IV. BIOCHEMICAL SYSTEMS 239 



iiihil)ition was^ eliminated in the presence of cysteine. ^^^ A suggestion that 

 (iehyclroascorhie aeid was the agent responsible for reacting with the SH 

 group of the enzyme'^'-* and that the protective elTect of thiol (compounds 

 was due to the reduction of dehj'^droascorl^ic acid was shown to be untenable 

 when it was shown that dehydroascorbic acid did not inactivate urease."" 

 In further studies'**" it was found that ascorbic acid itself does not inhibit 

 urease activity. In the presence of Cu++, however, it does so by effecting 

 the reduction of CU++ -^ Cu+, which latter ions have a much higher affinity 

 for — Sn groups than the former. With mercury salts the reverse effect of 

 ascorbic acid was observed, namely an activation of urease activity. This 

 was correlated with the fact that enzymic activity was reduced more by 

 Hg++ than by Hg+ salts. The action of ascorbic acid on urease could there- 

 fore be explained in terms of its reducing action on metallic ions present in 

 solution. The inhibiting action of ascorbic acid on plant /3-amylase is prol)- 

 ably explicable on a similar basis, since it has been shown that the inhibition 

 was increased in the presence of small amounts of copper salts. '*- 



It is of interest to note that the activating effect of ascorbic acid on 

 papain occurs only if ferrous ions are present; otherwise the effect of the 

 \'itamin is depressant.'^- There is evidence suggesting that the ascorbic 

 acid-iron complex activates by first reducing dithio compounds associated 

 with the enzyme, and that these thiol compounds in turn activate the 

 enzyme."*^ Similar reactions may be involved in the activation of arginase 

 by ascorbic acid-iron complex.'®^ 



The action of ascorbic acid on many enzymes appears to be conditioned 

 by other substances, notably metallic ions, present in reaction mixtures; 

 certainly where its action has been critically examined, this has been found 

 to be so. Whether ascorbic acid in vivo has any regulatory influence on these 

 enzymes is uncertain. Still more improbable is the view that the action of 

 ascorbic acid constitutes one of its essential roles in the living cell, since 

 in the case of urease it has been shown that other dienols which are bio- 

 logically inactive react similarly.'®" 



5. Ascorbic Acid and Phosphatase 



As early as 1932 it was observed that the alkaline phosphatase activity 

 of the plasma of infants and young children suffering from scurvy was low,'®^ 



160 L. W. Mapson, Biochem. J. 40, 240 (1946). 



!«' K. Giri and P. Seshagiri Rao, Nature 153, 253 (1944). 



'«2 L. Hellerman and M. Perkins, J. Biol. Chem. 107, 241 (1934); E. Maschmann 



and E. Helmert, Hoppe-Seyler's Z. physiol. Chem. 224, 56 (1934). 

 i«3 A. Purr, Biochem. J. 29, 513 (1935). 

 '^ A. Purr, Biochem. J. 27, 1703 (1933); A. Purr and L. Weil, Biochem. J. 28, 740 



(1934). 

 >65 J. Smith and M. IMaizels, Arch. Disease Childhood 1, 149 (1932); J. Smith, ibid. 8, 



215 (1933). 



