The half-life in plasma of immunoreactive insulin was determined after the injection of eight units of unlabeled human insulin. Six juvenile diabetics and nine maturityonset diabetics as well as eight and six nondiabetics of comparable ages were investigated. The average T½ for the groups was respectively: 10.0 ± 2.8; 12.9 ± 1.7; 10.6 ± 1.9 and 13.0 ± 2.5 minutes. There is no tendency to any difference in T½ between diabetics and nondiabetics of comparable age. However, the T½ in the older group is significantly longer than T½ in the younger age group.
Evidence from in vitro experiments, as well as from the literature, is presented which points to the fact that iodinated insulin cannot be used for the evaluation of the disappearance rate or secretion rate of endogenous insulin. Furthermore, it is emphasized that protein precipitation with TCA or Na2SO4 cannot be employed for the isolation of undestroyed iodinated insulin.