ENZYMES 45 



school, is that of selective adsorption. It has been found 

 that enzymes, in virtue of their colloidal and amphoteric 

 nature, are readily adsorbed by such materials as kaolin, 

 kieselguhr, charcoal and alumina. By carrying out 

 these adsorptions, using appropriate adsorbants and 

 appropriate conditions of acidity or alkalinity, it has been 

 found possible to adsorb one enzyme and leave others in 

 solution, and then, by altering the pH. value of the solu- 

 tion in which the adsorbed complex is suspended, to 

 wash out or " elute " the enzyme again and so obtain it 

 free from other enzymes or inactive accompanying sub- 

 stances. Kaolin is negatively charged and will adsorb 

 positively charged basic substances, whilst alumina is 

 positively charged and adsorbs negatively charged acidic 

 substances. It is obvious that by altering the 2^H of 

 the solution the charge on the adsorbant, the enzjrme, 

 and the complex of the two can be altered and the adsorp- 

 tion or elution of the enzyme controlled. Thus the enzyme 

 peroxidase, which has basic properties, is adsorbed on 

 kaolin from dilute acid solution, and can then be eluted 

 from the adsorbate (the complex of adsorbant and enzyme) 

 by dilute ammonia. Invertase which, together with 

 maltase, is adsorbed on alumina from acid solution can 

 be selectively eluted by a solution of acid phosphate 

 which does not remove the maltase. Or invertase can 

 be adsorbed on kaolin in acid solution and eluted with 

 dilute sodium hydroxide solution. 



The methods adopted for the crystallisation of enzymes 

 usually involve the use of fairly concentrated enzyme 

 in an appropriately buffered solution of a salt at fairly 

 low temperatures. For example, pepsin crystallises when 

 an alkaline solution is brought to 2^H 3 with sulphuric 

 acid. Pepsinogen can be crystallised, after preliminary 

 purification, from 0-4 saturated ammonium sulphate 

 solution at ^^H 6-5. After fractionation of a pancreatic 

 extract by ammonium sulphate, chymotrjrpsinogen can 

 be obtained as long needles by crystallisation at pK 5 



