title: An alpha-glucose-1-phosphate phosphodiesterase is present in rat liver cytosol.
abstract: UDP-glucose:glycoprotein glucose-1-phosphotransferase (Glc-phosphotransferase) catalyzes the transfer of alpha-Glc-1-P from UDP-Glc to mannose residues on acceptor glycoproteins. The predominant acceptor for this transfer in both mammalian cells and Paramecium is a cytoplasmic glycoprotein of 62-63 kDa. When cytoplasmic proteins from rat liver were fractionated by preparative isoelectric focusing following incubation of a liver homogenate with the 35S-labeled phosphorothioate analogue of UDP-Glc ([beta-35S]UDP-Glc), the acceptor was found to have a pI of about 6.0. This fraction, when not labeled prior to the focusing, became very heavily labeled when mixed with [beta-35S]. UDP-Glc and intact liver microsomes, a rich source of the Glc-phosphotransferase. In addition, it was observed that the isoelectric fractions of the cytosol having pI values of 2-3.2 contained a degradative activity, alpha-Glc-1-P phosphodiesterase, that was capable of removing alpha-Glc-1-P, monitored through radioactive labeling both in the sugar and the phosphate, as an intact unit from the 62-kDa acceptor. Identification of the product of this cleavage was substantiated by its partial transformation to UDP-Glc in the presence of UTP and UDP-Glc pyrophosphorylase. The alpha-Glc-1-P phosphodiesterase had a pH optimum of 7.5 and was not effectively inhibited by any of the potential biochemical inhibitors that were tested. Specificity for the Glc-alpha-1-P-6-Man diester was suggested by the diesterase's inability to degrade UDP-Glc or glucosylphosphoryldolichol. This enzyme may be important in the regulation of secretion since the alpha-Glc-1-P present on the 62-kDa phosphoglycoprotein appears to be removed and then rapidly replaced in response to secretagogue.
authors.affiliation: Department of Cell Biology and Anatomy, University of Alabama, Birmingham 35294.
authors.forename: C
authors.initials: C
authors.lastname: Srisomsap
authors.forename: K L
authors.initials: KL
authors.lastname: Richardson
authors.forename: J C
authors.initials: JC
authors.lastname: Jay
authors.forename: R B
authors.initials: RB
authors.lastname: Marchase
mesh_terms: D000818:Animals; D001345:Autoradiography; D003513:Cycloheximide / Q000494:pharmacology; D003600:Cytosol / Q000201:enzymology; D006023:Glycoproteins / Q000302:isolation & purification / Q000378:metabolism*; D007525:Isoelectric Focusing; D008099:Liver / Q000201:enzymology*; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D010727:Phosphoric Diester Hydrolases / Q000302:isolation & purification / Q000378:metabolism*; D010770:Phosphotransferases / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D013379:Substrate Specificity; D013462:Sulfur Radioisotopes; D017855:Transferases (Other Substituted Phosphate Groups); D014316:Tritium; D014532:Uridine Diphosphate Glucose / Q000378:metabolism
chemical_list: D006023:Glycoproteins; D013462:Sulfur Radioisotopes; D014316:Tritium; D003513:Cycloheximide; D010770:Phosphotransferases; D017855:Transferases (Other Substituted Phosphate Groups); C037061:UDPglucose-glycoprotein glucose-1-phosphotransferase; D010727:Phosphoric Diester Hydrolases; C061898:glucose-1-phospho-D-mannosylglycoprotein phosphodiesterase; D014532:Uridine Diphosphate Glucose