Preservation of immunological and biological activities of insulin by insulinase inhibitor in vitro
The relation of insulinase activity to the biological effect of insulin on isolated tissue was studied. Rat diaphragm in vitro caused the rapid disappearance of immunoreactive insulin (IRI) in physiological concentrations. IRI loss at time intervals was exponential. The fractional rate of loss of IRI was therefore independent of IRI concentration and was also approximately constant per milligram of tissue, the value being 0.0216%/mg∙mm. The value of the Michaelis constant (Km), obtained from initial velocities at five initial concentrations of IRI, was 1.85 × 10−8M, and of the maximal velocity (Vmax) was 2.32 × 10−11 mole/g∙min, based on insulin dimer. The addition of an insulinase inhibitor (a partial hydrolysate of insulin) to hemidiaphragm in vitro reduced the fractional rate of IRI loss by 60%. The increase in Km, without appreciable change in Vmax, indicated that the inhibition was competitive. The IRI preserved by means of the inhibitor was biologically active, since it increased the glycogen accumulation and the incorporation of 14C-U-glucose into glycogen in a second, fresh hemidiaphragm. In single incubation of hemidiaphragm with insulin, the gain in glycogen was correlated with the amount of inhibitor. The biological effect of insulin on diaphragm in vitro was therefore limited by tissue insulinase activity, and insulinase inhibitor potentiated to some extent the action of insulin.