The role of reduced glutathione in thyroglobulin proteolysis iv vitro.

During the process of secretion of the thyroid hormones, stored thyroglobulin (Tg) is digested within the lysosomes. An acid protease has been found, but this enzyme hydrolyses Tg at a relatively slow rate and has a low pH optimum (3.6). Since previous work has shown a stimulatory effect of reduced glutathione (GSH) on Tg digestion the following studies have been performed. Homogenates were obtained from dog thyroid and total homogenate or different subcellular fractions were used as enzyme source. Labelled thyroglobulin (125-I) was purufied from prelabelled dog thyroid and its hydrolysis, judged from the increase in butanol soluble radioactivity, was studied. The greatest stimulatory effect of GSH on Tg hydrolysis was found around pH 5.6. When butanol soluble radioactivity was considered as a function of incubation time and tissue concentration a linear relationship was found. GSH effect was evident at 2 mM. Subcellular distribution studies showed that the GSH-stimulated proteolytic activity was mainly found in the 15 000 times g pellet. Labelled Tg hydrolysis was progressively decreased with increasing amounts of non-labelled purified Tg. GSH also stimulated labelled insulin hydrolysis, but failed to alter haemoglobin or casein degradation. GSH could also be replaced by other reducing agents, like cysteine or dithiothreitol. The significance of these findings is discussed in relation to the process of thyroid hormone secretion.