thiol cofactor, whereas Km values in the nanomolar range were noted with 0.5 mM dithiothreitol, glutathione, or a reconstituted thioredoxin cofactor system. This complex pattern of 5'-deiodinase activity, which mimics that found in homogenates and subcellular fractions of rat liver and kidney, was reproduced exactly in oocytes by the microinjection of G21-derived RNA transcripts. Furthermore, hybrid arrest of translation in oocytes using a partial type I 5'-deiodinase cDNA completely inhibited the expression of both high and low Km activity after the injection of rat liver poly(A)+ RNA. These findings demonstrate that rat liver and kidney contain only a single 5'-deiodinase which manifests either high or low Km activity depending on the reduced thiol cofactor utilized in the reaction.

Animals↗
PubMedDate not supplied

Biliary excretion of iopanoate glucuronide by the rat.

Biliary excretion, plasma decay, and tissue distribution of 125I after a single intravenous dose (13 mumol/kg) of 125I-Iopanoate and 125I-iopanoate glucuronide were compared in control and phenobarbital-pretreated rats under either anesthesia. In control animals the plasma decay of iopanoate glucuronide following iopanoate glucuronide administration was significantly greater than that after iopanoate administration. Biliary clearance following iopanoate glucuronide was approximately 20 times greater than that after iopanoate; within 60 min, 82% of the dose of iopanoate glucuronide, compared to 23% of the dose of iopanoate, was recovered in bile. Phenobarbital pretreatment significantly increased the biliary excretion following iopanoate administration but had no effect on that following iopanoate glucuronide. The urinary excretion after both agents was less than 3% of the administered dose within the 60-min experimental period. The data suggest that intracellular metabolism and/or binding may account for the marked difference in the biliary excretion of iopanoate glucuronide following iopanoate as compared to that after administration of iopanoate glucuronide.

Animals↗
PubMed1989

Gastro-duodenal retention of Telepaque--a useful sign in oral cholecystography.

With the resurgence of oral cholecystography for assessing gallbladder function prior to new treatment methods, it is important to be aware of the possibility of gastro-duodenal retention of contrast medium, and its implications. Six patients who had retained Telepaque in the stomach or duodenum at oral cholecystography are described here. In each patient there was underlying pathology, including gastritis, peptic ulceration and neoplasm. The presence of retained contrast medium in the stomach or duodenum is a useful indication of disease in these areas.

Adult↗
PubMedDate not supplied

Competition for binding to multiple sites of human serum albumin for cholecystographic agents and sulfobromophthalein.

The binding of two cholecystographic agents, iophenoxate and iopanoate, to human serum albumin was studied with 11 putative competitors; the results were qualitatively consistent with competitive binding to common sites. A more precise analysis of competition was achieved with four pairs of compounds for which the free and bound concentration of each was determined. The results were analyzed by a computer program and the dissociation constants calculated for both binder and competitor at specified sites on albumin. With numbering based on the rank order of dissociation constants for iophenoxate, the highest binding of the four compounds occurs at different sites: iophenoxate at site I; iopanoate at site II, sulfobromophthalein at site III; and bromphenol blue at site II. For a given compound, there is close agreement in the calculated affinities at different sites regardless of the competitor.

Binding Sites↗
PubMed1988

Evidence for a large and flexible region of human serum albumin possessing high affinity binding sites for salicylate, warfarin, and other ligands.

The relations between the single high affinity binding sites for azapropazone, phenylbutazone, chlorpropamide, sulfathiazole, and iophenoxate and the binding regions of human serum albumin represented by the marker ligands diazepam, phenol red, salicylate, and warfarin were examined by a series of competition experiments. Binding was determined by equilibrium dialysis at pH 7.0. In order to ensure an accurate analysis of the competition experiment, the number of moles of ligand bound per mole of protein was usually 0.4 or less to minimize ligand binding to weaker sites. Furthermore, binding of both ligands was determined in all experiments (except for iophenozate). None of the test ligands competed with diazepam for a common high affinity binding site, but, surprisingly, they were all able to displace two or three of the other marker ligands according to a competitive scheme. These findings show, first, the existence of a particular serum albumin region for high affinity binding of diazepam. Secondly, they imply that it is not necessary to assume the existence of new drug binding regions beyond those existing for phenol red, salicylate, and warfarin. On the contrary, the relatively many examples of competitive binding indicate that the binding regions represented by the last-mentioned three marker ligands are placed quite close to each other in the albumin molecule in a common region, which is suggested to be located at subdomains 1C and 2A-B. The region must be relatively large, because in some cases independent high affinity binding of pairs of ligands is observed. It is probably also rather flexible, inasmuch as no clear relation could be found between the chemical structure of the test ligands and the two or three marker ligands with which they compete. Correlations between primary association constants and partition coefficients for both marker ligands and test ligands, in the unionized forms, between n-hexane or 1-octanol and aqueous media showed that hydrophobic forces are important for the binding processes. However, the data also showed that other attractive forces must be operative as well.

Apazone↗