Enzymatic Inactivation of Parathyroid Hormone by Rat Kidney Homogenate

Rat kidney homogenate corresponding to 50–100 mg wet tissue inactivated the calcium mobilizing activity of 40 USP U of Parathyroid Injection (Lilly) upon incubation in Robinson's buffer at 38 C for 1 hr more effectively than the liver, spleen and muscle homogenates of similar concentration. The parathyroid hormone (PTH)-inactivating activity was mainly found in the protein fraction of the kidney homogenate precipitated at 30–60% ammonium sulfate saturation and labile to heating at 65 C for 10 min. The PTH-inactivating effect was absent in the supernatant of the 0.25M sucrose homogenate of the kidney but present in the supernatant of the distilled water homogenate, suggesting the localization in the particulate fraction. PTH-inactivating effect was inhibited by mercaptoethanol at 4×10-1M, carbobenzoxy-Ltyrosin-p-nitrophenylester at 6 × 10-2M, tosyl-Larginine- o-methylester at 4×10-2M, and benzoyldl- arginine naphthylamide at 3×10-2M. EDTA inhibited PTH-inactivating effect at 1×10-2M but puromycin appeared to be ineffective at 1×10-3M. Fractionation of the subcellular components of rat kidney homogenate according to Shibko and Tappel revealed the localization of the PTH-inactivating enzyme (s) mainly in the microsomal fraction. (Endocrinology86: 42,1970)