24 Inside the Living Cell 



chemical changes, have been distinguished in every kind of living 

 organism and no doubt there are many others to be discovered. All 

 living things contain them, not only in their digestive organs, but 

 also in every tissue. The sap of the growing plant contains a host of 

 them. Even in the seed there are enzymes packed away, which will 

 act on the starch when germination begins and produce the sugar 

 which feeds the plant until it has roots and leaves. 



A large number of enzymes are also concerned with the oxidation 

 of carbohydrates. When conducted in the cell this is by no means a 

 simple process like the burning of wood. In the cell it is necessary 

 that the energy given out on oxidation should be obtained in a form 

 in which it is available for use. A great deal of work has been done 

 to elucidate the very complex sequence of reactions which occur. 

 The main result has been to find that the oxidation processes are 

 coupled with reactions involving phosphates, as the result of which 

 'high-energy' phosphate compounds are formed. In these compounds 

 the energy is available for bringing about the reactions which the 

 cell has to perform, and they are also used in muscle cells in the pro- 

 cesses by which mechanical work is performed. 



What are the enzymes which have these remarkable properties? 

 In 1926, Professor J. B. Sumner, of Ithaca, obtained from jack beans 

 a crystalline substance which had the property of breaking down 

 urea. It appeared to be the enzyme called urease, and further investi- 

 gation has confirmed this. It was the first enzyme to be prepared in a 

 crystalline condition as a pure substance, so that its nature could be 

 determined. It was found to be a protein. Since then a considerable 

 number of enzymes has been isolated and many have been obtained 

 in the crystalline state. Pepsin, the protein-splitting enzyme of the 

 stomach, was crystallized by Dr J. H. Northrop at Princeton in 

 1929. Later the animal pancreas was found to be a rich mine of 

 proteins, and Drs Northrop and Kunitz and their collaborators 

 isolated from it a succession of crystalline enzymes, such as trypsin 

 and chymotrypsin, which are poured into the intestine and complete 

 the digestion of the food. Enzymes have also been isolated from 

 varied materials such as yeast, malt, potatoes, fruit juices and milk. 



All these substances are very powerful bio-catalysts, i.e. they 

 bring about rapid chemical changes when present in very small 

 amounts, without being changed themselves. A solution containing a 

 ten millionth of an ounce of pepsin has a powerful effect on clotting 

 of milk; rennin, another enzyme present in calves' stomachs and 

 used for making junkets and cheese, can clot ten million times its 

 weight of milk in ten minutes. Urease crystals produce a hundred 

 times their weight of ammonia from urea in five minutes; it is said 



