AMINO-ACID CATABOLISM I9 



leucine, tyrosine, methionine, ornithine, tryptophan, 

 phenylalanine, cysteine and ethanol also act as H-donors, 

 whilst tryptophan, tyrosine, ornithine and arginine function 

 as H-acceptors [33, 65]. It will be noted that some amino- 

 acids serve both as an acceptor and as a donor, and whilst 

 the reaction is specific for the L-isomer of H-donors [cf. 56], 

 there is no stereochemical specificity w^ith respect to 

 H-acceptors [65]. 



If CI. sporogenes is grown in the presence of glucose, it 

 develops an active hydrogenase which enables the reducing 

 component of the Stickland system to be replaced by mole- 

 cular H2 [33]. The reaction can then be followed in terms 

 of an uptake of Hg and the end-products are only those 

 derived from the amino-acid added as the H-acceptor. In 

 an analogous manner, and perhaps unexpectedly since the 

 organisms are strict anaerobes, the H-acceptor part of the 

 system can be replaced by Og [46]. A number of amino- 

 acids, all H-donors in the Stickland reaction, were oxida- 

 tively deaminated by washed suspensions of CI. sporogenes 

 to the corresponding a-keto acid. The uptake of Og was 

 appreciably reduced by the presence of a H-acceptor amino- 

 acid and restored to its former value by the addition of 

 arsenite. The latter can readily be explained since although 

 arsenite completely inhibited H-transfer to a H-acceptor 

 amino-acid (e.g. from leucophenosafranine to proline) it 

 had no eflFect on the H-donor part of the system (e.g. the 

 reduction of phenosafranine by alanine) [47]. Hence the first 

 step in the Stickland reaction is probably catalysed by a 

 type of L-amino-acid oxidase which, perhaps with the aid 

 of one or more carriers, can transfer hydrogen to Og, a dye 

 or another amino-acid. In cell-free extracts, DPN but not 

 TPN was readily reduced by H-donor acids [44], but 

 whether reduced DPN is the natural carrier and reacts 

 directly with H-acceptor amino-acids is not known. Pyru- 

 vate and other a-keto acids are oxidized by CI. sporogenes to 

 CO 2 and a fatty acid containing one less carbon atom by a 

 mechanism which can lead to the fornjation of energy- 

 rich phosphate groups (cf. reaction h): 



CH3COCOOH+H3PO4— > CHgCO^POsHa+zH+CO., [47] 



