Oxidative stress results in increased sinks for metabolic energy during aging and sprouting of potato seed-tubers

Glutathione-mediated free-radical-scavenging and plasma membrane ATPase activities increase as sinks for metabolic energy with advancing tuber age. Plasma membrane ATPase activity from 19-month-old tubers was 77% higher than that from 7-month-old tubers throughout sprouting. The higher activity was not attended by an increase in the amount of ATPase per unit plasma membrane protein. Concentrations of oxidized (GSSG) and reduced glutathione more than doubled as tuber age advanced from 6 to 30 months, but the proportion of GSSG to total glutathione remained constant with age. The activity of glutathione transferase, an enzyme that catabolizes lipid-hydroperoxides, increased by 44 and 205% on a fresh weight and protein basis, respectively, as tubers aged from 6 to 30 months. Glutathione reductase activity also increased with advancing age, by 90% on a fresh weight basis and 305% on a protein basis. Older tubers had more glutathione reductase per unit of soluble and mitochondrial protein. The age-induced increase in cytosolic glutathione transferase activity was likely due to increased availability of lipid-hydroperoxides and/or a positive effector. Synthesis of glutathione requires ATP, and the increased reduction of GSSG resulting from catalysis of lipid-hydroperoxides is NADPH-dependent. Thus, increased plasma membrane ATPase and glutathione-mediated free-radical-scavenging activities likely constitute substantial sinks for ATP in older tubers prior to and during sprouting. Increased oxidative stress and loss in membrane integrity are central features of aging that undoubtedly contribute to the enhanced respiration of sprouting older tubers.