title: Enzymes of Choline Synthesis in Spinach (Response of Phospho-Base N-Methyltransferase Activities to Light and Salinity).
abstract: In spinach (Spinacia oleracea L.), choline is synthesized by the sequential N-methylation of phosphoethanolamine -> phosphomono- -> phosphodi- -> phosphotrimethylethanolamine (i.e. phosphocholine) followed by hydrolysis to release choline. Differential centrifugation of spinach leaf extracts shows that enzymes catalyzing the three N-methylations are cytosolic. These enzymes were assayed in leaf extracts prepared from plants growing under various light/dark periods. Under a diurnal, 8-h light/16-h dark photoperiod, the activity of the enzyme catalyzing the N-methylation of phosphoethanolamine is highest at the end of the light period and lowest following the dark period. Prolonged dark periods (exceeding 16 h) lead to a further reduction in the activity of this enzyme, although activity is restored when plants are reexposed to light. In contrast, the activity of the enzyme(s) catalyzing the N-methylations of phosphomono- and phosphodimethylethanolamine does not undergo comparable changes in response to light/dark treatments. Salt shock of plants with 200 mM NaCl results in a 2-fold increase in all three N-methylation activities relative to nonsalinized controls but only in plants exposed to light. Thus, light is required for the salt-responsive up-regulation of choline synthesis in spinach.
authors.affiliation: Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1.
authors.forename: E. A.
authors.initials: EA
authors.lastname: Weretilnyk
authors.forename: D. D.
authors.initials: DD
authors.lastname: Smith
authors.forename: G. A.
authors.initials: GA
authors.lastname: Wilch
authors.forename: P. S.
authors.initials: PS
authors.lastname: Summers