initially translated as a 110 amino acid
molecule, comprising a signal peptide and B-,
C- & A-chains. The signal
peptide (24 aa's) is
first clipped from amino terminus. The A- & B- Chains
of the proinsulin are
then joined by disulfide bridges. This stabilizes
the molecule so that the intervening C peptide (35 aa's) can be excised. The
biologically-active, tertiary structure of
insulin comprises an A chain (30 aa's) & B chain (21 aa's) held together by 3
disulfide bridges. [One S-S bridge that
joins two cys residues in the A-Chain is
A patient was admitted to the emergency room of a hospital
in insulin shock. Radioimmunoassay showed high levels of
insulin in the blood. Physicians first suspected an
insulinoma, a cancer of the pancreas that would cause it to
over-secrete insulin. However, the biopsy was negative, and
the patient was released. Several weeks later, the patient
again showed up in insulin shock. This time, the lab
adjusted the RIA to detect preproinsulin as well as insulin.
The patient tested postive for insulin as before, and tested
negative for preproinsulin.
hypothesis to explain these results.