glucagon, hormone secreted by the α cells of the islets of Langerhans, specific groups of cells in the pancreas. It tends to counteract the action of insulin, i.e., it raises the concentration of glucose in the blood. Glucagon was first purified and crystallized in 1955; the amino acid sequence of this 29-amino acid polypeptide (see peptide) was published in 1956-57. One of the most important actions of glucagon is the promotion of glycogenolysis, i.e., the degradation of glycogen to glucose, in the liver. Glucagon stimulates adenyl cyclase, the enzyme that catalyzes the conversion of adenosine triphosphate to 3'5'-cyclic adenosine monophosphate (cyclic AMP).