C1I. XXV.] THE CONJUGATED PROTEIDS 401 



The general properties of the acid-albumin or syntonin, and the alkali- 

 albumin, which are thereby respectively formed, are as follows : they 

 are insoluble in pure water, but are soluble in either acid or alkali, 

 and are precipitated by neutralisation unless certain salts, like sodium 

 phosphate, are present. Like globulins, they are precipitated by 

 saturation with such neutral salts as sodium chloride and magnesium 

 sulphate. They are not coagulated by heat. 



A variety of alkali-albumin (probably a compound containing a 

 large quantity of alkali) may be formed by adding strong potash to 

 undiluted white of egg. The resulting jelly is called Lieberkiihris 

 jelly. A similar jelly is formed by adding strong acetic acid to 

 undiluted egg-white. 



The halogens (chlorine, bromine, and iodine) also form albumin- 

 ates, and may be used for the precipitation of proteids. 



B. THE CONJUGATED PKOTEIDS. 



These complex substances are compounds of albuminous substances 

 with other organic materials, which are, as a rule, also of complex 

 nature. They may be divided into the following groups : 



1. HAEMOGLOBIN and its allies. These are compounds of proteid 

 with an iron-containing pigment. All will be fully discussed under 

 Blood. 



2. GLUCO-PEOTEIDS. These are compounds of proteids with 

 members of the carbohydrate group. This class includes the mucins 

 and substances allied to mucins called mucoids. 



Mucin. This is a widely distributed substance, occurring in 

 epithelial cells or shed out by them (mucus, mucous glands, goblet 

 cells). 



There are several varieties of mucin, but all agree in the following 

 points : 



(a) Physical character. Viscid and tenacious. 



(b) Precipitability from solutions by acetic acid. They are soluble 

 in dilute alkalis, like lime water. 



(c) They are all compounds of a proteid with a carbohydrate 

 radicle, which by treatment with dilute mineral acid can be hydrated 

 into a reducing but non-fermentable sugar. 



It is probable that the carbohydrate radicle may differ in different mucins ; 

 in some cases it is certainly the case that the so-called sugar derived from it is not 

 sugar, but a nitrogenous derivative of sugar called glucosamine (C 6 H n O 5 NH 2 ) 

 i.e., glucose in which HO is replaced by NH 2 . 



The mucoids generally resemble the mucins but differ from them 

 in minor details. The term is applied to the mucin-like substances 

 found in the ground substance of connective tissues (tendo-mucoid, 

 chondro-mucoid, etc.). Another (ovo-mucoid) is found in white 



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