A SENSITIVE AND RELIABLE METHOD FOR ASSAYING TRUE HUMAN INSULIN WITHOUT INTERACTION WITH HUMAN PROINSULIN-LIKE MOLECULES

In the present study our interests focused on the evaluation of a high capacity assay (MEIA) which allows true insulin determinations in the absence of cross-reactivity with proinsulin-like molecules. This meth od was compared to a commercially available radioimmunoassay (RIA) for insulin determination, As the latter gives insulin levels which repre sent a mixture of insulin and proinsulin-like molecules, the proinsuli n-like molecules were quantitated by subtracting the true insulin leve ls measured using MEIA from the total insulin levels obtained using RI A. These methods were applied for the analysis of blood samples drawn in 63 normal subjects, 16 obese subjects, 3 patients submitted to isle t transplantation and 4 patients with insulinoma, The MEIA was precise , fully automated and time-saving, making its application on a routine basis particularly attractive. MEIA and RIA were equally able to corr ectly quantify human insulin molecules. On the contrary, the antibody present in the true insulin assay did not interact with proinsulin-lik e molecules, which were recognized even in the presence of increasing insulin levels. In normal subjects, the true and total insulin levels in the fasting state and at the time peak after glucose- or arginine-i nduced endogenous insulin release were well correlated at r=0.88 and 0 .89, respectively. Interestingly, total insulin values were overestima ted by 10%-16% as compared with true insulin levels, which represent p roinsulin values superimposable on previously reported data. Proinsuli n-like molecules made up 50% of the total insulin in obese and transpl anted patients, and about 70% in patients with insulinoma. In conclusi on, the present study describes a precise, sensitive, fully automated and time-saving method for true insulin determination which is competi tive with previously published methods. By subtracting the immunoreact ivity measured in the insulin RIA and the insulin MEIA, we obtained th e proinsulin-like molecule levels in the range previously reported for normal and obese subjects and patients with insulinoma, and provided a new insight into islet-transplanted patients.