76 INTRODUCTION TO GENERAL PHYSIOLOGY 



removes the NH 2 group and the hydrogen from their combination 

 with the carbon atom next the carboxyl group (the a-position), and 

 replaces them by oxygen, thus forming a ketonic acid. In the 

 same process the carboxyl is reduced to an aldehyde group. Taking 

 alanine (a-amino-propionic acid) as an example : 



HCNH 2 becomes CO + N H 3 



I I 



OCOH OCH 



A ketonic aldehyde is produced, in this case pyruvic aldehyde 

 corresponding to the particular ammo-acid affected by the process. 

 known as " examination" Further changes are produced in these 

 compounds by one of three kinds of reaction, under the control 

 of enzymes. Most of those enzymes with which we have been 

 concerned so far have been those dealing with the removal or 

 addition of the elements of water. If a ketonic aldehyde is 

 hydrolysed, we obtain the corresponding hydroxy-acid. In our 

 example it would be lactic acid : 



CH 3 CH 3 



C0 + H 2 = HCOH 



I I 



OCH COOH 



But there are also enzymes, as we shall see later, which bring 

 about oxidation or reduction. In the first case we get : 



CHo CHo 



i i 



CO + O = CO (i.e., pyruvic acid) 



I I 



OCH OCOH 



In the latter case we have : 



CH 3 CH 3 



I I 



CO + H 2 = CH 2 (i.e., propionic acid). 



I I 



OCH OCOH 



Any one of these three acids, passing on to the tissues is 

 oxidised to carbon dioxide and water, giving off energy. 



The ammonia from the deamination reaction combines first with 

 carbon dioxide to form the carbonate, which is then converted into 



