Subcellular redistribution of liver α-glucan phosphorylase during alterations in glycogen content

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Abstract
About 0-80% of [alpha]-glucan phosphorylase was recovered in the microsomal fraction of livers from fed rats. After subfractionation of microsomes by sucrose-density-gradient centrifuging, the majority of the re- coverable enzyme activity was associated with the "particulate" glycogen fraction. Starvation for 18-20 hr. (overnight) led to a drop of 15-20% in total phosphorylase activity per liver and 80% of the enzyme was now present in the 105 000 g supernatant. Then, 24 hr. after the re-feeding of starved rats the enzyme reappeared in the particulate fraction, with a 15-20% increase in total phosphorylase. The reversible redistribution of phosphorylase between the particulate and "soluble" fractions paral-leled the depletion and reappearance of glycogen. The administration of puromycin to fed rats caused a rapid and reversible subcellular redistribution of the enzyme; this followed the course of depletion and reappearance of glycogen in the liver. With 3,3[image],5-tri-iodo-L-thyronine, the redistribution was incomplete but its effect persisted for a longer time than after puromycin. The association between particulate glycogen and phosphorylase appears to be a phenomenon of reversible binding. It was studied by measuring, inartificial systems, the distribution of phosphorylase present in the 105 000 g supernatant from starved rats after the addition of liver microsomes and particulate glycogen from fed rats or chemically isolated glycogen preparations. Particulate glycogen in livers from fed rats was about one-third saturated with respect to phosphorylase. The binding of phosphorylase to glycogen exhibited a specificity with regard to the size of the glycogen molecule. Some physiological implications of this association have been discussed.