487 Pyridines 



Attachment to the apoenzyme in vivo was assumed to 

 be at the pyridine nitrogen atom. Spectral data from such 

 model systems, however, when applied to purified en- 

 zymes, indicate that pyridoxal phosphate is bound to the 

 apoenzyme as a Schiff base in glutamate-aspartate amino- 

 pherase'-'* and in homoserine deaminase-cystathionase.^^ 

 In crystalline muscle phosphorylase pyridoxal is bound to 

 the apoenzyme, probably at a lysine e-amino group, as an 

 aldamine, involving an additional side-chain of the protein 

 (perhaps — SH).-'' -' 



Protein 



Protein HN X ^ 



o© \ / o® 



,N^ XH 



CH. 



H' ^CH (-H_o_p_OH ®0 I ' CHo— O— P— OH NaBH4 



o 



CH3 CH3 H® 



O 



Schiff Base Aldamine (X = S?) 



Protein 



HN XH 



O© 



CH I 



©O I ' CH.— O— P— OH 



i 



o 



Reduced Enzyme 



Glutamate-aspartate aminopherase contains 2 moles of 

 bound pyridoxal phosphate and muscle phosphorylase 4. 

 It is rather surprising to find the vitamin in an enzyme, 

 such as the latter, unrelated to its ordinary function. 

 Doubt has been cast on its function as a prosthetic group 

 in phosphorylase by several experiments, one of them the 

 reduction shown, which should have inactivated the 

 pyridoxal, but which did not inactivate the enzyme.-'^ It 



-^ Yoshihiko Matsuo and David M. Greenberg, /. Biol. Chem. 230 

 545, 561 (1958); idem., ibid. 234 507, 516 (1959). 



-° Alan B. Kent, Edwin G. Krebs and Edmond H. Fischer, /. Biol. 

 Chem. 232 549 (1958). 



-" Barbara Illingworth, Hendrlk S. Jansz, David H. Brown and 

 Carl F. Cori, Proc. Nat. Acad. Sci. 44 1180 (1958). 



■-** Edmond H. Fischer, Alan B. Kent, Eloise R. Snyder and Edwin G. 

 Krebs, /. Am. Chem. Soc. 80 2906 (1958). 



