ACCUMULATION OF SUCCINATE 103 



a phenomenon seen with most ionic substances, and this must account for 

 the poor accumulation of succinate in this organ and the relative lack of 

 effect of malonate on central nervous system function. 



The low concentration of succinate in the blood is interesting since it 

 implies that succinate does not leave the tissues readily. The conclusion 

 that must be reached is that the rate at which succinate diffuses out of the 

 tissues into the blood is slower than the rate of renal excretion of the 

 succinate. Substances that are not resorbed by the renal tubules are excret- 

 ed rapidly and their concentrations in the blood can be maintained at a 

 low level despite a continuous influx. Nevertheless, it shows that succinate 

 leaves the tissue cells rather slowly under physiological conditions. The slow 

 penetration of malonate into the tissues is suggested by the fact that the 

 peak levels in the blood occur around 30 min after administration whereas 

 the peak levels in liver and tumor occur 30 min later (Fig. 1-11), the kid- 

 ney concentration paralleling the blood levels because of the excretory 

 function of this organ. 



The degree of succinate accumulation does not necessarily reflect the 

 cycle activity in a tissue. For one reason, succinate can often be formed 

 from other pathways. In certain tissues the amino acid content rises mark- 

 edly (e.g. + 215% in thymus and + 160% in spleen) during malonate 

 inhibition, while in others, especially the Flexner-Jobling carcinoma, the 

 amino acids decrease as the succinate increases. It is likely in the latter 

 tissues that some of the succinate is derived from amino acids, probably 

 mainly glutamate. Thus the cycle activity in this tumor may be quite low 

 and the normal accumulation of succinate due to other sources for the suc- 

 cinate. The other tumors do not show such marked decreases in amino acid 

 content and this was attributed to their greater necrosis. It may be recall- 

 ed that the incorporation of acetate by Flexner-Jobling tumor is slower 

 than in most tissues and very little labeled succinate is formed from la- 

 beled acetate (Busch and Potter, 1953), indicating a low degree of cycle 

 activity. We have seen that this tumor also differs from normal tissues in 

 the nonlinearity of the plot of tissue succinate against tissue malonate 

 (Fig. 1-12). At low levels of malonate the ratio (succinate)/(malonate) is 

 near 3 but at high concentrations diminishes to 0.5. This means that as 

 the malonate concentration rises, the ability of the tumor to accumulate 

 succinate decreases. This is the type of curve expected if malonate at higher 

 concentrations is inhibiting the reactions forming succinate. If the supply 

 of cycle substrates in this tumor is low, a relatively small block of the cycle 

 might reduce the formation of succinate through the cycle to zero. The 

 slope approaches that of the blood, and it is possible that above the inflection 

 point the slow rise in succinate may be due only to the rise in the blood. 



This type of investigation could well be applied to other inhibitors 

 and certain chemotherapeutic agents. First, one is able to correlate tissue 



