Immunoluminometric assay (ILMA) for intact human proinsulin and its conversion intermediates

Area of Science:

• Biochemistry
• Endocrinology
• Assay Development

Background:

• Proinsulin is a precursor to insulin, and its elevated levels can indicate certain medical conditions.
• Accurate measurement of intact proinsulin and its conversion intermediates is crucial for diagnosing endocrine disorders.

Purpose of the Study:

• To develop and validate an immunoluminometric assay (ILMA) for quantifying intact proinsulin and its split intermediates in human serum.
• To establish reference ranges for serum proinsulin in healthy individuals.
• To evaluate the assay's utility in identifying patients with insulinoma.

Main Methods:

• Development of a sensitive immunoluminometric assay using a guinea pig anti-human insulin antibody and a monoclonal anti-human C-peptide antibody labeled with acridinium ester.
• Incubation of serum samples with coated beads to form a sandwich complex.
• Detection limit established at 0.11 pmol/l.

Main Results:

• The assay demonstrated a detection limit of 0.11 pmol/l.
• Mean serum proinsulin in healthy fasting subjects was 7.3 pmol/l, with levels not exceeding 40 pmol/l after glucose stimulation.
• 18 out of 20 patients with insulinoma had proinsulin concentrations above 50 pmol/l, with significantly elevated mean levels (261 pmol/l).
• Serum proinsulin levels normalized post-surgery in patients with successfully treated B-cell adenoma.

Conclusions:

• The developed ILMA is a sensitive and specific method for measuring intact proinsulin and its intermediates in serum.
• Elevated serum proinsulin levels, as detected by this assay, are indicative of insulinoma.
• The assay shows potential for clinical application in the diagnosis and management of insulin-secreting tumors.