Plants glucose-6-phosphatase

In plants, inositol is biosynthesized from glucose 1-phosphate via inosose 1-phosphate, which is reduced into ID-inositol 3-phosphate (INOl pathway). After dephosphorylation by inositol phosphate monophosphatase, myo-inositol is fed into biosynthesis of inositol phospholipids via a Kennedy-like sequence. In mammalian organisms, inositol is acquired from the diet and is also biosynthesized via INOl. The latter pathway, rather than the inositol phosphate phosphatase, seems to be a more promising target of antipsychotic drugs (17, 18) (see Fig. 2). 

Sucrose is a highly soluble disaccharide that provides a mobile energy source for all the plant cells. Sugar cane stores large amounts of sucrose in its leaves and stalk, whereas sugar beet stores it in roots. All plants make sucrose from two molecules of fructose 6-phosphate. One molecule is activated with UDP and isomerized to UDP-glucose. Sucrose 6-phosphate synthase reacts with UDP-glucose and fructose 6-phosphate to make sucrose 6-phosphate. The latter then reacts with a phosphatase to produce sucrose (Fig. 2.9). 

The synthesis of sucrose by Leloir and collaborators has been discussed previously. These workers found that UDP-D-glucose is the D-glucose donor for formation of sucrose. The synthesis takes place by means of two separate enzymes, one utilizing D-fructose as the acceptor, and the other, D-fructose 6-phosphate. The sucrose phosphate formed in the second reaction is hydrolyzed by a phosphatase, resulting in the formation of free sucrose. As the equilibrium of the reaction for the formation of sucrose phosphate lies to the right, and inasmuch as the large accumulation of sucrose in some plants could be better accounted for by hydrolysis of the sucrose phosphate with phosphatase (which is a practically irreversible reaction), it was suggested that sucrose in plants is most likely synthesized by way of the sucrose phosphate intermediate. 

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