Increased levels of adenine nucleotides modify the interaction between starch synthesis and respiration when adenine is supplied to discs from growing potato tubers

Abstract.

To investigate the importance of the overall size of the total adenine nucleotide pool for the regulation of primary metabolism in growing potato tubers, freshly cut discs were provided with zero or 2 mM adenine in the presence of 1 or 100 mM [U-¹⁴C]glucose or 100 mM [U-¹⁴C]sucrose in the presence and absence of 20 mM orthophosphate (Pi). Adenine led to a 150–250% increase of the total adenine nucleotide pool, which included an increase of ADP, a larger increase of ATP and an increase of the ATP:ADP ratio. There was a 50–100% increase of ADP-glucose (ADPGlc), and starch synthesis was stimulated. Respiratory oxygen uptake was stimulated, and the levels of glycerate-3-phosphate, phosphoenolpyruvate and α-ketoglutarate decreased. The response to adenine was not modified by Pi. It is proposed that increased ATP stimulates ADPGlc pyrophosphorylase, leading to a higher rate of starch synthesis. The impact on starch synthesis is constrained, however, because increased ADP can lead to a stimulation of respiration and decline of glycerate-3-phosphate, which will inhibit ADPGlc pyrophosphorylase. The quantitative impact depends on the conditions. In the presence of 1 mM glucose, the levels of phosphorylated intermediates and the rate of starch synthesis were low. Adenine led to a relatively large stimulation of respiration, but only a small stimulation of starch synthesis. In the presence of 100 mM glucose, discs contained high levels of phosphorylated intermediates, low ATP:ADP ratios (<3) and low rates of starch synthesis (<20% of the metabolised glucose). Adenine led to marked increase of ATP and 2- to 4-fold stimulation of starch synthesis. Discs incubated with 100 mM sucrose already had high ATP:ADP ratios (>8) and high rates of starch synthesis (>50% of the metabolised sucrose). Adenine led to a further increase, but the stimulation was less marked than in high glucose. These results have implications for the function of nucleotide cofactors in segregating sucrose mobilisation and respiration, and the need for energy conservation during sugar-starch conversions.