On the stability of nucleoside diphosphate glucose metabolites: implications for studies of plant carbohydrate metabolism

HILL, BENJAMIN L. (IGUAL CONTRIBUCIÓN); FIGUEROA, CARLOS M. (IGUAL CONTRIBUCIÓN); ASENCION DIEZ, MATÍAS D.; LUNN, JOHN E.; IGLESIAS, ALBERTO A.; BALLICORA, MIGUEL A.

JOURNAL OF EXPERIMENTAL BOTANY

OXFORD UNIV PRESS

Año: 2017 vol. 68 p. 3331 - 3337

0022-0957

Nucleosidediphosphate sugars (NDP-sugars) are the substrates for biosynthesis of oligo-and polysaccharides, such as starch and cellulose, and are also required forbiosynthesis of nucleotides, ascorbic acid, several cofactors, glycoproteinsand many secondary metabolites. A controversial study that questions thegenerally accepted pathway of ADP-glucose and starch synthesis in plants isbased, in part, on claims that NDP-sugars are unstable at alkaline pH in the presenceof Mg2+ and that this instability can lead to unreliable resultsfrom in-vitro assays of enzymeactivities. If substantiated, this claim would have far-reaching implicationsfor many published studies that report activities of NDP-sugar metabolizingenzymes. To resolve this controversy, we investigated the stability of UDP- andADP-glucose using biophysical (NMR) and highly specific enzymatic methods. Resultsobtained with both techniques indicate that NDP-sugars are not as unstable aspreviously suggested. Moreover, their calculated in vitro half-lives are significantly higher than estimates oftheir in planta turnover times. This indicatesthat the physico-chemical stability of NDP-sugars has little impact on their concentrationsin vivo, and that NDP-sugar levelsare determined primarily by the relative rates of enzymatic synthesis andconsumption. Our results refute one of the main arguments for the controversialpathway of starch synthesis from imported ADP-glucose produced by sucrosesynthase in the cytosol.