306 ASCORBIC ACID 



sulfate or collagen fractions isolated from cartilage and skin. Essentially 

 all the C^^ found in the nasal septum was present as ascorbic acid. In ex- 

 periments with rats, Jackel et al}'^ used C^* glucose labeled uniformly in all 

 positions and employed chloretone as a stimulant to ascorbic acid syn- 

 thesis. The total transfer of C^^ from glucose to ascorbic acid in 24 hours 

 was about equal to the total conversion of administered carbohydrate to 

 to ascorbic acid. It appeared that the glucose carbon chain was used in 

 ascorbic acid synthesis without changing the ratio of activity in positions 

 1 or 2 to the total activity. 



A number of investigators have suspected that ascorbic acid may be 

 concerned directly with the synthesis of glucuronic acid. A recent study 

 of the problem by Mosbach and King^"^"" indicates, however, that the vi- 

 tamin is probably not the precursor of this substance. 



The results of these experiments are of considerable interest and appear 

 to indicate that ascorbic acid is not used as such or in a modified form, at 

 least in appreciable amounts, in the building of structural elements in cells. 



(b) As a Component of an Enzyme. Evidence that ascorbic acid may 

 function as a coenzyme in the oxidation of tyrosine has been presented by 

 Sealock and coworkers-"^ • -°''<= and is discussed in the sections on metabo- 

 lism and respiration (pp. 312, 333). 



2. Relation to Growth 



a. In Plants 



Direct proof of a growth relation of ascorbic acid in green plants is diffi- 

 cult because it is always present in metabolically active tissues, since, like 

 the rat, they have the capacity to synthesize it. Unfortunately there is no 

 equivalent of the guinea pig in the realm of green plants. The highest con- 

 tent of the vitamin is found in regions of highest metabolic acti\'ity as in 

 the leaves, in certain stages of some of the floral organs, and in the growing 

 regions of shoots and roots. The concentration in regions of high metabolic 

 activity suggests a possible function in growth. A number of investigators 

 have found that very weak concentrations of ascorbic acid in the nutrient 

 medium have a stimulatory effect on germination, but, on the other hand, 

 some have obtained negative results.-^^--^^ Experiments conducted by 



207a E. H. Mosbach and C. G. King, /. Biol. Chem. 185, 491 (1950). 



^o'b R. R. Sealock and R. L. Goodland, Science 114, 645 (1951). 



2"c R. R. Sealock, R. L. Goodland, W. N. Sumerwell, and J. M. Brierly, /. Biol. 



Chem. 196, 761 (1952). 

 207d ciegg, R. E., and Sealock, R. R., J. Biol. Chem. 179. 1037 (1949). 



208 L. Havas, Nahire 136, 435 (1935). 



209 S. von Hansen, Nature 136, 516 (1935). 



210 W. Davics, G. A. Atkins, and P. C. B. Hudson, Ann. Botany (London) [N.S.] 1, 

 329 (1937). 



