V. SPECIFICITY OF ACTION 



241 



all ai)par«'iitl>' spccilic type of (raii.sainiiiatioii lias also hecn observed.-^' -^ 

 Not nuu'li is known about the enzymes carrying out tlie series of reactions 

 illustrated, but there is adequate evidence that they exist. 



C. SPECIFICITY AND MECHANISM OF ACTION 



In all cases so far known, the enzymatically active form of vitamin Be 

 is pyridoxal-5-phosphate and the activity of pyridoxamine phosphate ob- 

 ser\ed oi-casionally^^-^ is due to the presence of auxiliary enzyme systems 

 converting it to pyridoxal phosphate. It appears that the principal storage 



CHs-xXHj CHO 



HOfT^CHjOH ^ HOjr^CH^OH 



H,C^H - H3CI^H 



N N 



Pjridoxamine Pyridoxal 



CHO 

 HOfj^CHjOPOjHj 

 HsCl^H 



N 



Pyridoxal- 5- phosphate 

 (coenzyme form) 



CH2OU 

 ^ HOrr^CH^OH 



N 



Pvridoxine 



CH2-NH: 

 HOif^CHjOPOiH 

 H,C^H 

 N 



CH2OH 

 HOr^CHjOPOsH. 

 HjCL^H 

 N 



Pyridoxamine Pvridoxine 



pliosphate phosphate 



Interconversion of Members of the Vitamin Be Group 



product of the phosphorylated vitamin is pyridoxamine phosphate but that 

 it is itself inactive. The fact that only the p3'ridoxal-5-phosphate is active 

 in a variety of enzyme systems listed previously makes any general postu- 

 late of a mechanism of action somewhat premature. Such mechani.sms as 

 have been proposed for either decarboxylation or transamination have 

 postulated the formation of a Schiff's base between the 4-aldehyde group 

 of pyridoxal phosphate and the a-amino group of the amino acid. ^2. 30-39 



" W. W. Umbreit, D. J. O'Kane, and I. C. Gunsalus, J. Biol. Chem. 176, 629 (1948). 



" A. Meister and S. V. Tice, J. Biol. Chem. 187, 173 (1951). 



" A. Meister, H. A. Sober, and S. V. Tice, J. Biol. Chem. 189, 577-591 (1951). 



" S. R. Ames, P. S. Sarma, and C. A. Elvehjem, J. Biol. Chem. 167, 135 (1947). 



^° H. Blaschko, Advances in Enzymol. 5, 67 (1945). 



