Total plasma insulin as measured by radioimmunoassay can be divided into at least two distinct components by gel fitration. One component is indistinguishable from crystalline pancreatic insulin (insulin component); the second component more closely resembles proinsulin (proinsulin-like component).
The proinsulin-like component may be heterogenous in different disease states. Differences in gel filtration properties have been demonstrated between islet cell tumor patients and nontumor patients.
The percentage proinsulin-like component is highest in the basal state; acute stimulation of insulin release predominantly increases the concentration of the insulin component with a resultant decrease in the percentage proinsulin-like component. The same dynamics are seen both in islet cell tumor patients who have high percentages of proinsulin-like component and in nontumor patients. In severely hypoin-sulinemic patients, the percentage proinsulin-like component increases regardless of the etiology of the hypoinsulinemia.