Translocated sorbitol is converted to fructose by NADSDH
or to glucose by NADP-SDH. NAD-SDH activity
in apple fruit was high in young fruit, decreased with
fruit enlargement and increased again with fruit
maturation (Yamaki and Ishikawa, 1986). Because the
fluctuation pattern of NAD-SDH activity during apple
fruit development corresponded to that of mRNA
expression, NAD-SDH activity is controlled by
transcriptional regulation (Yamada et al., 1999).
Nosarzewski et al. (2004) found high activity and high
amount of NAD-SDH protein during fruit setting and in
the early development stages by ELISA. SDH1 and
SDH3 localized in flesh and seeds, SDH2 in flesh and
SDH6 and SDH9 only in seeds, and NAD-SDH activity
in seeds was clearly higher than in flesh for 2 to 5 weeks
after fruit setting in apple (Nosarszewski and Archbold,
2007). Suzuki et al. (2001) suggested that accumulation
of fructose in apples fruit is not produced only by an
increase in NAD-SDH activity, but also by a decrease
in fructokinase activity.