Acetaldehyde Stimulation of Net Gluconeogenic Carbon Movement from Applied Malic Acid in Tomato Fruit Pericarp Tissue

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Abstract
Applied acetaldehyde is known to lead to sugar accumulation in fruit including tomatoes (Lycopersicon esculentum) (O Paz, HW Janes, BA Prevost, C Frenkel [1982] J Food Sci 47: 270-274) presumably due to stimulation of gluconeogenesis. This conjecture was examined using tomato fruit pericarp discs as a test system and applied i-[U-14C]malic acid as the source for gluconeogenic carbon mobilization. The label from malate was recovered in respiratory CO2, in other organic acids, in ethanol insoluble material, and an appreciable amount in the ethanol soluble sugar fraction. In Rutgers tomatoes, the label recovery in the sugar fraction and an attendant label reduction in the organic acids fraction intensified with fruit ripening. In both Rutgers and in the nonripening tomato rin, these processes were markedly stimulated by 4000 ppm acetaldehyde. The onset of label apportioning from malic acids to sugars coincided with decreased levels of fructose-2,6-biphosphate, the gluconeogenesis inhibitor. In acetaldehyde-treated tissues, with enhanced label mobilization, this decline reached one-half to one third of the initial fructose-2,6-biphosphate levels. Application of 30 micromolar fructose-2,6-biphosphate or 2,5-anhydro-d-mannitol in turn led to a precipitous reduction in the label flow to sugars presumably due to inhibition of fructose-1,6-biphosphatase by the compounds. We conclude that malic and perhaps other organic acids are carbon sources for gluconeogenesis occurring normally in ripening tomatoes. The process is stimulated by acetaldehyde apparently by attenuating the fructose-2,6-biphosphate levels. The mode of the acetaldehyde regulation of fructose-2,6-biphosphate metabolism awaits clarification.