Reduced proteolytic activity in vitro of lysosomes isolated from "cold" thyroid nodule.

The proteolytic activity of lysosomes isolated from solitary "cold" thyroid nodule and morphologically normal perinodular thyroid tissue was examined in parallel in an in vitro system using 125I-labelled rat Tg as substrate. Lysosomes were isolated by centrifugation of the tissue homogenates between 800 and 20,000 x g. The optimal proteolytic activity of acid proteases from human thyroid tissue was observed between pH 3.6 and 4.8. The evident differences in the proteolytic activity of lysosomal acid proteases between nodular and perinodular tissues were observed. It was found that the activity of lysosomal proteases from "cold" nodule in both the same amount of wet weight thyroid tissue and the same amount of lysosomal proteins was significantly lower (29-54%) than in the morphologically normal perinodular tissue used as a control. The reduced proteolytic activity of lysosomes from patients with "cold" nodule provides further confirmation on the low metabolic activity of thyrocytes in non-functioning thyroid nodules.