Immunological Stability of Calcitonin in Plasma

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Abstract
Relative rates of decline in the concentration of immunoassayable calcitonin from several species were compared by serial specific radioimmunoassay measurements during incubations in vitro in plasmas or serums obtained from a number of different animals. The immunological activities of porcine, bovine and ovine calcitonins were rapidly inactivated (50% inactivation in 2-3 hr at 25 C) while salmon and human calcitonins, structurally distinct forms of calcitonin, were relatively stable (50% inactivation in 30 or more hr at 25 C). That the inactivation of calcitonin in plasma, in vitro, may be due to an enzymatic process was suggested by: 1) dependence of rate of inactivation upon temperature, pH and amount of plasma, 2) prevention of inactivation by prior heating of plasma at 56 C for 30 min, and 3) gel filtration analysis showing that loss of calcitonin immunoreactivity correlates with the appearance of smaller fragments of the hormonal peptide. Primary structural difference among the calcitonins appear to determine their susceptibility to degradation in vitro. The findings have numerous practical implications in the storage, treatment and assay of plasma samples containing calcitonin. (Endocrinology90: 952, 1972)

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