Level of knowledge References

Kinetics Purified Gene" (not exhaustive)

Sorbitol NADPH-dependent aldose-6-phosphate reductase

sorbitol-6-pliosphate phosphatase not assigned

NADPH-dependent ketose reductase (sorbitol dehydrogenase)

Mannitol Mannose-6-phosphate isomerase

NADPH-dependent mannose-6-phosphate reductase

mannitol-1-phosphate phosphatase

Ribitol NADPH-dependent ribose-5-phosphate reductase not assigned

Galactitol NADPH-dependent galactitol-synthesizing not assigned aldose reductase

;l Kinetics, refers to in vitro studies of the kinetics of the enzyme; Purified, means that the protein was purified partially or to homogeneity; Gene, refers to the publication of the gene sequence.

activity (sorbitol synthesis) increases [20, 21]. Similar results were obtained for mannitol in celery leaves [42], Although both mannitol and sucrose are translocated out of celery leaf tissues, leaf age differences indicate that, unlike sucrose, mannitol utilisation is restricted to active sink tissues. Mannitol represents a more rigorously sequestered form of transport carbohydrate which is specifically devoted to the carbon supply of heterotrophic sinks [5],

3.1. Biosynthesis

Sugar alcohols are synthesised from hexoses or hexose-phosphates via reductases, or reductases and phosphatases (Table 1). The controlling step involves the reductase when there are two steps. With the exception of ketose reductase, the biosynthesis of sugar alcohols requires NADPH.

Lewis [4] suggested that the synthesis of sugar alcohols was cytoplasmic. This hypothesis was in line with the results of cell fractionation experiments performed for studying the enzymes of mannitol synthesis in celery [34], although some studies reported that sugar alcohol synthesis was chloroplastic [43. 44], In a later study, mannose-6-phosphate reductase, the central enzyme of mannitol biosynthesis was found predominantly in the cytosol of mature leaf mesophyll cells [38] using immunocytochemical analyses.

Recently, the expression of aldose-6-phosphate reductase, the key enzyme of sorbitol synthesis, was studied in terms of protein and RNA in developing peach leaves [45], The results confirmed the association of this enzyme with the source function.

3.2. Degradation

Sugar alcohols are degraded via dehydrogenases or oxidases (Table 2). Mannitol dehydrogenase, which catalyses the oxidation of mannitol to mannose in celery, was shown to be a cytoplasmic enzyme [46]. In a later study, it was shown to be present in the cytosol and nuclei [47], It is found in the meristems of celery root apices, in young expanding leaves, in the vascular cambium, and in the phloem [46], Pharr et al. [48] proposed a model of sugar repression of mannitol dehydrogenase expression. In a later study, evidence was presented that mannitol dehydrogenase activity is repressed by sugars in cultured celery cells [49], Furthermore, this sugar repression appears to be mediated by hexokinases [49, 50] in a way that is comparable to the reported sugar repression of photosynthetic genes [51].

The protein level of sorbitol dehydrogenase (a key enzyme in sorbitol degradation in Rosaceae trees) was studied during apple fruit development [52], The amounts of enzyme protein were very low in young fruit and rose during fruit maturation. A weak correlation was observed between enzyme protein level and activity, suggesting post-translational modifications. In a later study, the gene expression of sorbitol dehydrogenase was studied at the RNA level [53], Expression at the RNA level was proportionate to the levels of enzyme activity and protein content during apple fruit development.

In Thesium humile, a parasitic weed, mannitol is utilised as a respiratory substrate in darkness. In a recent study, the highest rate of mannitol metabolism occurred in the young leaves, but this metabolism was not totally restricted to growing organs. Mannitol dehydrogenase was detected in the young leaves [54],

3.3. Transport across the plasma membrane

The membrane transport of sugar alcohols, mainly sorbitol and mannitol, was studied to a

Table 2 Catabolism of

some sugar alcohols in higher plants (modified from Loescher and Everard [ I ] and updated).

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