SUBSTRATES OTHER THAN SUCCINATE 105 



On the other hand, 4-10.5 millimoles/kg of malonate injected intra- 

 venously into rabbits does not increase plasma citrate appreciably (Forss- 

 man and Lindsten, 1946), and a number of reports have indicated a depres- 

 sion of citrate formation by malonate. Eat brain and liver homogenates oxi- 

 dizing oxalacetate, or pyruvate and oxalacetate, form less citrate in the 

 presence of 10 mM and 30 milf malonate, respectively, this being attributed 

 to an inhibition of oxalacetate decarboxylase (Pardee and Potter, 1949). 

 The formation of citrate from acetate in yeast is inhibited 73% by 17 mill 

 malonate, while simultaneously succinate accumulates markedly (Barron 

 and Ghiretti. 1953). The incorporation of C^^ from glucose into citrate in 

 potato tuber slices is also depressed 71% by 50 mM malonate (Table 

 1-19) (Romberger and Norton, 1961). Although there is an increase in 

 citrate in excised tobacco leaves during culture with malonate, this increase 

 is generally less than in the controls, so that this probably represents an 

 inhibition of citrate formation (Table 1-20) (Vickery, 1959; Vickery and 

 Palmer, 1957). Finally, malonate inhibits the formation of citrate from a 

 variety of substrates in kidney and testis breis, the effects being surprisingly 

 large for the reasonable concentrations of malonate used (Table 1-21) 

 (Hallman, 1940). It is, therefore, evident that citrate levels may be affected 

 by malonate in a variety of ways, depending on the malonate concentration, 

 the type of preparation, and the conditions of the experiment. It is not dif- 

 ficult to explain the falls in citrate level brought about by malonate, since 

 this could arise either by a depression of succinate oxidation (reducing the 

 rate of entry of acetyl-CoA into the cycle) or inhibitions of other reactions 

 (such as the condensation of oxalacetate and acetyl-CoA), especially at the 

 high malonate concentrations often used. It is, on the other hand, difficult 

 to interpret the accumulation of citrate and to this end we must direct 

 our efforts, although only suggestions can be offered because of the lack 

 of sufficient data. 



Citrate is being formed and metabolized continuously and thus, generally 

 speaking, a rise in the citrate level implies an inhibition of citrate utili- 

 zation or an acceleration of its formation, or both. Although there is little 

 evidence for a direct affect of malonate on isocitrate utilization, we have 

 noted that an inhibition of succinate oxidation can interfere with isocitrate 

 oxidation by depletion of NADP mediated by a fall in malate concentration 

 (Jones and Gutfreund, 1964). This could certainly contribute to the accu- 

 mulation of the tricarboxylates in some instances. 



The inhibitions of citrate oxidation in Table 1-14 can be mostly explained 

 on the basis of a block at the succinate oxidase step. On the other hand, 

 there is certainly no evidence that the formation of citrate via the cycle 

 can be stimulated by malonate, most data pointing instead to a depression 

 if there is any effect. It would appear that effects of malonate on the cycle 

 alone are not sufficient to explain an accumulation of citrate. Other path- 



