IV BIOSYNTHESIS OF AMINO ACIDS 63 



Threonine aldolase. Liver and kidney en/.ymes catalyze the breakdown of threo- 

 nine to glycine and acetaldehyde (Braunstein and Vilenkina, 1949; Lin and Green- 

 berg, 1954) (Fig. 24): 



COOH COOH 



I I 



CH — NHo CHo 



I I 



CH-OH ^ NH3 4- CHO 



CH 



CH. 



3 *-M3 



Threonine Glycine Acetaldehyde 



Fig. 24. Threonine aldolase reaction. 



It is possible that this reaction is of some significance in mammalian tissues. 

 Thus, after administering threonine-4-''*C, ^^N to rats, ^^N glycine and '"^C 

 acetate were isolated (Meltzer and Sprinson, 1952) while with threonine- 1, 2-^'*C, 

 glycine-^'^C was obtained (Lien and Greenberg, 1952). 



{c) Cysteine 



The carbon atoms of cysteine are derived from serine in microorganisms and 

 in mammals. Thus, in E. coli, non-labelled serine suppresses the incorporation of 

 labelled glucose to cysteine (Abelson, 1954) while in Neurospora (Abelson and 

 Vogel, 1955), serine-3-''*C is incorporated into protein cysteine. The nitrogen is 

 also derived from serine since in the rat, serine-^^N is converted to cysteine-'^N 

 (Stekol, 1954). 



Methionine is a potential source of the sulfur needed for cysteine synthesis. 

 Homocysteine, and cystathionine are intermediates in this process: 



Methionine — > homocysteine + serine 



> 1 



cystathionine 



T 

 i i 



homoserine + cysteine 



The evidence is as follows: a) methionine, homocysteine, and L-cystathionine can replace 

 cysteine in the diet of the rat, h) the feeding of methionine plus bromobenzene increases 

 the excretion of mercapturic acids, c) the feeding of methionine increases the urinary 

 cysteine of cystinurics, d) methionine-''S is converted to cysteine-^'S of rat hair and 

 to L-cystathionine-'^S of rat liver (Tabachnick and Tarver, 1955), e) when methionine 

 labelled with sulfur-34 and carbon- 13 on the beta and gamma carbons, respectively, was 

 administered to rats, only the labelled sulfur was converted to cysteine. L-cystathionine-"S 

 was also converted to cysteine in the rat but the '■'Q of cystathionine was converted to 

 cysteine only when the label was present on the aminopropionic acid portion of the 

 cystathionine molecule, J) cysteine formation from homocysteine and serine or from 

 L-cystathionine takes place in rat liver homogenates while the reverse reaction, the for- 

 mation of cystathionine from serine and homocysteine is also catalyzed by liver enzymes. 

 Pyridoxal phosphate is required in the synthesis of cysteine from serine and homocysteine 

 (Stekol, 1954). 



The conversion of sulfate to cysteine. The intermediary steps in the conversion of 

 sulfate to cysteine have been investigated by Hockenhull (Hockenhull, 1949). 



Literature p. 124 



