Formation and breakdown of glycogen in the liver

UNTIL a few years ago it was generally accepted that glycogen breakdown in animal tissues is initiated by an enzymic hydrolysis, giving glucose, or as some authors thought, a special "reactive form of hexose".Then Parnas & Baranowski [1936] discovered a new reaction of glycogen.They found that autolysed rabbit muscle extract forms from glycogen and phosphate a difficultly hydrolysable hexosemonophosphate -which Ostern et al. [1936] showed to be the Embden equilibrium ester.This reaction, called by Parnas "phosphorolysis " of glycogen, differs from hydrolysis of glycogen in that phosphoric acid replaces water as the disruptive agent: Hydrolysis of glycogen: (C6H10O5)" + nH20 = nC6H12O6.Phosphorolysis of glycogen: (C6HI005)n + nH3PO4 = nC6H10O6 .PO3H2.The only other carbohydrate which reacts similarly in muscle extract is starch.Simple sugars and disaccharides are not phosphorylated.The extent of phosphorylation depends on the concentrations of glycogen and phosphate; with excess of phosphate present, all the glycogen is converted into hexosemono- phosphate.With excess of glycogen, its breakdown is limited by the amount of phosphate available [Ostern et at.1937].Phosphorolysis seems to be the only method of breakdown of glycogen in muscle, and is the first step of the glycolytic cycle.A most important contri- bution to our knowledge of the mechanism of phosphorolysis was made by Cori et al. [1937], who showed that the hexosemonophosphate initially formed from glycogen is a new ester, glucose 1-phosphate (Cori ester), which is then converted into glucose 6-phosphate by the action of a second enzyme.Cori et al. [1938] further showed that this enzyme system also occurs in heart, brain, liver and yeast.Liver extracts, dialysed for a short time, convert glycogen into Cori ester which is partly dephosphorylated to glucose.Since Davenport [1926] found that after removal of blood by perfusion little amylase could be found in liver, Cori & Cori [1938] suggested that glucose was formed in the liver by phosphorolysis rather than by the action of an amylase.Later the presence of the phosphorylating enzyme in yeast was further confirmed by Schaiffner [1939] and by Kiessling [1939], and Hanes [1939] has found in extracts of pea meal an enzyme which converts starch into Cori ester in the presence of inorganic phosphate.Kiessling [1939] also showed that the reaction glycogen + phosphate = Cori ester is reversible.He isolated a protein fraction ("Protein C") from yeast which formed glycogen from Cori ester, and Cori ester from glycogen and phosphate, the same equilibrium being reached in both cases.An analogous reaction has very recently been described by Cori et al. [1939, 3] in muscle extracts from which ( 1858 )