718 14. EFFECTS OF pH ON ENZYME INHIBITION 



electrodes for intracellular recording will allow more accurate measurements 

 to be made. The thorough review by Caldwell (1956) covers these problems 

 and presents a complete summary of the intracellular pH values that 

 have been obtained in a variety of cells. Examining the data carefully, 

 eliminating the results from inaccurate methods and from probably ab- 

 normal cell preparations, one may arrive at reasonably consistent values 

 for the most likely pH/s in important types of cells. It would appear that 

 the following cells generally have a pHj between 6.8 and 6.9: skeletal 

 muscle, cardiac muscle*, liver, pancreas, kidney, gastric mucosa, tumors 

 (various sarcomas and carcinomas), and amebas. The pH, in some cells is 

 a little higher — frog and invertebrate nerve (7.05) and erythrocytes (7.2) — 

 and in others lower — marine invertebrate eggs (6.65) and yeast (5.8). 

 The vacuolar fluid in large algal cells and the higher plants is usually be- 

 tween 5.8 and 6.0, but there are no reliable data for the pH of plant proto- 

 plasm. In any event, the majority of cells may be considered to have a 

 pH, in the neighborhood of 6.8 and this value will be assumed in the exam- 

 ples and calculations of this chapter. 



Although the normal pH^ probably does not fluctuate widely under phys- 

 iological conditions, it drops fairly quickly when the cell is injured. As 

 muscle becomes fatigued and passes into rigor, the pH, usually falls 0.8- 

 1.0 units, and cells that have sustained enough injury to produce cytolysis 

 will often show a drop of 1.2-1.8 units. The pH, is closely related to the me- 

 tabolism for several reasons — the release of protons in oxidations, the for- 

 mation and utilization of many weak acids, the ionization reactions of 

 nucleotides, etc. — and thus anoxia appears to be capable of altering pHj 

 readily. In experiments involving living cells with regard to the pH de- 

 pendence of inhibition it is necessary to ensure i)hysiological conditions, 

 particularly to provide as normal a medium as possible and to maintain 

 oxygenation of the cells. 



Effects of Inhibitors on Intracellular pH 



There are two important ways in which inhibitors can alter the pH,. 

 The first is by entry into the cell of undissociated acid or dissociated base, 

 as illustrated by the case of the unbuffered cells discussed above. The 

 second is through their effects on the cell metabolism. Inasmuch as anoxia 

 readily causes a fall in pH,, one might expect many inhibitors to do likewise. 

 In part this would be due to the accumulation of acid intermediates, such 

 as lactic acid, and in part due to shifts in the equilibria of the adenine 

 nucleotides and creatine phosphate. If the formation of lactic acid by gly- 



* Preliminary and unpublished experiments by Dr. Philip Hollander using a micro- 

 pH-electrode in rat atrium and ventricle indicate the normal pH, for actively con- 

 tracting tissue to be 6.8. 



