Metabolism of human calcitonin in vitro in the dog

In vitro loss of human calcitonin (HCT) in dog tissues and plasma was assessed by radioimmunoassay. Whole tissues were homogenized, centrifuged at 22,000 g for 20 min., and added HCT (200 ng/ml) incubated with supernatants at 37°C for 120 min. The kidney supernatant removed 76% of immunoassayable HCT, liver 34%, but lung, heart, and spleen negligible amounts. The factor in kidney responsible for this loss was non dialyzable, temperature dependent (max. 37.5°C), pH dependent (max. 6 to 9), and present in greater concentrations in cortex than medulla. Loss of HCT was associated with the microsomal fraction in renal cortex and with the microsomal supernatant in liver. Subcellular fractions were incubated with ¹²⁵I HCT and chromatographed on BioGel P 10. The radioactivity in incubates eluted later from the column than nonincubated ¹²⁵I HCT, presumptive evidence that immunologic loss of HCT is due to enzymatic degradation. In dog plasma, loss of HCT was pH dependent (max. 6.5 to 7.5), and temperature dependent (max. 37.5°C). Likewise progressive production of fragments was demonstrated when labeled hormone was incubated in plasma. These studies support the view that HCT is destroyed by one or more enzymes in renal cortical microsomes. In addition it is degraded by one or more soluble enzymes in liver and these, in part, may be responsible for loss of hormone in plasma.