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The presence of the seven carbon (7C) sugar D-manno-heptulose
(D-manno-2-ketoheptose) and its corresponding sugar alcohol perseitol
(D-glycero-D-galacto-heptitol) as the predominant sugars in
avocado fruits is of considerable interest for two reasons. First, D-manno-heptulose
is a potent inhibitor of plant and animal hexokinase and second, nothing is
known about the role of 7C sugars in avocado fruit growth and development. In
this paper the contribution of avocado 7C sugars was explored. Results show
that D-manno-heptulose and perseitol are major soluble sugars in phloem
sap, leaf petiole exudates, and in seed and mesocarp of avocado. Whereas levels
of perseitol remained constant throughout fruit growth, D-manno-heptulose
levels increased linearly until maturity. D-manno-heptulose does not
appear to be a precursor of perseitol in fruit tissue. However, exogenous D-manno-heptulose
increased the endogenous pool of perseitol in avocado seed and enhanced
activity of microsomal 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR,
EC 1.1.1.34). Although perseitol did not affect seed HMGR when applied to whole
fruit, this 7C sugar effectively negated the deleterious effects of tert-butylhydroperoxide
and hydrogen peroxide in in vitro assays. It is concluded that D-manno-heptulose
and perseitol fulfill several important functions during avocado fruit growth.
These are: modulation of carbon flux, and as a protectant against reactive
oxidative damage of key enzymes such as HMGR required for growth and
development of the fruit.

South African Journal of Botany
2004, 70(1): 75–82