688 6. ARSENICALS 



media point out once more how the experimental conditions can modify 

 the results obtained with inhibitors, but are difficult to understand since 

 the addition of pyruvate and glutamate would be expected to make the 

 respiration more sensitive to arsenite. Actually, the respiratory rates in 

 medium III were not much higher than in the KRP medium, and the highest 

 rates were seen in the tissues respiring endogenously on the mats. In any 

 event, these two investigations demonstrate clearly that near-lethal doses 

 of arsenite can quite markedly disturb respiratory activity in most tissues. 



Mechanism of Respiratory Inhibition 



A relatively selective depression of keto acid oxidation by the arseni- 

 cals has been observed in many organisms and tissues, if one assumes that 

 the accumulation of pyruvate and a-ketoglutarate is valid evidence for 

 this. It has thus been generally thought that respiratory inhibition by the 

 arsenicals is mainly or completely due to the action on these steps in the 

 cycle, and arsenicals have been used to demonstrate the existence of the 

 cycle and the quantitative importance of the cycle in the total respiration. 

 There certainly are better means of establishing the presence or absence 

 of the cycle, but the use of the arsenicals to determine the contribution by 

 the cycle requires further comment. This problem has been touched on 

 previously (page 662) and it was concluded that the nature of the concen- 

 tration-inhibition curves, and the high concentrations often required to 

 inhibit respiration markedly, make it difficult to decide how much of the 

 inhibition arises from depression of the cycle and how much is contributed 

 from actions elsewhere of the arsenicals. If one is using these inhibitors 

 to estimate the cycle activity, one must choose a concentration, or range 

 of concentrations, and how is this to be done ? It might be suggested that 

 the concentration producing the maximal accumulation of keto acids be 

 used; however, if this is determined, there is little point in measuring 

 respiratory depression. Furthermore, one cannot be sure that even at 

 arsenite concentrations below 1 m.M there is no inhibition of enzymes 

 other than the keto acid oxidases; for example, lipoate functions in several 

 systems and a few other enzymes are quite sensitive (page 655). It may 

 well be possible to find a concentration which in a particular tissue and under 

 proper conditions will selectively interfere with the oxidation of keto acids 

 and the operation of the cycle, but there is no way to do this without a 

 through investigation of the tissue's metabolism and its response to the 

 arsenical. The results in Table 6-7 do not, I feel, reflect the cycle contribution 

 to the respiration, particularly for those tissues, such as kidney or heart, 

 in which the cycle is probably the major oxidative pathway.* 



* Tissue preparations usually are functionally inactive and this may reduce the 

 relative importance of the cycle. 



