136 NITROGEN METABOLISM 



base and sugar in the complete absence of phosphate and 

 other nitrogenous bases has been observed with prepara- 

 tions from bakers' yeast [12] and Lb. pentosus [29]. 



The removal of amino groups from the nitrogenous bases 

 is accomplished by highly specific deaminases, but apart 

 from the cytosine deaminase of Esch. coli and yeast [cf. 14], 

 the adenosine deaminase oi Neurospora [35] and the cytidine 

 deaminase of brewers' yeast and Esch. coli [53], little is 

 known about the occurrence and properties of these enzymes 

 in micro-organisms. 



Oxidation and fermentation of purines and pyrimidines 



Application of the enrichment culture technique has led 

 to the isolation of organisms capable of utilizing purines and 

 pyrimidines as sole sources of carbon, nitrogen and energy. 

 Thus CI. cylindrosporum ferments uric acid to NH3, CO 2 

 and acetic acid, while CI. acidi urici, in addition to uric acid, 

 can also utilize xanthine, guanine and hypoxanthine [2]. 

 Since neither of these species attacks allantoin, it has been 

 suggested that they degrade uric acid by a route diiferent 

 from that found in animal tissues, and there is some evidence 

 that glycine is an important intermediate in these fermenta- 

 tions. Unlike the Clostridia, Micrococcus aerogenes attacks 

 neither uric acid nor glycine, and lactic acid is the main 

 acidic end-product in the fermentation of adenine or gua- 

 nine [54]. On the other hand, Micrococcus lactilyticus is 

 unable to utilize adenine, guanine or uric acid but ferments 

 hypoxanthine and xanthine to H2, COg, NH3 and urea, 

 together with propionic and acetic acids [55]. A number of 

 aerobic bacteria (species of Nocardia, Corynehacterium^ 

 Mycobacterium and an unidentified soil organism) obtain 

 carbon, nitrogen and energy by the oxidation of pyrimi- 

 dines [30, 52]. Uracil and thymine are both oxidized to 

 barbituric acid, which is then split into urea and malonic 

 acid: the urea is subsequently decomposed by urease into 

 CO 2 and NH3, but the further steps in the metabolism of 

 malonic acid are unknown. The oxidation of thymine pro- 

 ceeds by way of 5-methylbarbituric acid rather than by 

 direct demethylation to uracil. 



