Insulinoma with hyperproinsulinemia during hypoglycemia and loss of expression of vacuolar-type H(+)-ATPase (V-ATPase) in the tumor tissue.

Hypoglycemia with a low serum immunoreactive insulin (IRI) level and serum immunoreactive C-peptide (IRC) level was found in a 74-yr-old female. Although a fasting test induced hypoglycemia, the responses of IRI and IRC during the fasting test, and the results of a glucose tolerance test, glucagon test, and secretin test did not indicate the presence of an insulinoma. However, the serum proinsulin level before the fasting test was 130.5 pmol/L (N: 3.0-10.0 pmol/L), and this high level was maintained throughout the test. Soon after surgical enucleation of the tumor, the patient's blood glucose levels increased. Postoperatively, the hypoglycemic status resolved, and the serum proinsulin levels returned to normal (2.8 pmol/L). Histopathological studies revealed a typical insulinoma. Immunohistochemical studies by the recently developed method for vacuolar-type H+ (V-ATPase), which is responsible for acidification of the intracellular compartments in eukaryotic cells, showed that normal islets stained positive, but not the tumor. This finding indicates that the insulin-secretory granules in the insulinoma cells existed in a microenvironment in which V-ATPase activity had been lost. This suggests that the reduced activity of V-ATPase on the endomembrane of the insulin-secretory granules in insulinomas may result in loss of the acidic microenvironment and impaired conversion of proinsulin by converting enzymes.