Studies on glycogen synthesis in pigeon liver homogenates. Incorporation of hexose into glycogen

Liver homogenates of avian species, but not of mammals, form glycogen from glucose, mannose, fructose and galactose. Incorporation of labelled glucose, fructose and mannose, but not of labelled galactose, into glycogen is diluted isotopically by unlabelled glucose. Except for fructose, glycogen formation from other substrates by pigeon liver homogenates compares favourably with that from the same substrates in pigeon liver slices. Optimum conditions for glycogen synthesis from glucose by pigeon liver homogenate are: medium of incubation, 0·175m-sucrose–45mm-potassium chloride−15mm-glycylglycine buffer, pH 7·5; concentration of substrate, 15mm; concentration of tissue, less than 120mg./ml.; temperature of incubation, 37–43°; atmosphere, oxygen. Uncouplers of oxidative phosphorylation, Ca²⁺, EDTA, PP_i, 2-deoxyglucose 6-phosphate and microsomal fraction of rat liver are inhibitory to glycogen synthesis from glucose. Starvation of pigeons for 24 and 48hr. leads to a slight stimulation of glycogen synthesis in their liver homogenates as compared with fed controls. Pigeon liver homogenates can be separated into subcellular fractions that on reconstitution can synthesize glycogen. All the enzymes of the glycogen pathway except soluble high-K_m glucokinase are present in pigeon liver.