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Conversion of Labeled Substrates to Sugars, Cell Wall Polysaccharides, and Tartaric Acid in Grape Berries

Zenzaburo Kasai

The Radioisotope Research Center, Kyoto University, Kyoto 606, Japan, The Research Institute for Food Science, Kyoto University, Uji 611, Japan

[U-¹⁴C]Sucrose, myo-[U-¹⁴C]inositol, [6-¹⁴C]- and [U-¹⁴C]glucuronate, UDP-[U-¹⁴C]glucuronate, [U-¹⁴C]gluconate, and L-[1-¹⁴C]ascorbic acid were fed into grape berries, Vitis labrusca L. cv. Delaware, at intervals throughout the ripening process and incorporation of ¹⁴C into several metabolites was studied.

[U-¹⁴C]Sucrose was the most effective precursor of cellulose in young grape berries and of glucose and fructose in mature berries. On the other hand, UDP-[U-¹⁴C]glucuronate was the best precursor of pectic substance, followed by [¹⁴C]glucuronate and myo-[U-¹⁴C]inositol. L-[1-¹⁴C]Ascorbic acid was the most effective precursor of tartaric acid. In young berries, [U-¹⁴C]sucrose and [U-¹⁴C]gluconate also produced labeled tartaric acid, the latter a somewhat better precursor in the 3 weeks following flowering. The remaining test compounds were only poor sources of ¹⁴C for tartaric acid although all three, glucuronate, UDP-glucuronate, and myo-inositol, were utilized by the grape berry for pectin biosynthesis.

These results strongly indicate that tartaric acid is synthesized by a C-1 oxidation mechanism of hexose in young grape berries.