Abstract
An inhibitor of alkaline (pH 7.4) ribonucleases is present in the rat thyroid. The properties of rat thyroid and rat liver inhibitors are similar in that both inhibit bovine pancreatic ribonuclease (ribonucleate pyrimidine nucleotido-2′-transferase(cyclizing), EC 2.7.7.16), both have similar pH optima, are inactivated by heat and sulfhydryl blocking agents, and appear in similar fractions following density gradient centrifugation. Measures designed to stimulate thyroid function such as thiouracil administration, low iodine diet, or thyroid-stimulating hormone administration to hypophysectomized rats increase the thyroidal concentration of the inhibitor as measured against pancreatic ribonuclease. Decreased thyroidal stimulation results in decreased inhibitor concentration. These observations on rat thyroid fit in with the finding by others of increased inhibitor concentrations in rapidly growing or stimulated tissues, and suggest a regulatory function for alkaline ribonuclease inhibitor in the thyroid.
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