144 PHOSPHATES AND THERAPEUTIC AGENTS 



Depression of phosphorus/oxygen ratios is clearly therefore not 

 a general property of depressants nor yet of all barbiturates. It 

 seems probable that in several instances much of the depression is 

 due to the activation of mitochondrial adenosine triphosphatase. 

 Thus, it has been shown (Bain, 1957; Maxwell and Nickel, 1954; 

 Abood, 1955; Bernsohn et aL, 1956; Century and Horwitt, 

 1956) that depressants and convulsant barbiturates and chlorpro- 

 mazine, at concentrations decreasing the phosphorus/oxygen ratios 

 markedly, increase the adenosine triphosphatase activity of the 

 mitochondrial preparations. Since an increased activity of adeno- 

 sine triphosphatase results in an increased formation of inorganic 

 phosphate, measurement of oxidative phosphorylation by deter- 

 mining the overall quantity of inorganic phosphate removed from 

 the medium under such conditions will yield a decreased phos- 

 phorous/oxygen ratio. However, since the function of adenosine 

 triphosphatase is not known such activation in itself does not 

 preclude the possibility that the action of some therapeutic agents 

 in the mitochondrial system is an " uncoupling " of oxidative 

 phosphorylation. Difficulties in ascribing to them such an action 

 arise from a lack of knowledge of the mechanisms by which the 

 " classical " uncoupling agents such as dinitrophenol, produce 

 their effects. In some interesting experiments designed to compare 

 the action of the two types of agents Bain (1957) found that 

 thiopental at concentrations sufficient to decrease the phosphorus/ 

 oxygen ratios by 50% had no effect upon the incorporation of 

 radioactive phosphate into the intra-mitochondrial adenosine 

 triphosphate nor into adenosine triphosphate in the medium. 

 Dinitrophenol, also at concentrations completely suppressing a 

 total uptake of phosphate, nevertheless failed to prevent a rapid 

 exchange of inorganic phosphate with adenosine triphosphate 

 in the mitochondria. Neither of these phenomena are satisfac- 

 torily explained by a process involving an activation of adenosine 

 triphosphatase since in such a case, the system might be expected 

 to turnover faster and thus increase the amount of labelled phos- 

 phate incorporated. However, an increase in activity of adenosine 

 triphosphatase is not likely to be a simple stimulation of the 

 enzyme(s) for depressant barbiturates at concentrations of 10"^ M 

 neither increased nor decreased its activity when measured in 

 aqueous dispersions of brain (Gore, 1951). The phenomenon is 



