Abstract
Plants synthesize a number of different oligomeric or polymeric sugars containing galactose. During growth and development some of these carbohydrates are metabolized or remodeled releasing galactose as a breakdown product. All plants have established recycling pathways for such sugars, for which they seem to have a limited capacity to cope with. Exceeding these limits results in sugar toxicity, which is observed already at concentrations as low as 1 mmol·l−1 for galactose. The mechanism of galactose toxicity is poorly understood but it seems plausible that the enzymes involved in carbohydrate metabolism also might be the targets responsible for the adverse effects. Data from yeast and bacteria suggests that the enzyme phosphoglucomutase (PGM) is inhibited by galactose-1-phosphate. To test this hypothesis for plants we expressed recombinant cytosolic PGM3 from Arabidopsis in E. coli. Intriguingly, the enzyme was not inhibited by galactose-1-phosphate at physiological concentrations. Furthermore, PGM3 did not convert galactose-1-phosphate to galactose-6-phosphate, which was suggested as the inhibitory mode of action in yeast. In addition, metabolite levels in Arabidopsis roots were analyzed for their galactose-1-phosphate concentration by means of GC–MS. Seedlings grown on MS-media with sucrose contained less than 10 nmol·g FW−1 of galactose-1-phosphate. However, seedlings from plates, in which the sucrose was replaced by galactose, showed a strong increase of Gal-1-P to levels of up to 200 nmol·g FW−1.
Highlights
Galactose is an abundant sugar in plants, fungi and animals
Mutant plants with an insertional TDNA (GabiKat_489_D10) in the galactokinase gene were unable to recycle galactose (Egert et al, 2012). They grew very similar to WT plants on MS-plates containing sucrose, demonstrating the necessity of intermediate Gal-1P formation for developing galactose toxicity (Figure 1)
Among those was the assumption that the enzyme phosphoglucomutase (PGM) is inhibited by Gal-1P or at least contributes to galactose toxicity by, e.g. the conversion of Gal-1P to Gal-6P
Summary
Galactose is an abundant sugar in plants, fungi and animals. The de novo synthesis involves the formation of UDP-glucose, which is epimerized to UDP-galactose by the activity of UDP-glucose-4epimerase (Seifert et al, 2002). Beside the de novo pathway, which always leads to nucleotide sugars, most organisms have developed a recycling pathway for galactose. Lactose uptake in humans leads to high amounts of galactose as result of lactose cleavage. Individuals with a mutation in the galactose utilization pathway develop the disease galactosemia already in newborns (Timson, 2016)
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