28 SUBCELLULAR PARTICLES 



accordance with a concentration of hydrogen ions at the surface which is greater 

 than in the surrounding buffer medium (29). The chemical potentials of the hy- 

 drogen ions in the ambient solution and in the double layer about the kaolinite 

 particles are of course the same. 



Actually, a ApH ot this magnitude may also be calculated from the data in 

 table I. Alderton et ^?/. (i) give moving boundary values for the mobility of 

 lysozyme in bulk at pHij = 8.05 of about 4ju,/sec/v/cm, giving pHs = 8.05 + 0.87 

 = 8.9. For kaolinite, pHs = 8.05 — 1.05 = 7.0, and for lysozyme on kaolin ApH 

 = 0.53 at an ionic strength of 0.05. Thus the effective pH at the surface of 

 lysozyme molecules in solution is 8.9, whereas on kaolin it is somewhere between 

 7.0 and 7.5. The zeta potential measurements serve to show that there should be 

 a pH shift of between 1.4 and 1.9 units of pH between the surfaces of adsorbed 

 and unadsorbed lysozyme. 



Apparently the action of an enzyme at an interface can serve the role of a 

 'molecular pH meter.' The following considerations will show that this action can 

 be used to give an idea of the activity coefficient of hydrogen ions at the interface 

 of lysozyme-on-kaolinite and an aqueous solution. It has been shown that the in- 

 fluence of hydrogen ion concentration on chymotrypsin activity, with an inflection 

 point at about pH 6, may be attributed to the dissociation of histidine residues in 

 the enzyme molecule (30). Histidine is part of the active center of chymotryp- 

 sin (19), so the hydrogen ion concentration at the surface or in bulk solution de- 

 termines the degree of dissociation of histidine residues and hence the fraction of 

 maximum activity of the enzyme, the left hand portions of the activity curves in 

 figure 2. 



In dilute solutions fi.-^'i, so that Cs^C|,/f\j. In the experiment cited, Ci, is 

 known and the action of chymotrypsin on adsorbed substrate shows a shift of 2 pH 

 units compared with activity in solution. Now pH^ — pHi, for lysozyme in solution 

 was 0.87, which leaves about i.i units of ApH for the substrate-clay surface, or in 

 other words Cs^sio Ci, and f^=^o.i. In such experiments the order of addition of 

 enzyme and substrate to kaolinite greatly influences the rate of reaction. At pH 

 = 8.5 the rate is slower if the enzyme is adsorbed first (31 ). This could mean that 

 the H^ ion concentration is higher nearer the surface, as is to be expected, or that 

 the enzyme is not oriented with active centers all outward toward the substrate, or 

 that the mobility of the enzyme is reduced. The first reason must be important 

 since the difference in rates becomes less at pHi, = 9.i, at which the pH of the 

 surface of the kaolin must be high enough to support nearly a maximum enzyme 

 activity by an almost fully active population of enzyme molecules regardless of the 

 order of addition of enzyme and substrate to kaolin (40). 



A locality in nature where the above phenomena are probably important is the 

 soil. In soil, microorganisms liberate enzymes which hydrolyse organic com- 

 pounds of many varieties. If such compounds, or the enzymes, are adsorbed on 



