AMINO-ACID CATABOLISM 23 



found in Esch. colt, Ps. fluorescens, Serratia marcescens, Pr. 

 vulgaris, Lb. casei, and perhaps in yeasts [67, 17, cf. 21]: 



COOH.CH2CH(NH2)COOH # COOH.CHrCH.COOH+NHs 



The system is reversible and by using cells treated with 

 cyclohexanol to prevent conversion of the fumarate to suc- 

 cinate or malate, it can be shown that the equilibrium 

 favours the synthesis of L-aspartic acid [67]. Serine and 

 threonine are deaminated anaerobically to pyruvic and 

 a-ketobutyric acid respectively, though whether one de- 

 aminase catalyses both reactions is not yet known. Unlike 

 aspartase, the reaction does not appear to be reversible and 

 the first step is thought to be the removal of the elements 

 of water, followed by the spontaneous hydrolysis of the 

 resulting imino-compound [i4<2]: 



CH2(OH)CH(NH2)COOH > (CH2:C(NH2)COOH) -^ 



CH3C(:NH)COOH 



CH3C(:NH)COOH ^ CH3COCOOH+NH3 



The dehydration step is analogous to that catalysed by 

 enolase, and cysteine desulphurase may likewise be regarded 

 as first removing the elements of HgS: 



CH2(SH)CH(NH2)COOH+H20=CH3COCOOH+H2S+NH3 



Serine and threonine deaminase activity is found in Esch. 

 colt, CI. welchii, Ps. pyocyanea, Proteus OX-ig and staphylo- 

 cocci: cysteine desulphurase occurs in Sac. cerevisiae, Esch. 

 colt, Pr. vulgaris, B. subtilis and Propionibacterium pentosea- 

 ceum [24]. 



Aspartase, cysteine desulphurase, and the serine and 

 threonine deaminases are all alike in that their activity is 

 dependent on the presence of certain co-factors, the identity 

 of which has not yet been completely established. For 

 example, there was a marked reduction in the aspartase 

 activity of washed cell suspensions of Esch. coli after they 

 had been kept standing in water or buffer. This decay in 

 activity could be prevented by the addition of a small amount 

 of either adenylic acid (AMP) or orthophosphate, together 

 3 



