114 1. MALONATE 



(c) The accumulation of succinate due to malonate inhibition would, of 

 course, not be reversed by fumarate; instead, it is usually increased. If some 

 response to malonate is dependent on the rise in succinate concentration 

 (e.g., a direct effect of succinate on other enzymes or some cell function, or 

 the increased formation of some substances derived from the succinate), 

 this would not be antagonized by fumarate. 



(d) The response to fumarate will often depend on what is being measured. 

 One example will be used here to illustrate this and others will be men- 

 tioned later. Malonate inhibits the oxidation of trilaurin and octanoate 

 in liver and kidney slices, and also reduces the amount of C^^Og formed from 

 labeled substrates (Geyer et al., 1950 a). It was found that fumarate is very 

 ineffective in counteracting the inhibition of C^^O, production, and this 

 might be attributed to a direct effect of malonate on fatty acid oxidations. 

 However, the results can be explained on the basis of an inhibition of suc- 

 cinate oxidase. Much of the C^* taken into the cycle from acetyl-CoA 

 would accumulate in succinate and fumarate would have no effect on this. 

 For the full release of CO2, the operation of the entire cycle is required, and 

 thus fumarate would increase the C^^Og formation only moderately in the 

 presence of malonate. If the oxygen uptake had been determined, fumarate 

 could well have shown a complete reversal of the malonate inhibition. 



(e) Absence of a reversal by fumarate, or the failure to achieve a complete 

 reversal, can be due to a variety of causes. In an experiment, such as the one 

 shown in the following tabulation, done on Aplysia muscle slices: 



Additions Qq^ 



although it was stated that fumarate was unable to relieve the malonate 

 inhibition (Ghiretti et al., 1959), it may simply be that fumarate would not 

 have been oxidized if added to the uninhibited tissue. Certainly the direct 

 inhibition of succinate oxidase would not be expected to be overcome by 

 fumarate, but in any case a control with fumarate alone must be run to 

 give any significance to the results. It is even possible for fumarate to in- 

 crease the inhibition produced by malonate. This was observed for the respi- 

 ration of Helix hepatopancreas (see accompanying tabulation) (Rees, 1953). 

 Here the endogenous respiration apparently is little dependent on the cycle, 

 whereas upon the addition of fumarate, through the formation of oxalace- 



