152 1. MALONATE 



the dicarboxylic amino acids particularly might be expected to bind mal- 

 onate to some extent, but there is only indirect evidence for this. The 

 dehydrogenation of glutamate with methylene blue as an acceptor in 

 toluene-treated E. coli is inhibited about 20% by 71 vaM malonate (Quastel 

 and Wooldridge, 1928), but it may be that all of the hydrogen atoms do 

 not arise from glutamate here and that some other reaction is inhibited. 

 In Walker carcinosarcoma, kidney, and liver, glutamate is metabolized 

 readily to succinate in the presence of 6.3 milf malonate (Nyhan and 

 Busch, 1957), but no controls are avilable so that some inhibition is pos- 

 sible. Aspartate and glutamate are metabolized by Hemophilus parain- 

 fluenzae; malonate does not interfere with the oxidation of the former but 

 does inhibit glutamate oxidation (Klein, 1940). Contrary to these results, 

 malonate completely inhibits aspartate oxidation in rat liver homogenate 

 (Nakada and Weinhouse, 1950). It was believed rightly that this could not 

 be entirely attributed to an inhibition of succinate oxidase. 



Glutamate may be converted to a-ketoglutarate by either glutamate 

 dehydrogenase or transamination, or it may be decarboxylated to y-amino- 

 butyrate; the decarboxylase is limited mainly to certain bacteria and the 

 nervous system of animals, so the major product is usually a-ketoglutarate, 

 which can be oxidized through the cycle or participate in transaminations 

 whereby it is reconverted to glutamate (this occurs also with y-amino- 

 butyrate so that the net reaction forms succinic semialdehyde, ammonia, 

 and CO2 from glutamate). The pattern of glutamate metabolism will depend 

 on the relative activities of these various enzymes, the availability of other 

 amino acids for transamination, and the supply of NAD for the glutamate 

 and a-ketoglutarate dehydrogenases; likewise, the response to malonate 

 inhibition will depend on these factors. If malonate selectively inhibits 

 succinate oxidation, the O2 uptake due to glutamate should be reduced 

 moderately (perhaps around 25-50%) unless much of the a-ketoglutarate 

 formed is transaminated and does not enter the cycle. Malonate, however, 

 occasionally inhibits the formation of ammonia from glutamate, indicating 

 some effect on the oxidative deamination. Malonate also inhibits the oxi- 

 dation of glutamate by guinea pig mammary gland mitochondria completely 

 (Jones and Gutfreund, 1961), which would not be the case if only succinate 

 oxidation were blocked. The glutamate respiration of rat brain mitochon- 

 dria is depressed 88% by 17.3 mM malonate (see note in Table 1-14) 

 (Levtrup and Svennerholm, 1963), which would indicate that glutamate 

 is being converted mainly to a-ketoglutarate by transamination (glutamate 

 decarboxylase is not present in brain mitochondria). Similar high inhibi- 

 tions by 20 mM malonate are observed in the mitochondria from pigeon 

 muscle, rat heart, rat liver, and ascites cells (64-99%) (Borst, 1962). Con- 

 clusions as to the pathway of glutamate catabolism based on the results 

 with malonate depend on the assumption that the inhibition is specifically 



