Kenneth Sterling

Conversion of Thyroxine to Triiodothyronine in Normal Human Subjects

The conversion of thyroxine to triiodothyronine, previously demonstrated in athyreotic human subjects, has been investigated in normal subjects who were given intravenous injections of purified thyroxine labeled with carbon-14 in ring A and in the alanine side chain. Evidence for the conversion of T4 to T3 was provided by the finding of carbon-14 in the T3 fraction isolated from serums. It is estimated that an appreciable fraction of T4 may be transformed to T3 in normal man.

Conversion of thyroxine to triiodothyronine by cultured human cells.

Human liver and kidney cells convert 6 to 10 percent of added thyroxine to triiodothyronine in vitro at 37�C. This extent of conversion is ten times greater than that in control studies with killed cells. Conversion is evident within 10 minutes and appears to be maximal within 1 hour. Greater net triiodothyronine formation results if greater amounts of exogenous thyroxine are added to the system, with no plateau evident even at very high thyroxine concentrations. The addition of high concentrations of nonradioactive triiodothyronine resulted in no evident inhibition of the conversion.

Mitochondrial binding of triiodothyronine (T3)

SummaryTo assess the distribution of the thyroid hormone triiodothyronine (T3) within intact living cells, freshly prepared dispersed rat hepatocytes were incubated with [125I]-T3 for periods of 5 min and 30 min. Lightand electron-microscopic (EM) radioautography was carried out to determine the distribution of grains over the isolated cells. Both procedures showed the grains distributed almost entirely over the cytoplasmic matrix rather than the nucleus. Grain counts under the EM were compared with expectation based on established quantitative methods. Only the mitochondria showed obvious and statistically significant grain counts, whereas the nucleus failed to accumulate grains in excess of expectations by chance alone based on area. The findings support the existence of mitochondrial binding of T3, presumably a prerequisite for its action in direct stimulation of the mitochondria.

Thyroid Hormone Action: Early Calorigenic Effect on Dispersed rat Liver Cells in the Absence of Protein Synthesis

Isolated dispersed rat liver cells were prepared by hypothyroid Sprague-Dawley rats. The cells were incubated under 95% O2/5% CO2 in Krebs-Ringer-bicarbonate buffer at pH 7.3-7.4 at 37 degrees C. The medium had been enriched with 2% bovine serum albumin (previously stripped of thyroid hormone) and 5-10 mM alanine as substrate. Two hour incubations were carried out with or without added triiodothyronine (T3) at 3 nM or 300-1,000 nM concentrations. Oxygen consumption determined at the end of the period of incubation with the Clark oxygen electrode showed stimulation above control values in the hormone treated flasks; parallel studies in which cycloheximide (100 microM) had been added to cells to block protein synthesis also showed enhanced oxygen consumption in response to T3. The results indicated a response to the hormone not dependent on new protein formation.

Direct Thyroid Hormone Stimulation of Mitochondrial Oxidative Phosphorylation: The Role of Adenine Nucleotide Translocase (AdNT)

Since the first reports 30 years ago of direct thyroid hormone action on target cell mitochondria (1–3), Bronk observed that the oxidative capacity of rat liver mitochondria was markedly reduced by thyroidectomy, and substantially restored within three hours after triiodothyronine (T3) injection in studies on the electron transport system (4,5) with no change whatever in coupling of oxidative phosphorylation; that is, phosphorus/ oxygen (P/0) ratios remained constant.