AMINO-ACID CATABOLISM I7 



although it was noted that in addition to pyridoxal phos- 

 phate, the enzyme preparations contained riboflavin, a pos- 

 sible carrier of hydrogen. The bond joining indole to the 

 side chain appears to be susceptible to reduction since indole 

 is readily formed in vitro when tryptophan is either refluxed 

 with Raney nickel and absolute alcohol containing a little 

 HCl, or boiled with aqueous NaOH and catalytic amounts of 

 Cu"^"^ or Co"^"*". A summary of the other mechanisms which 

 have been proposed to explain the mode of action of trypto- 

 phanase will be found in the review by Happold [29]. 



With the possible exception of tryptophanase, all the 

 enzyme systems discussed above accomplish the oxidative 

 catabolism of amino-acids in association with molecular O2 

 as the ultimate and natural H-acceptor. In anaerobic organ- 

 isms either amino-acids themselves or compounds derived 

 from them may fulfil this function. 



Stickland reaction 



Stickland was the first to demonstrate that amino-acids 

 take part in anaerobic oxido-reduction reactions, certain 

 acids acting as H-donors whilst others function as Pi- 

 acceptors. The original experiments were done with the 

 strict anaerobe CI. sporogenes which is capable of growing 

 in an amino-acid medium in the absence of carbohydrates, 

 and he suggested that the organism derived its energy from 

 reactions of this type [56]. Nutritional studies later revealed 

 that the organism only grew well in a medium containing 

 adequate amounts of the amino-acids shown by Stickland 

 to be H-acceptors and H-donors [22]. In Stickland's experi- 

 ments, washed cell suspensions were incubated anaerobi- 

 cally with the appropriate substrates in Thunberg tubes. 

 Hydrogen-donor amino-acids were detected by their ability 

 to reduce methylene blue or cresyl blue to the colourless 

 leuco form. Alanine, valine, leucine and pyruvate were all 

 active H-donors whilst phenylalanine, aspartic and glutamic 

 acids showed some activity, reduction of the dye being 

 accompanied by deamination of the amino-acid. Hydrogen 

 acceptors were detected by their ability to accept hydrogen 

 from the leuco form of a dye of suitable redox potential 



