Il8 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



identified chemical composition. According to this concept, the metabolic event 

 is directly coupled with the transport of electrolyte. However, other mecha- 

 nisms are equally plausible. Conditions suitable for active metabolism might 

 permit the mitochondrial unit to accumulate anionic metabolic intermediates 

 which in turn would demand the accumulation of a cation for electro-neutrality. 

 Such a situation is suggested by the reported accmulation of added substrates 

 (i) and, as described below, by the accumulation of certain phosphate esters. 

 Under these conditions the uptake of a specific cation may be only remotely 

 related to the metabolic event and may depend primarily upon certain inherent 

 characteristics of the membrane which are sufficiently stable as to be inde- 

 pendent of continued metabolism. Although other possibilities might also be 

 cited, these examples may serve to emphasize the hazard of over-simplifying 

 the nature of ion transport in the mitochondrial unit which, at present, can 

 only be defined in descriptive terms. 



RELATION OF POTASSIUM EXCHANGE TO PHOSPHATE METABOLISM 



In dealing with in vitro experiments a procedure commonly employed has 

 been that of reconstituting the system by the addition of various components. 

 If an added factor has a demonstrable effect, it may then be considered either 

 to be essential for the reaction under study or to participate in it in some 

 manner. Because of the role of the adenine nucleotides in the generation of 

 energy rich phosphate bonds, these compounds have been examined for their 

 effect on electrolyte transport by adding them to suspensions of respiring mito- 

 chondria. MacFarlane and Spencer (7) found an adenylic acid requirement 

 for the maintenance of mitochochondrial potassium, while no effect could be 

 demonstrated in our laboratory. The difference in the results is probably at- 

 tributable to the experimental conditions, namely the addition of phosphate 

 buffer to the incubation medium. Because of its stimulating effect on the res- 

 piration of washed particles, orthophosphate has been widely employed in 

 studies on oxidative metabolism, and hence has been incorporated routinely 

 by several investigators in the incubation medium used for the study of potas- 

 sium metabolism of mitochondria (i, 3, 7). However, controlled studies reveal 

 that orthophosphate markedly depresses the level of mitochondria potassium 

 (17). This inhibitory effect is shown in table 4. The addition of adenylic acid 

 in the absence of added orthophosphate has little effect on the level of mito- 

 chondria potassium, but in the presence of orthophosphate adenylic acid is defi- 

 nitely stimulatory, i.e. it reverses the inhibitory effect of the phosphate ion. 

 Although the mechanism of this action has not been defined, simple considera- 

 tions of stoichiometry indicate that it cannot be attributed to a lowering of 

 the orthophosphate concentration by the formation of phosphate esters. In 

 addition, it may be emphasized that the essentiality of adenine nucleotides for 

 electrolyte transport is not established by these experiments, due to the fact 



