Mathematical model applied to decomposition rate of RIA radiotracers: 125I-insulin used as sample model

A mathematical model is described to fit the decomposition rate of labelled RIA compounds. The model was formulated using four parameters: one parameter correlated with the radioactive decay constant; the chemical decomposition rate 'K*' of the radiolabelled molecules; the natural chemical decomposition rate 'K' and; the fraction 'f*' of the labelled molecules in the substrate. According to the particular values that these parameters can assume, ten cases were discussed. To determine one of these cases which fit the experimental data, three types of samples were need: radioactive; simulated radiotracer ('false radiolabelled') and; on labelled common substrate. The radioinsulin 125I was used as an example to illustrate the model application. The experimental data substantiate that the insulin labelled according to the substorchiometric procedures and kept at freezer temperature were degraded with K=0.45% per day. (Author)