Enzymatic Digestion of Desiccated Thyroid

Abstract

In the course of studies on the calorigenic activity of various fractions derived from the enzymatic digestion of desiccated thyroid, 1 we have observed that the organic iodine in the gland is rapidly split into acid soluble and acid insoluble iodine fractions. The 2 fractions are separated by adjusting the pH to 5.0.

A single large lot of desiccated thyroid was used for these experiments (Lot No. 2∗). The desiccated thyroid was insoluble at pH 5 and contained 2% of the total iodine in the form of “preformed inorganic”iodine as determined by the method of Lawson. 2 When like volumes of enzyme digest were precipitated in dilutions of one, 2 and 4 volumes, the ratio of acid soluble iodine to total iodine was not increased, indicating that the acid insoluble iodine fraction has a negligible solubility at pH 5. The iodine soluble at pH 5 consisted of organic iodine digested off in the acid soluble form and a small amount of “preformed inorganic”iodine. It was thus possible to measure the rate of hydrolysis of desiccated thyroid into, acid soluble and acid insoluble iodine fractions, by measuring the ratio of acid soluble to total iodine in the digest.

Incubation of the desiccated thyroid in one and 2% pepsin at pH 2 resulted in a liberation of 55% of the total iodine in an acid soluble form in 4 hours. (The acid soluble fraction is not increased after several months incubation in pepsin solutions.) In one and 2% trypsin at pH 8, 60% of the total iodine was rendered acid soluble in 4 hours. With each enzyme, about 70% of the total hydrolysis into acid soluble and acid insoluble iodine fractions occurred during the first 15 minutes of digestion.