Enzymatic Breakdown of Glycogen in Liver Extracts

Abstract

It has been shown in previous papers 1 that dialyzed liver extracts contain an enzyme which forms glucose-1-phosphoric acid (1-ester) from glycogen and inorganic phosphate and that this enzyme is activated by adenylic acid. When 1-ester is added to liver extract, inorganic phosphate is split off due to the presence of a phosphatase. It is shown in this paper that the combined action of these two enzymes converts glycogen to glucose, a reaction which has hitherto been ascribed exclusively to a diastatic enzyme. A typical experiment is recorded in Table I. The liver of a fasted rabbit was cooled, ground in a mortar and extracted twice with ice-cold distilled water. The extract was dialyzed for 4 hours in thin collodion sacs against running tap water of 10° it was then centrifuged at high speed for 10 minutes and used at once. Additions were made to the extract as shown in Table I, and analyses were performed before and after incubation. The glycogen, after digestion in 30% NaOH, was precipitated from boiling alcohol, centrifuged, redissolved in water and again precipitated. The fermentable sugar was determined in HgSO₄-BaCO₃ filtrates, inorganic phosphate in trichloracetic acid filtrates. The formation of hexosemonophosphate was calculated from the amount of inorganic P which was esterified during incubation.

Table I shows that addition of phosphate to the reaction mixture increases very markedly the disappearance of glycogen and that addition of adenylic acid causes a further increase. In the latter case the rate of disappearance of glycogen corresponds to 1.4 g per 100 g liver per hour which would be sufficiently rapid to meet physiological needs of blood sugar formation in that organ. Phlorhizin, which is known to inhibit the disruptive phosphorylation of glycogen in muscle hash or extract, 2 also has an inhibitory effect in liver extracts.