A fast photoisomerization method for the preparation of tritium-labeled 9-cis-retinoic acid of high specific activity.

A method is described by which tritium-labeled all-trans retinoic acid of high specific activity (up to approximately 51 Ci/mmol corresponding to 85% of theoretical) is converted photolytically within a fraction of a second to a mixture of retinoic acid stereoisomers. One of these isomers, 9-cis-retinoic acid, was obtained in high radiochemical purity by reverse-phase HPLC of the stereoisomer mixture. This fast photolysis was obtained by using a high-pressure 100-W mercury lamp operated at 86 +/- 2 W.

Thyroid hormone binding to isolated human apolipoproteins A-II, C-I, C-II, and C-III: homology in thyroxine binding sites.

Thyroid hormone binding to lipid-free apolipoprotein (apo) A-II, C-I, C-II, and C-III isolated from human plasma was investigated by photoaffinity labeling with [125I]T4 and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Both the monomeric and polymeric forms were specifically labeled. Inhibition by 10 microM unlabeled L-T4 was > or = 50%, suggesting affinity constants in the nM to microM range; the least inhibition was seen with apoA-II.

Characterization of thyroid hormone binding to apolipoprotein-E: localization of the binding site in the exon 3-coded domain.

Apolipoprotein-E (apoE) has been shown by noncovalent binding and photoaffinity labeling with [125I]T4 to possess a single L-T4 binding site with a K5 of about 3 x 10(7) M-1 and a relative affinity for analogs of L-T4 = D-T4 = rT3 = triiodothyroacetic acid > L-T3. T4 binding was not affected by the flavonoid EMD 21388 or heparin, but was inhibited by diclofenac = mefenamic acid > furosemide. Localization of the T4 site to the N-terminal 62-amino acid region of the mature peptide coded by exon 3 was deduced from the following evidence.

5'- and 5-deiodinase activities in adult rat cecum and large bowel contents inhibited by intestinal microflora.

Enzymatic mechanisms for deiodination of 3,5,3'-triiodothyronine or thyroxine in the phenolic ring (5'-deiodinase) or tyrosyl ring (5-deiodinase) are found in cells of many organs, including the intestinal wall. Deiodinases are highly active in intestinal tissue of developing rat fetuses and relatively inactive in adult intestinal cells, but little is known about these systems in the luminal contents of intestines. We have found both 5- and 5'-deiodinase activities in adult rat intestinal contents and have shown that their expression is inhibited by resident intestinal microflora, which are normally present in the adult but not in the fetus, possibly because they are bound by intestinal bacteria in the adult.