The growth of Clostridium sticklandii on the substrate pair L-alanine-L-proline (redoctanthsidant each 40 m31) in a medium containing 2 g/l yeast extract was completely inhibited by equiniolar amounts of glycine, although glycine itself should be used as oxidant by the cells. The effect, of glycine was t.he same, whether L-alanine. L-arginine, or L-serine were used as reducdants. Performance of the growth experiments in media of high osmolarity excluded the possibility that the inhibition effect,ed by glycine was caused by the synthesis of defective cell wall peptidoglyran. I n cell-free extracts an inhibit.ion of L-proline reduction by glycine was observed that did not belong to anyone of the known types of kinet,ic inhibition. It depended upon the presence of a functioning glycine-reducing enzyme system, besides glycine itself, and was lost after the purificat'ion of Dproline reductase. It was concluded from t,hese results that' a protein. besides glycine, participated in the inhibition of L-proline reduct,ion. The regulatory implicat'ions of the inhibition for the energy metabolism of C . sticklandii are discussed.Clostridium sticklandii, and other peptolytic clostridia, may nietabolize pairs of anlino acids, which are not fernieritable as single substrates. in pathways coupled by a sequence of redox reactions: Electrons subtracted from one ainino acid, the reductant or hydrogen donor, are transferred to the other aniino acid, the oxidant or hydrogen acceptor (STICKLAND 1934(STICKLAND , 1935. STADTMAN 1954, for a suniniary see BBRKER 1961). This type of fermentation was called STICKLAND reaction by WOODS (19:36) and later workers, and two-substrate fermentation with amino acids by us. The classical clectron-accepting reactions of C . sticklandii comprise the reduction of glycine by the enzyme complex of glycine reductase (STADTMAN 1969. CONE et al. 1977), and the reduction of proline by D-prohne reductase (STADTMAN 1956, STADTMAN and ELLIOTT 1957, SETO and STADTMAN 1976), besides other electron-accepting reactions presently under investigation (SCHWARTZ and ISCHAFER 1973, SCHAFER and SCHWARTZ 1976).The reduction of glycine, either by NADH, or by dithiothreitol as reductants has been reported to be accoinpanied by the foriiiation of ATP froin ADP and inorganic phosphate (STADTMAN et nl. 1958, STADTMAN 19CiS), whereas the reduction of D-proline by either reductant does not seem to be coupled to ATP synthesis (nnpublished experinients).The existence of several electron-accepting enyziiie reactions in C . sticklaiidii poses a regulatory problem: I n the presence of more than one oxidant in the medium, that oxidant should be preferentially used for growth which is most favourable for the organism in energetic terms. This situation applies to oxidants as well. The occurrence of the preferred reaction, and the prevention of the unwanted reaction may be ensured by several regulatory principles, as it has been amply demonstrated in other cases : These are induction and repression on the level of the genes, or activati...
