Abstract
The deiodination of L-MIT-125I was measured in rat thyroid homogenates and slices before and after acute TSH stimulation.
Slices and homogenates were incubated with identical concentrations of tissue and
substrate in the presence and absence of NADPH. 1 USP unit TSH added in vitro to thyroid
slices failed to stimulate deiodination; a single in vivo ip injection of 3 USP units
TSH was also unable to raise deiodinating activity. In contrast to TSH, NADPH added
to homogenates and slices enhanced deiodination significantly. However, several arguments,
including a review of the literature, strongly militate against the hypothesis of
an increased intracellular concentration of the coenzyme NADPH being the prerequisite
to enhanced deiodination.
The results suggest that deiodinase activity in acutely stimulated thyroids is not
limited by the intracellular concentration of the enzyme itself nor by the availability
of coenzyme. Therefore, the increased iodide release induced by acute TSH stimulation
is a mere consequence of the enhanced thyroglobulin proteolysis and does not require
higher enzyme concentration. It will be shown subsequently that a different conclusion
must be drawn in experiments with chronic TSH stimulation.
Key words
Deiodinase Activity - Iodotyrosines - TSH - NADPH Concentration - Thyroid Slices -
Thyroid Homogenates
1 This work was supported by a grant from the Schweizerischer Nationalfonds zur Förderung
der Wissenschaftlichen Forschung
