EFFECTS ON METABOLIC PATHWAYS 163 



Rhodopseitdomonas spheroides is inhibited 50% by 20 mif malonate and 

 75% by 40 mM malonate (Lascelles, 1956), indicating again some inhibition 

 of the formation of succinyl-Co A. 



The incorporation of iron into heme, as demonstrated with Fe^^, is not 

 inhibited by 10 mM malonate in canine reticulocytes (Yoshiba et al., 1958) 

 but is inhibited 26% in chicken erythrocytes (Kagawa et al., 1959). It is 

 possible in the latter case that the inhibition is due to the chelation of part 

 of the Fe^^, making it unavailable for incorporation. 



EFFECTS OF MALONATE 

 ON MISCELLANEOUS METABOLIC PATHWAYS 



There have been many reports on the actions or lack of action of malonate 

 on enzyme reactions or metabolic pathways of varying degrees of impor- 

 tance. Some of these are worth mentioning, either because they indicate 

 areas where further study might be profitable or because they provide some 

 evidence for noncycle actions of malonate. 



One might expect very little effect of malonate on photosynthesis but, 

 although very little work has been done, in every case some effect has been 

 observed. Even the Hill reaction is susceptible to inhibition (Ehrmantraut 

 and Rabinowitch, 1952). This reaction is the photochemical oxidation 

 of water with the production of oxygen and the reduction of a substance, 

 usually quinone, other than COg. In Chlorella this reaction is inhibited 

 30% by 6 mM malonate and 50% by 60 mM malonate. The inhibition is, 

 surprisingly, prevented by fumarate, indicating that the site of action is 

 succinate dehydrogenase and that this enzyme takes part in the transport 

 of hydrogen in the Hill reaction, which would not be the case if quinone 

 were serving as the immediate hydrogen acceptor. It may also be that 

 malonate does not inhibit the Hill reaction directly, but depletes the cells 

 of cycle intermediates or other cycle products necessary for the Hill reaction 

 to proceed. Malonate not only inhibits the total incorporation of C^^Og in 

 Scenedesmus by about 20%, but almost completely blocks the formation of 

 labeled malate (Bassham et al., 1950). This was taken as evidence that mal- 

 ate is not on the direct line of phosphoglycerate synthesis, but it also dem- 

 onstrates that by some mechanism malonate can inhibit COg incorporation. 

 An inhibition of glucose formation in Chlorella by malonate has also been 

 reported (Kandler, 1955), although there is less inhibition in the light than 

 in the dark. The synthesis of glucose was believed to be closely related to 

 the formation of high energy phosphate intermediates, and it is thus inter- 

 esting that malonate inhibits the photosynthetic phosphorylation of ADP 

 in Rhodospirillum, although the inhibition is only 17% at the very high 

 concentration of 100 mM (Smith and Baltscheffsky, 1959). With these lim- 

 ited observations on the effects of malonate, it must be admitted that it 



