title: Phosphatidylcholine synthesis in castor bean endosperm: characteristics and reversibility of the choline kinase reaction.
abstract: Choline kinase (EC 2.7.1.32) was measured in concentrated 100,000gav supernatants from castor bean endosperm (Ricinus communis L. var. Hale). Initial velocity analysis, along with competitive inhibitor (hemicholinium-3) and product inhibition (ADPMg2+) studies suggested that the forward reaction followed a sequentially ordered mechanism with ATPMg2+ binding to the enzyme first, followed by choline and then activation of the ternary complex by free Mg2+. The kinetic constants of the forward reaction are reported. A reverse reaction was measured which had a pH optimum of 6.5 and produced 1 mol of ATP for every mole of choline phosphate. The estimated maximum possible Keq at 7.25 was 5 X 10(-3) which suggested that this reaction is highly reversible in this tissue. The possible physiological significance of this is discussed.
authors.affiliation: Department of Botany, Louisiana State University, Baton Rouge 70803-1705.
authors.forename: A J
authors.initials: AJ
authors.lastname: Kinney
authors.forename: T S
authors.initials: TS
authors.lastname: Moore
mesh_terms: D002367:Ricinus communis / Q000201:enzymology / Q000378:metabolism*; D002797:Choline Kinase / Q000378:metabolism*; D006868:Hydrolysis; D007700:Kinetics; D010713:Phosphatidylcholines / Q000096:biosynthesis*; D010767:Phosphorylcholine / Q000096:biosynthesis; D010770:Phosphotransferases / Q000378:metabolism*; D010947:Plants, Toxic; D012278:Ricinus / Q000378:metabolism*
chemical_list: D010713:Phosphatidylcholines; D010767:Phosphorylcholine; D010770:Phosphotransferases; D002797:Choline Kinase