GENERAL CHEMISTRY OF LIFE PHENOMENA 



2. Reversible Enzyme Actions 



Reversible chemical processes are characterized by the fact that 

 the reaction comes to a standstill before all the substances on one side 

 of the equation are transformed into those on the other side. The 

 reason is, that a point is reached when, in the unit of time, the change 

 in one direction is just as great as the change in the opposite direction. 

 When this occurs we say that chemical equilibrium has been estabhshed. 

 Inasmuch as, according to Ostwald, enzymes do not inaugurate chem- 

 ical reactions, but only accelerate them, it follows that the action of 

 enzymes must also be reversible, if the process itself is reversible. It 

 is the merit of Arthur Croft Hill to have first shown a few years ago 

 that an enzyme, maltase, which accelerates the hydrolysis of maltose 

 into dextrose, also accelerates the synthesis of dextrose into maltose 

 when added to pure dextrose. It is no exaggeration to say that Hill's 

 paper entirely changed the conceptions of the physiology of metabolism. 

 We shall return to Hill's experiments later, and first discuss the revers- 

 ible action of a fat-splitting enzyme, lipase. 



It had been known for some time that the pancreas secretes an 

 enzyme which digests fat in the intestinal canal. Kastle and Loeven- 

 hart* showed that in all tissues and liquids of the body which contain 

 fat, lipase can be found. A watery extract of pancreas contains a sub- 

 stance in solution which is capable of hydrohzing fats, i.e. of splitting 

 fats into fatty acid and alcohol. Kastle and Loevenhart showed, more- 

 over, that the watery extract of any tissue which contains fat acts in a 

 similar way. The chemical character of this catalytic substance is 

 unknown, except that its efficiency is rapidly destroyed if it is heated 

 in water. According to Taylor,f it does itself, at a high temperature, 

 undergo a hydrolytic cleavage. 



Kastle and Loevenhart showed that lipase not only accelerates the 

 hydrolysis of fat, but also the synthesis of fat, when added to a mixture 

 of fatty acid and alcohol. Their experiments were made on ethylbuty- 

 rate. If an extract from the pancreas or Hver was added to a mixture 

 of ethylalcohol and butyric acid, ethylbutyrate was formed. This 

 reversible action of lipase has the effect that the process of digestion of 

 fat can only be completed if the products of digestion are removed. 

 In the intestine this occurs through absorption. 



The velocity of the hydrolysis of ethylbutyrate was found to be in 

 proportion to the concentration of the hpase. This explains the fact 



* Kastle and Loevenhart, Am. Cheni. Journal, Vol. 24, p. 491, 1900.' 



t A. E. Taylor, University of California Publications, Pathology, Vol. I, p. 33, 1904. 



