682 



6. ARSENICALS 



ical inhibition. The data are limited quantitatively by the fact that arsen- 

 ical inhibition is usually progressive; most of the values are for inhibition 

 over 1-hr periods, but in some cases the interval is not stated in the report. 

 Various factors quite markedly modify arsenical inhibition (see page 684) 

 and thus it is difficult to compare results from different investigations. The 



KIDNEY CORTE) 



o.oor 0.01 



ARSENITE »- mM 



Fig. 6-4. Inhibition of the respiration of rat tissues by arse- 

 nite. (From Elhott and Norris, 1945.) 



data in the table indicate that mammalian tissues are inhibited to approxi- 

 mately the same extent, their respiration being depressed 35-75% by 

 arsenite at concentrations around 1 mM. One might tentatively write the 

 tissues in the order of their sensitivities as 



Kidney > brain > testis > muscle > liver > heart 



based not only on data in the table but work on minces and other prepara- 

 tions, but this does not seem to correlate with any known differences in 

 metabolic patterns. The microorganisms vary a great deal more in the 

 response to the arsenicals but in general are fairly resistant, with the 

 exception of the trypanosomes, which is surprising because these protozoa 

 apparently do not readily metabolize pyruvate. 



Arsenical-Resistant Respiration 



Concentration-inhibition curves for cellular respiration are typically 

 sigmoid with low slopes, often covering a 1000-fold range of concentration 

 (Fig. 6-4). These curves are much less steep than those obtained with 



