166 1. MALONATE 



into the cycle and thereby lead to a greater availability of acetyl-CoA for 

 choline acetylation. However, it is not known if the acetyl-CoA pool is 

 common to both the cycle and the synthesis of acetylcholine. In many 

 cases the effects of malonate are due only to a depression of the cycle oper- 

 ation and a decreased formation of ATP. For example, in the synthesis of 

 chondroitin sulfate in tibial condyles of chick embryos, the fixation of sulfate 

 is inhibited by malonate in a parallel fashion to the inhibition of respiration 

 (Boyd and Neuman, 1954). The fixation of sulfate requires ATP, as shown 

 by the marked inhibition with 2,4-dinitrophenol, so that here the mechanism 

 of malonate action is simply an inhibition of energy formation. Other proc- 

 esses, such as calcium deposition in tibial cartilage (Hiatt et al., 1953), 

 do not require energy and are not inhibited by malonate. 



EFFECTS OF MALONATE 

 ON THE ENDOGENOUS RESPIRATION 



The alterations of the most important metabolic pathways by malonate 

 have been discussed, and we shall now conclude this aspect of the subject 

 with a survey of the effects on the total oxygen uptake of cells respiring in 

 the absence of any external substrate. Although the interpretation of the 

 results of such studies is very difficult, the changes in the endogenous respi- 

 ration have been examined more frequently than any other response to 

 malonate. There is, thus, a vast and variable mass of data, some of which 

 is summarized in Table 1-26. The aim of most of these investigations has 

 been to demonstrate the absence or presence of the cycle in the types of 

 cells tested, and we must attempt to assess the validity of conclusions based 

 on the response to malonate. The most unsatisfactory work has been done, 

 and the most unjustified conclusions have been drawn, in studies of this 

 type, inasmuch as the inherent complexities of the situations have seldom 

 been appreciated. Although the cycle has a wide distribution in the cells of 

 microorganisms, plants, and animals, its operation during the metabolism 

 of endogenous substrates is quite variable and dependent on the state and 

 past history of the cells. 



Factors That May Determine the Degree of Malonate Inhibition 



Certain basic factors should be considered in every investigation of the 

 susceptibility of the endogenous respiration to malonate. Although some 

 of these have been mentioned previously and some will be taken up in 

 greater detail later, it may be convenient to enumerate here the most im- 

 portant. 



(a) Intrinsic susceptibility of succinate dehydrogenase to malonate. This en- 

 zyme from different species varies a good deal in its ability to bind mal- 



