Nucleotide Sequence of a cDNA Encoding NADP-Sorbitol-6-Phosphate Dehydrogenase from Apple.

Sorbitol plays a key role in the translocation of photosynthate in the Rosaceae family, including apple (Malus domestica), pear, and stone fruits (4, 7). NADP-S6PDH2 plays the most important role in biosynthesis of sorbitol in apple because its activity increases before accumulation of sorbitol in seedlings (8), and high activity is maintained in leaves (10). This enzyme reduces G6P to S6P using only NADPH (1, 6). Therefore, S6P dehydrogenase (EC 1.1.1.140), which catalyzes NAD-dependent conversion between fructose 6-phosphate and S6P in microorganisms (3), is different from NADP-S6PDH described in this paper. We report the isolation of a cDNA clone encoding NADPS6PDH from apple (Table I). To our knowledge, this is the first report of the complete primary structure of a sorbitolrelated enzyme from plants. A cDNA library constructed from apple seedlings was screened with the antibody against NADP-S6PDH purified from loquat leaves (1). The nucleotide and deduced amino acid sequences of the clone cDNA is shown in Figure 1.