Described by Kimball and Murlin in the 1920s, but not sequenced until decades later, this substance has a 62 kilodalton membrane bound receptor which initiates a cascade that ends in the activation of phosphorylase a.  A polypeptide with 29 amino acids, it is generated from a precursor which also includes oxyntomodulin and two peptides considered to be "like" this molecule.  
Severely elevated levels of this may result in the skin disorder necrolytic migratory erythema; these levels are generally only seen in cases of (*) endocrine tumor in the cells that express it.  Described by Kimball and Murlin in the 1920s, but not sequenced until decades later, this substance has a 62 kilodalton membrane bound receptor which initiates a cascade that ends in the activation of phosphorylase a.  A polypeptide with 29 amino acids, it is generated from a precursor which also includes oxyntomodulin and two peptides considered to be "like" this molecule.  Synthesized and released by the alpha cells of the islets of Langerhans, for 10 points, name this hormone that directs the liver to break down glycogen into glucose and which acts in opposition to insulin.
Severely elevated levels of this may result in the skin disorder necrolytic migratory erythema; these levels are generally only seen in cases of (*) endocrine tumor in the cells that express it.  
{glucagon} (1) [AU]