title: Hexokinase activity alters sugar-nucleotide formation in maize root homogenates.
abstract: Two pools of hexokinase activities differing in sensitivity to ADP inhibition were characterised in maize roots. In order to evaluate how glucose utilisation could be affected by these hexokinases, glucose-6-P and NDP-5'-sugar levels were measured after a D-[U-14C]glucose pulse in root extracts in the presence of 0 or 1 mM ADP. Analysis of radio-labelled activated sugars by paper chromatography revealed that: (1) without ADP, nearly 20% of the 14C appeared in NDP-5'-sugars; (2) 0.1 mM ADP inhibited 14C-NDP-5'-sugar formation by 85%; and (3) with 1 mM ADP, 14C-NDP-5'-sugars were undetectable, but substantial (14%) 14C accumulated as glucose-6-P. Mannoheptulose, a hexokinase inhibitor, blocked the NDP-5'-sugar formation, but did not modify the amount of 14C-glucose-6-P in root extracts either with or without ADP. The analysis of the hexokinase activities with 0.8 mM glucose in maize root extracts showed that: (1) mitochondrial hexokinase activity was totally inhibited by 30 mM mannoheptulose; and (2) the cytosolic hexokinase was inhibited by only 30%. These data suggest that NDP-5'-sugar synthesis is sensitive to ADP fluctuations and that mannoheptulose affects preferentially the mitochondrial-bound hexokinase, but the cytosolic form is less sensitive. We propose that the mitochondrial hexokinase is the main energy charge sensor in this pathway in maize.
authors.affiliation: Departamento de Bioquímica Medica, Instituto de Ciências Biomédicas, Universidad Federal do Rio de Janeiro, RJ, Brazil. galina@bioqmed.ufrj.br
authors.forename: A
authors.initials: A
authors.lastname: Galina
authors.forename: W S
authors.initials: WS
authors.lastname: da Silva
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D019298:Glucose-6-Phosphate / Q000096:biosynthesis*; D006593:Hexokinase / Q000378:metabolism*; D008356:Mannoheptulose / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism; D009702:Nucleoside Diphosphate Sugars / Q000096:biosynthesis*; D018517:Plant Roots / Q000378:metabolism; D003313:Zea mays / Q000378:metabolism*
chemical_list: D009702:Nucleoside Diphosphate Sugars; D019298:Glucose-6-Phosphate; D000244:Adenosine Diphosphate; D008356:Mannoheptulose; D006593:Hexokinase; D005947:Glucose