Sorted By Test Name - Mayo Medical Laboratories

PRLPM
84462
Clinical Information: Proinsulin is the precursor of insulin and C-peptide. Following synthesis,
proinsulin is packaged into secretory granules, where it is processed to C-peptide and insulin by
prohormone convertases (PC1/3 and PC2) and carboxypeptidase E. Only 1% to 3% of proinsulin is
secreted intact. However, because proinsulin has a longer half-life than insulin, circulating proinsulin
concentrations are in the range of 5% to 30% of circulating insulin concentrations on a molar basis, with
the higher relative proportions seen after meals and in patients with insulin resistance or early type 2
diabetes. Proinsulin can bind to the insulin receptor and exhibits 5% to 10% of the metabolic activity of
insulin. Proinsulin levels might be elevated in patients with insulin-producing islet cell tumors
(insulinomas). These patients suffer from hypoglycemic attacks due to inappropriate secretion of insulin
by the tumors. The biochemical diagnosis of insulinoma rests primarily on demonstrating non-suppressed
insulin levels in the presence of hypoglycemia (blood glucose 95%) can
be achieved using a 5 pmol/L cutoff, and this is the cutoff recommended by the Mayo Clinic's highly
experienced hypoglycemia team to avoid missing cases. However, the lower cutoff results in a reduced
specificity (approximately 40%), emphasizing the need for a combination of different tests to assure
accurate biochemical diagnosis. Patients with PC1/3 deficiency have low, or sometimes undetectable,
insulin levels and substantially elevated proinsulin levels, exceeding the upper limit of the reference range
substantially in the fasting state and rising even higher after food intake. Many other hormonal
abnormalities are also present, including cortisol deficiency (because of lack of processing of
pro-opiomelanocortin to adrenocorticotropic hormone and other peptides), infertility and, often, morbid
obesity. This assay demonstrates no cross-reactivity with insulin or C-peptide.
Reference Values:
3-20 pmol/L
Clinical References: 1. Kao PC, Taylor RL, Service FJ: Proinsulin by immunochemiluminometric
assay for the diagnosis of insulinoma. J Clin Endocrinol Metab 1994;78(5):1048-1051 2. Service FJ:
Hypoglycemic disorders. N Engl J Med 1995;322(17):1144-1152 3. Steiner DF: The proprotein
convertases. Cur Opin Chem Biol 1998;2(1):31-39 4. Vezzosi D, Bennet A., Fauvel J, Caron P: Insulin,
C-peptide and proinsulin for the biochemical diagnosis of hypoglycemia related to endogenous
hyperinsulinism. Eur J Endocrinol 2007;157(1):75-83
Prolactin, Pituitary Macroadenoma, Serum
Clinical Information: Prolactin is secreted by the anterior pituitary gland. The major chemical
Current as of January 4, 2013 7:15 pm CST 800-533-1710 or 507-266-5700 or MayoMedicalLaboratories.com Page 1478