Action of thyrotropin on phosphate incorporation into thyroid proteins in vitro.

Abstract

The kinetics of [32P]phosphate incorporation into dog thyroid slices, in vitro, has been studied. Phosphate penetration was saturable which suggests a mediated transport. This transport appeared to be the limiting step in the slow incorporation of [32P]phosphate in the trichloroacetic‐acid‐soluble fraction of the slices. The incorporation of [32P]phosphate in thyroid proteins was a fast process (half equilibrium with the trichloroacetic‐acid‐soluble fraction radioactivity in 30 min). About 50%, of this incorporation took place in the nuclear proteins, 25%, of it in the histones (mostly in the arginine‐rich histones). The turnover of protein P was fast in all classes of proteins.Thyrotropin, in vitro, often enhanced the incorporation of [32P]phosphate in the trichloro‐acetic‐acid‐soluble fraction and into total proteins of the slices. However, this effect was delayed and rather small. It is ascribed mainly to an enhancement of phosphate penetration into the cells. This suggests, that, in dog thyroid cells, most of protein phosphorylation is independent of thyrotropin and cyclic AMP.Thyrotropin markedly enhanced the phosphorylation of F1 (lysine‐rich) histones. This effect was fast, specific and mimicked by dibutyryl cyclic AMP; it required higher concentrations of thyrotropin than the activation of iodide binding to proteins. The role of this action of thyrotropin is discussed; the hypothesis is proposed that it is related to thyrotropin‐induced growth of thyroid tissue.NaF (5 mM) enhanced total protein phosphorylation in the thyroid slices. This effect, which may be due to unspecific inhibition of protein phosphatases, may explain the fact that NaF mimics several effects of thyrotropin.