Alkaline ribonuclease inhibitor in human thyroid.

Thyroids of goitrogen-treated rats contain increased amounts of a protein inhibitor of ribonuclease activity at pH greater than 7 (alkaline ribonuclease inhibitor, ARI). We report here that thyroids from hyperthyroid patients contain more ARI than normal human thyroids. This increase parallels RNA concentration. The inhibitor is heat labile, inactivated by sulfhydryl blocking agents, and has a molecular weight near 50, 000 daltons. ARI is quantitated by its activity against bovine pancreatic RNase, but it also inhibits human thyroid RNase. Analyses of a solitary toxic nodule and its surrounding suppressed tissue confirm in tissues from a single patient our results in tissue from numbers of thyrotoxic and euthyroid individuals and decrease the likelihood that changes are induced by antithyroid medication. A possible regulatory role for ARI is suggested.