Hexose Sugars as Precursors

There are a number of papers in the literature, the results of which suggest that the vitamin is synthesized from a hexose sugar in both plants and animals. Studies on this subject have been concerned with supplying possible precursors and ascertaining the effect on the ascorbic acid content of the tissues. With plant tissue, seeds have been selected by many workers as the best material, because they are devoid of ascorbic acid but produce it during germination, even in the absence of light. The synthesis can therefore be studied under conditions in which photosynthesis cannot occur, and thus must take place at the expense of a carbon source derived from the reserves of the seed. 

In many cases, however, the effect of such treatments on the concentration of ascorbic acid in the cells is small. Hence interpretation of the data is complicated by the fact that an increase in the concentration of the vitamin might arise either (1) by an increase in the rate at which it is being synthesized, or (2) by a decrease in its rate of destruction. In other words, the increase in concentration may be related to conditions favoring stability of the vitamin, rather than to those favoring synthesis. 

Of the other commonly occurring hexose sugars, glucose and fructose, together with sucrose, appear to be equivalent as sources from which ascorbic acid may be formed (Butkewitsch, 1938 ltd and Mizumo, 1948 Mapson et al., 1949). 

Negative results have been obtained with pentose sugars and with L-sorbose (Mapson et al., 1949 Abei, 1951) this is of interest in view of the structural similarity of that sugar and L-ascorbic acid. 

The early evidence suggesting that ascorbic acid may be formed from a hexose sugar in animals is very unconvincing. Widenbauer and Koschor-rech (1937) claimed that the ascorbic acid content of slices of intestinal 

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