AbstractsChemistry

Glucose-1, -2 and -6-C¹⁴ substrates were incorporated into individual tomato fruits in order to study the catabolic fate of substrate glucose, especially with regard to the possible occurrence of triose recombination, as well as to investigate the pathways leading to the biosynthesis of C₄ acids in the fruit. After metabolizing the administered glucose-C¹⁴ substrates for a period of 12 hours, the fruits were individually processed for the separation of organic acids and carbohydrates. Subsequently, both glucose and malic acid isolated from the tomatoes were degraded in order to determine their isotopic distribution pattern. Data obtained from the carbohydrate fractions reflected the presence of a large endogenous pool of carbohydrates in the fruit, and a comparison of the specific activities of fruit glucose and fructose indicated that glucose-6-phosphate and fructose-6-phosphate probably are not in isotopic equilibrium in this organism. The isotopic distribution pattern in fruit glucose clearly established the occurrence of triose recombination via both the Embden-Meyerhof-Parnas (EMP) and the pentose phosphate (PP) pathways. A theoretical analysis of the impact of triose recombination on the equations derived for the estimation of pathway participation indicated that recombination processes do not significantly alter the previously calculated values. The examination of organic acids isolated from the fruits indicated that the EMP pathway, in conjunction with the tricarboxylic acid cycle, is the primary source of carbon skeletons for the biosynthesis of fruit acids. The isotopic distribution pattern of malic acid demonstrated that a CO₂-fixation reaction of the C₃ + C₁ type is largely responsible for the net biosynthesis of C₄ acids in tomato fruit.