160 ILLINOIS STATE ACADEMY OP SCIENCE 



the same formula, which had hitherto been considered 

 identical, differed in their effect on polarized light. 



For example, lactic acid from beef extract rotated the 

 plane to the right, being dextro-rotatory ;.lactic acid from 

 the fermentation of milk sugar, with a certain ferment, 

 was levo-rotatory, while the synthesized product was in- 

 active. The chemist usually explains such isomers by 

 a difference in arrangement of the atoms, but in the case 

 of lactic acid only one structure could be conceived, un- 

 less, as Van't Hoff suggested, spatial relations were con- 

 sidered. So the immediate importance of the polari- 

 meter was the incentive to study organic structure. 



The inactive lactic acid referred to was found to be a 

 mixture of the dextro and levo forms, so that the problem 

 arose of separating them. In most cases the chemical 

 and physical properties of two optically active isomers 

 are the same except in their effect on polarized light, and 

 so the usual method of separation based on differences 

 of properties was difficult to apply. But here biology 

 made an important contribution in the form of 'certain 

 bacteria, which, when introduced into the mixture, would 

 destroy one and leave the other. These living cells se- 

 crete complex proteins called enzymes which catalyze or- 

 ganic decomposition in order to use the energy liberated. 

 Now in accordance with laws of evolution each organ- 

 ism will be provided with the enzyme which can attack 

 the substances it finds in its environment, and conse- 

 quently one which will be indifferent to most other sub- 

 stances. The mould penicillum glaucum is thus capable 

 of destroying levo-lactic acid but is indifferent to the 

 dextro-form, so that when it is introduced into the mix- 

 ture, it will leave only the latter acid. 



Aside from this practical value the study of enzyme 

 catalysis is of the greatest importance in biological chem- 

 istry. The value of a compound as a food is largely de- 

 pendent upon its ability to undergo decomposition, and 

 since these decompositions are brought about in many 

 cases only through the agency of certain enzymes, the 

 question of food value becomes largely one of whether 

 the necessary enzyme is present. The enzyme 's selection 

 of compounds has been shown to be according to the 



