ANALYSIS OF DRUG ACTION 



8i 



The analysis is further compHcated by the occurrence 

 of very marked species differences. For example, when 

 we take the inhibitory action of HCN on cellular processes 

 or say the lethal action of HgS, it is tempting to work 

 on the hypothesis that in all cases the drug is acting on 

 the same enzyme system, but Figs. 15 and 16 show that 

 the concentrations required to produce a given degree of 



100 



log [HiS] 



Fig. 16. The relationship between concentration of H2S and its lethal 

 action on the spores of eight different species of fungi, (a) Venturiain- 

 equalis and Uromyces caryophyllinus; (b) Puccinia antirhini; (c) Sclerotina 

 americana: (d) Macrosporidium sarcinaeforme; (e) Pestolatia stellata; (f) 

 Glomerella cingulata; (g) Botrytis 



action vary widely from species to species. Various ex- 

 planations of this are possible. It may be that the drug 

 does not produce its effect by acting on the same en- 

 zyme in all species. Or it may be that a given enzyme in 

 different species varies in its susceptibility to a given 

 drug. A third possibility is that the effective concen- 

 tration of the drug which arises in the vicinity of the 

 enzyme is different in different cells, although the ex- 

 ternal concentration of drug is the same. 



