T-4 ACCUMULATION IN LYSOSOMES OF RAT-THYROID REMNANTS AFTER SUBTOTAL THYROIDECTOMY

In chronically stimulated rat thyroids after subtotal thyroidectomy, l ysosomes increased in number and volume. They contained iodocompounds and did not appear in iodine-deficient animals. In this study, we anal yzed the subcellular localization and the nature of these intracellula r iodocompounds. Classical subcellular fractions were isolated from ho mogenates of rat thyroids and remnants 14 weeks after sham-operation o r subtotal thyroidectomy. Two lysosome subpopulations of increasing de nsity, a light fraction, lysosomes 2 (L2, density 1.065-1.08 g/ml) and a dense fraction, lysosomes 1 (L1, density > 1.08 g/ml) were separate d from crude lysosomal particulate fractions (ML) by centrifugation in Percoll gradients. Results obtained with thyroids of normal rats were used as controls. In TSH-stimulated thyroid remnants, total activitie s of three lysosomal enzymes and iodine concentration were increased b y 1.6-fold compared with thyroids of sham-operated rats. Total iodopro tein-derived T-3 and T-4 concentrations, measured after pronase hydrol ysis, were slightly decreased. Thyroglobulin (Tg) concentration in the supernatant was reduced by 50 %. Iodine, T-3 and T-4 contents of Tg w ere not modified. After differential centrifugation, the iodine excess of remnants sedimented with subcellular particulate fractions. The co ncentration of iodine in dense lysosomes (L1) was 6 times that in sham L1. Intact Tg did not accumulate in L1. Two thirds of the iodine in L 1 was soluble in methanol, double the normal proportion, with twice as much iodine included in hydrophobic peptides eluted after T-4 by reve rse-phase HPLC. Although iodoprotein-derived T-4 and T-3 concentration s were decreased in the remnant homogenate, they were increased in par ticles, particularly in L1 where they were increased by 8 and 4-fold, respectively. In contrast, specific activities of lysosomal enzymes in ML and L1 remained unchanged. It is concluded that the chronic TSH st imulation of thyroid remnants in subthyroidectomized rats receiving a normal iodine supply induces the endocytosis of a normal Tg with iodin e kept in dense lysosomes. The expansion of the lysosomal compartment resulted from a limitation in iodopeptides degradation as though secon dary lysosomes would be overloaded with Tg. The accumulation in L1 of hydrophobic iodopeptides and of more iodoprotein-derived T-4 than T-3 suggests that exopeptidases involved in the liberation of T-4 become r ate-limiting.