Subcellular Localization of Thyrocalcitonin1

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Abstract
Homogenates of rat thvroids were fractionated sequentially by differential contrifugation on 0.25[image] sucrose. Initial homogenates and isolated particulate and supernatant fractions were assayed biologically for thyrocalcitonin hypocalcemic activity using young intact male rats. In these assays, linear log dose-responses were obtained with doses of initial homogenate ranging from 0.05 to 0.4 of a single rat thyroid gland. Hypocalcemic responses parallel to those obtained with the homogenate were seen with the supernatant fraction after centrifugation at 8000 X g and with the particulate material recovered at 100,000 Xg. These 2 active fractions contained 78[image] 1.12 (SE) and 66[image] 1.20%, respectively, of the hypocalcemic activity found in the initial'' homogenate. Hypocalcemic activity was also found in the 8000 Xg supernatant and 100, 000 Xg particulate fraction obtained from porcine and calf thyroid tissue. It is concluded that thyrocalcitonin is found chiefly in the small particle ("microsomal") fraction of the thyroid gland.