32 SUBCELLULAR PARTICLES 



independent of pHj, over a wide range (24). The buffering power resides more in 



salts than in proteins, and on prolonged fermentation the interior pH value may 



exceed 6 whereas at the outer layers a drop to 4.2 may occur. An example 



wherein local, point-to-point variations of [H*] may play a part in the rate of a 



reaction is the steady state of reduced pyridine nucleotides in yeast (5). Assum- 



, ,, , , . ,. [HM IDPNHl [acetaldehyde] . , . . 



mg that k for the reaction k= ' ^. ,^;' , 7^ is the same in vivo 



^ [DPN+] [ethanol] 



as /« vitro, Chance calculated an intracellular 'pH' about two units higher than 

 expected from solution studies. 



Enzyme Reactions in Gels and at Liquid-Liquid Interfaces. With such prob- 

 lems in mind it is clear that the study of enzyme reactions in solution, although 

 a preliminary step, cannot be expected to be sufficient for a thorough understanding 

 of the enzymology of a living cell. Enzymology must eventually develop more 

 closely in companionship with cell morphology. Cytochemical localizations of 

 enzymes represent a primitive beginning in this direction; the effort must be made 

 to correlate reaction rates with the ion-exchange properties and three-dimensional 

 arrays of macromolecules as gels and membranes (4). 



Katchalsky and colleagues (37) have been studying the potentiometric behavior 

 of simple gels which can be characterized by two factors — the electrostatic inter- 

 action among the ionic constituents and the contractility of the polymer network. 

 These studies should provide useful models for describing cell structures of inter- 

 est to the enzymologist. In substance, in order to relate the pH of bulk solution to 

 the degree of ionization, a, of the gel network and the number of small ions in 

 the gel per monomer unit, p, equation 7 is revised to give 



pH,-pH,= '/, log [^ = •/, log £+" 



[A„ J p — a 



where the subscript g applies to the gel and X^ represents molal fractions of 

 univalent small ions. The pH differences between the two phases can thus be 

 evaluated from ionic concentrations. The differences were found to be of the order 

 of 0.2 to 1.2, depending on the ionic strength of the external solution. Although 

 the Donnan theory cannot be expected to apply well to living cells, since they are 

 not at thermodynamic equilibrium but rather in steady state equilibrium (4), such 

 equations may be pertinent to two-phase systems within a cell. Mazia (35) has 

 prepared molecular fibers of albumin plus pepsin which undergo self-digestion at 

 about the same pH), optimum at which pepsin digests albumin in solution, and 

 the theory could presumably be checked enzymatically by careful quantitative 

 comparisons. 



If, in the organized cell, enzymes operate as parts of structures, then a new kind 

 of enzyme kinetic theory must be formulated. The theory must take into account 

 the diffusion of substrate to and from the structure, or else the diffusion of the 



