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Iodoamino acid synthesis in thyroid lobes in vitro with excellent yield of iodothyronines.

Thyroid lobes of male Sprague-Dawley rats on an iodine-sufficient diet were incubated in our improved in vitro system with 0.01 microM 127I and 5mU/ml of bovine TSH. Thyroidal 131I-uptake and the relative incorporation of iodine into iodothyronines increased with time. The average yield of each iodoamino acid after 8 h of incubation was: monoiodotyrosine 28.0%, diiodotyrosine 46.5%, triiodothyronine 1.9% and thyroxine 13.9%, which showed a striking resemblance to the values obtained in vivo. The yields of iodotyrosines and iodothyronines, the latter in particular, were strikingly high, and this system is considered to be useful in the study of thyroidal iodine metabolism. Effects of TSH, temperature and pH of the medium were examined and a unique effect of pH was observed on the iodoamino acid synthesis. As the pH was elevated from 6.8 to 7.9, 131I-uptake, MIT/DIT ratio, T4/DIT ratio and T3/T4 ratio increased. The effect of slightly alkaline pH was considered to be similar to that observed in iodine-deficiency. It was found that the rubber stoppers which are commonly used in short-term incubation contain a kind of potent inhibitor of thyroid hormone synthesis. The pattern of inhibition was similar to that of thionamide compounds.