28 HUMAN PHYSIOLOGY 



atoms of carbon in each molecule. Some substances, containing 

 no sulphur, are composed of hexo-bases and give real proteid 

 reactions. These substances are therefore regarded as the simplest 

 proteids and are called protamine. From arginine urea may be 

 obtained by boiling with baryta water, proving that urea may be 

 formed from proteids by a simple process of splitting up. 



By means of putrefaction, ether products may also be formed 

 which are not simple decomposition products of the proteid, but 

 which must be regarded as metabolic products of the bacteria 

 causing the putrefaction. These products are called ptomaws, 

 some of which are very poisonous. 



Potassium-permanganate oxidizes proteids to oxyprotosulphonic 

 acid, which has still the character of a proteid but contains more 

 oxygen and the sulphur in a completely oxidized form. 



Because of their decomposition products and their relation 

 to acids and bases with which they form salt-like combina- 

 tions, the proteids may be considered as condensation 

 products of^various, in part aromatic, amido acids. Otherwise 

 nothing is known as to their chemical constitution. 



(c) PJiysical Properties of Proteids. 



Most proteids are soluble in water or dilute salt solutions, 

 but insoluble in alcohol and ether. 



They are levorotatory. 



They dialyze (except peptone) with difficulty or not at all 

 through animal and vegetable membranes. 



They do not readily crystallize. Crystals have, however, 

 been obtained from haemoglobin, vitellin, egg and serum 

 albumin, and from many vegetable proteids. 



The fact that crystallizable proteids do not dialyze contradicts 

 the old classification of crystalloid and colloid bodies (although 

 this had already been proven untenable). Proteids do not dialyze 

 because their molecules are too large for the pores of the mem- 

 branes. 



(d) Reactions of Proteids. 



A. Precipitants of proteids. 



i. Many proteids are rendered insoluble by heat: they 

 coagulate. 



The coagulation temperature lies between 50 and 80. The 

 exact temperature does not only depend on the nature of the 

 proteid but also on the concentration, the amount of salts present, 



