42 CHOLINE 



formate becoming the /3-carbon of the new serine molecule, appears defi- 

 nitely established. In in vitro experiments with rat liver homogenates, 

 Winnick et al}^^ isolated carboxyl-labeled serine after addition of C^^-car- 

 boxyl-labeled glycine and serine labeled in the a- and jS-carbons after addi- 

 tion of C^'*-a:-carbon-labeled glycine. Following the feeding of C^^-carboxyl- 

 labeled glycine and radioformate (C^'^) to rats, Sakami^^^ isolated serine 

 from liver tissue and found C^'^ in the serine carboxyl group and C^^ in the 

 iS position. Confirmatory evidence for the formation of serine from glycine 

 was supplied by Greenberg and his coworkers as a result of in vivo studies 

 in rats^"" and in vitro experiments with rat liver slices. ^"^ Ehrensvard et al}^"^ 

 noted a rapid conversion of C^^-carboxyl-labeled glycine to serine by yeast. 



Serine is also a source of formate, possibly by reversal of the reaction in 

 which formate is added to glycine. Shemin^f*^ isolated hippuric acid after 

 the administration of benzoic acid and C ^^-carboxyl-labeled, N^^-labeled 

 serine in rats and guinea pigs and found the same N^^ : C^* ratio in the gly- 

 cine component as in the serine. C^''-formate is produced from C^^-/3-carbon- 

 labeled serine by rat liver in in vitro experiments. ^^^ 



The mechanisms of the glycine-serine reactions are not known. Shemin^"^ 

 suggested the possibility of a removal of two hydrogens from the /3-position 

 of serine with the formation of a-amino-/3-ketopropionic acid as the immedi- 

 ate precursor of glycine and formic acid. On the other hand, a serine dehy- 

 drase has been reported in mouse^"* and rat liver.^"^ In the latter instance 

 a-aminoacrylic acid was postulated as the product of the removal of water 

 from the /3-carbon of serine. Ratner et al}^^ have prepared a glycine oxidase 

 from liver and kidney of several species. Its prosthetic group is 

 flavin adenine dinucleotide, and the enzyme converts glycine to glyoxylic 

 acid plus ammonia and sarcosine to glyoxylic acid plus methylamine. 

 Paretsky and Werkman^"'^ concluded that formaldehyde is an intermediate 

 in the dissimiliation of glycine by Achromohacter . 



The evidence is clear-cut also for the decarboxylation of serine to amino- 



138 T. Winnick, I. Moring-Claesson, and D. M. Greenberg, J. Biol. Chem. 175, 127 



(1948). 

 199 W. Sakami, /. Biol. Chem. 176, 995 (1948). 

 ^^o P. D. Goldsworthy, T. Winnick, and D. M. Greenberg, /. Binl. Chem.. 180, .341 



(1949). 



291 P. Siekevitz and D. M. Greenberg, J. Biol. Chem. 180, 845 (1949). 



292 G. Ehrensvard, E. Sperber, E. Saluste, L. Reio, and R. Stjernholm, J. Biol. Chem.. 

 169, 759 (1947). 



203 D. Shemin, J. Biol. Chem. 162, 297 (1946). 



204 F. Binkley, J. Biol. Chem. 150, 261 (1943). 



295 E. Chargaff and D. B. Sprinson, J. Biol. Chem. 151, 273 (1943). 



296 S. Ratner, V. Nocito, and D. E. Green, J. Biol. Chem. 152, 119 (1944). 



297 D. Paretsky and C. H. Werkman, Arch. Biochem. 25, 288 (1950). 



