276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



When evidence has been obtained for the existence of some interest- 

 ing new natural product, one of the first aims of the research worker is 

 to try and isolate it in the pure state. Stage by stage, inert matter is 

 removed from the crude product, and the progress of purification can 

 often be followed by observing an increase in a particular biological 

 activity per unit weight. If the worker is lucky, he may eventually 

 obtain the pure material which shows a constant and maximum bio- 

 logical activity, unchanged after going through the motions of a fur- 

 ther purification. The research worker is most happy if his product 

 turns out to be a crystalline solid, as purification by recrystallization is 

 often one of the easiest techniques, especially if it becomes necessary 

 to work on a small scale because of scarcity of material. Crystals usu- 

 ally have well-defined physical properties which are valuable criteria 

 of purity. As soon as it was realized that biological material, such as 

 yeast juice or gastric juice from the stomach, probably owed its activi- 

 ties to the presence of specific enzymes, efforts were made to isolate 

 these for precise examination. For many years, the final products ob- 

 tained were noncrystalline solids, and it was suspected that they were 

 still impure, in spite of showing very high enzymatic activity. These 

 products consisted largely of protein material, and it has always been 

 rather difficult to resolve mixtures that contain proteins into pure com- 

 ponents. However, so consistently was high enzymatic activity in 

 these final products associated with protein material, that many work- 

 ers believed that the enzymes were themselves proteins. A German 

 school, on the other hand, held that the enzyme was really a relatively 

 simple chemical substance which was absorbed onto the protein mate- 

 rial which acted as an inert carrier. However, in 1926, the first enzyme 

 was obtained as crystals and found to be a pure protein. The method 

 used in this particular case was remarkably simple. There is an 

 enzyme, urease, which breaks down urea into ammonia, water, and 

 carbon dioxide. This enzyme is fairly widespread in nature; soya 

 beans are quite a good source, but the best is the seeds of the jack bean 

 {Canavalia ensiformis) . The ground beans from which the fat has 

 been extracted are available commercially as a source of urease, under 

 the name of jack-bean meal. The American chemist Sumner ex- 

 tracted the meal with 31.6 percent aqueous acetone and filtered the so- 

 lution in a cold room. After standing overnight, very small octahe- 

 dral crystals separated out, which were about 730 times as active as the 

 original meal in splitting urea. 



CRYSTALLINE ENZYMES 



Since that time about 20 other enzymes have been obtained in crys- 

 talline form by various workers, but usually only by more elaborate 

 and more lengthy methods. Attempts to crystallize many other en- 



