186 1. MALONATE 



Significance of Respiratory Inhibition 



Does the degree of malonate inhibition indicate the contribution of the 

 cycle to the total oxygen uptake? This must certainly be answered in the 

 negative. Lack of inhibition can be due to a failure to penetrate, the me- 

 tabolism of malonate, a source of oxalacetate external to the cycle, met- 

 abolic adaptations of the cells, and many other factors. Positive evidence 

 of inhibition is more valuable than absence of inhibition, but even when 

 definite inhibition is observed the possibility of actions other than in the 

 cycle must be considered, especially when the malonate concentration must 

 be high to achieve an effect. It is doubtful if anyone examining Table 1-26 

 would attempt to correlate the inhibitions with the importance of the 

 cycle in the organisms and tissues. For example, in general there is greater 

 inhibition of mammalian endogenous respiration than of the respiration of 

 microorganisms or plants. This might indicate a greater role of the cycle 

 in mammalian tissues, but it could also be attributed to a poorer penetration 

 in the plants and microorganisms, or to a greater metabolic flexibility and 

 adaptability in these more resistant forms. It must also be clear that the 

 degree of respiratory inhibition bears no necessary relationship to the de- 

 gree of inhibition of succinate dehydrogenase. A significant inhibition by a 

 reasonable concentration of malonate is evidence for the operation of the 

 cycle, but the quantitative aspects of the contribution cannot be derived 

 from these data alone. The effects of malonate on the endogenous respira- 

 tion are sometimes of greater physiological significance than effects on the 

 oxygen uptake in the presence of high concentrations of often abnormal 

 substrates, since the endogenous metabolism may be representative of a 

 more normal balance of substrates. In this connection, studies of inhibitors 

 would often be improved if the attempt were made to provide the cells 

 with a mixture of physiologically pertinent substrates at the concentrations 

 normally occurring in the cellular environment. 



PERMEABILITY OF CELLS TO MALONATE 



One of the major problems in the use of malonate has always been the 

 degree of penetration of the inhibitor into the cells or tissues, and it has 

 been frequently stated that this is the primary factor responsible for the 

 low inhibitions observed in many cases. It is true that the plasma mem- 

 brane is relatively impermeable to most ions, particularly anions and those 

 carrying two or more charges, but if this is so how can one explain the 

 marked respiratory stimulations usually seen with succinate or other di- 

 carboxylate ions? Furthermore, malonate is often metabolized readily by 

 tissues and this presupposes entrance into the cells. Since there are other 

 possible reasons for a resistance to malonate, the permeability hypothesis 

 must be examined critically. 



