the common observation that GS requires a reducing environment for full activity suggests that this en-
zyme may be under some form of redox control. Certainly, further study is warranted.
F. Cyclitols and Galactosylcyclitols
Cyclitols are similar to the linear polyols already discussed except that their carbon skeletons are cyclized
to form a ring structure (Figure 9). The most common cyclitols are inositols, of which myo- and chiro-in-
ositol are the most prevalent. In addition to these simple forms, these cyclitols are often encountered as
their methylated derivatives, pinitol and ononitol, and may be further complexed with galactose to form
galactosylcyclitols [52]. The most prevalent galactosylcyclitol in nature is galactinol, which is probably
a reflection of its important role in synthesis of raffinose oligosaccharides.
The methylated derivatives often accumulate in source leaves during periods of water stress [53–56],
suggesting that like the linear polyols, cyclitols may play some as yet unknown role in stress tolerance.
G. Seven-Carbon Sugars
Whereas most plants synthesize carbohydrates based on a hexose framework, certain plant species are
able to synthesize and translocate significant amount of novel seven-carbon (C7) sugars. Despite their in-
frequent occurrence in the plant kingdom, these sugars appear to have important physiological functions
in the species in which they are found. The best studied of the C7 sugars are the ketoheptuloses, sedo-
heptulose and mannoheptulose, and their polyol forms, volemitol and perseitol (Figure 10). Sedoheptu-
lose, in its mono- and bisphosphorylated form, is an important intermediate in the Calvin cycle. Its polyol
478 PATTANAGUL ET AL.Figure 9 Structures of some common galactosylcyclitols.
Figure 10 Structures of common seven-carbon heptuloses and their polyhydroxy derivatives.