Role of nuclear glycogen synthase and cytoplasmic UDP glucose pyrophosphorylase in the biosynthesis of nuclear glycogen in HD33 Ehrlich-Lettré ascites tumor cells.

Open Access
Publisher Website
Google Scholar
Abstract
Biochemical and autoradiographic evidence show glycogen synthesis and the presence of glycogen synthase (UDP glucose [UDPG]: glycogen 4-.alpha.-D-glucosyltransferase; EC 2.4.1.11) in isolated nuclei of Ehrlich-Lettre mouse ascites tumor cells of the mutant subline HD33. Five days after tumor transplantation, glycogen (average 5-7 pg/cell) is stored mainly in the cell nuclei. The activity of glycogen synthase in isolated nuclei is 14.5 mU/mg protein. At least half of the total cellular glycogen synthase activity is present in the nuclei. The nuclear glycogen synthase activity exists almost exclusively in its b form. The Km value for (a + b) glycogen synthase is 1 .times. 10-3 M UDPG, the activation constant is 5 .times. 10-3 M glucose-6-phosphate (Glc-6-P). Light microscopic and EM autoradiographs of isolated nuclei incubated with UDP-[1-3H]glucose show the highest activity of glycogen synthesis not only in the periphery of glycogen deposits but also in interchromatin regions unrelated to detectable glycogen particles. Together with earlier findings on nuclear glycogen synthesis in intact HD33 ascites tumor cells, the results of tests on isolated nuclei suggest a predominantly appositional mode of nuclear glycogen deposition, without participation of the nuclear membrane system. In intact cells, synthesis of UDPG for nuclear glycogen synthesis depends on the activity of the exclusively cytoplasmic UDPG pyrophosphorylase (UTP: .alpha.-D-glucose-1-phosphate uridylyltransferase; EC 2.7.7.9). Glycogen synthesis is apparently not exclusively a cytoplasmic function and the mammalian cell nucleus is capable of synthesizing glycogen.