PROTEINS OR ALBUMINOUS SUBSTANCES 427 



mineral acids and by the salts of practically all the heavy 

 metals. 



The precipitate in each case consists of a compound or mixture 

 of compounds of the albumin with the acid or salt used. 



A curious point about the albumins is the fact that they are 

 optically active and that they all rotate the polarised ray to the 

 left. In the case of the sugars it has already been mentioned 

 that the most physiologically active and fermentable rotate the 

 ray to the right. 



Conjugated proteins consist of one or more molecules of albumin 

 associated with some other substance of a different nature such 

 as sugar. They do not always contain sulphur, but phosphorus 

 is in many of these compounds a prominent constituent. Several, 

 such as haemoglobin, the red colouring matter of blood, contain 

 iron. 



The derived proteins represent the first stages in the process of 

 degradation which the proteins, simple or conjugate, undergo 

 in contact with almost any reagent. Indeed the process of 

 extraction which involves any change in the composition of the 

 fluid in which the natural albumin is normally found probably 

 produces some modification. The solubility is affected even by 

 the comparatively simple operation of diffusion by which the 

 sodium chloride or other saline component of the albuminous 

 fluid is removed. Contact with acids causes incipient hydrolysis, 

 and the same effect is induced by alkalis, by metallic salts and 

 probably also by the enzymes concerned in the process of digestion 

 in the stomach. The products, called albumoses and peptones, 

 are very nearly allied to the albumins and answer to many of the 

 chemical tests which are supposed to characterise those bodies. 

 They are also of high molecular weight. More severe treatment 

 with hydrolysing agents leads to a break up of these complicated 

 molecules, and the products are for the most part comparatively 

 simple. It is of course significant that these products of hydro- 

 lysis consist principally of substances generally known as amino- 

 acids. 



One of the first cases of the kind was observed by Braconnot 

 nearly one hundred years ago in the production of what was then 

 called " gelatine sugar " or the " sugar of glue," glycocoll (from 

 y\luKv<}, sweet ; /co'AAa, glue) in allusion to its sweet taste. This 

 substance which is now called glycine is amino-acetic acid 

 CH 2 NH 2 -CO-OH. Two similar substances, namely alanine o,r 



