The effect of varying iodine content on the proteolytic activity of rat thyroid lysosomes

Abstract

Abstract. The possibility that the iodine supply modulates thyroid lysosomal activity was investigated in rats receiving chronic or acute increasing doses of iodide. The lysosomal activity or the various experimental groups was determined by measuring the activity of β-glycerophosphatase and cathepsin D both in thyroid homogenates and in semipurified thyroid lysosomal preparations, and the degradation of labelled thyroglobulin by the various lysosomal fractions. In the chronic experiments β-glycerophosphatase and cathepsin D activities increased with the iodide supply of the animals up to 100 μg I/rat and decreased slightly thereafter. In the acute experiments the activity of these enzymes increased up to 1000 μg I/rat and decreased above 5000 μg I/rat. The proteolytic activity of lysosomal fractions from the various experimental groups towards thyroglobulin decreased slightly with increased iodide supply both in chronic and acute experiments. The results suggest that thyroid lysosomal activity may participate in the autoregulation of thyroid secretion by inducting synthesis of new enzymes and modulating thyroglobulin degradation.