74 PHYSIOLOGY 



a crystalline precipitate is produced consisting of whetstone-shaped crystals 

 which are doubly refracting. On treating these crystals with dilute acetic 

 acid this acid extracts calcium phosphate from the crystals. The original 

 shape of the crystals is, however, retained. The only difference under the 

 microscope consists in the fact that they have now lost their doubly refracting 

 power on polarised light. They consisted of a mixture of calcium sulphate 

 and calcium phosphate, from which, on treatment with acid, only the calcium 

 phosphate was dissolved out. 



The Mokcular Weight of Proteins. We may arrive at an approximate idea 

 of the minimum size of the protein molecule in various ways, though in all 

 cases our calculations are apt to be vitiated by the difficulty of obtaining a 

 preparation which is homogeneous, i.e. chemically pure, and by the ease 

 with which molecules of the size which we must assume for proteins form 

 adsorption combinations in varying proportions with other substances. 

 If we assume that each molecule of the respective protein contains only one 

 atom of sulphur, we can calculate its molecular weight. It is evident that 

 the protein which contains 1 per cent, of sulphur will have a molecular weight 

 of 3200. In this way the following molecular weights have been arrived at 

 (Abderhalden) : 



Sulphur per cent. Molecular weight. 



Edestin . . . 0-87 .. 3680 



Oxyhsemoglobin . . 0-43 . . 7440 



(horse) 



Serum albumin . . 1-89 .. 1700 



(horse) 



Egg albumin . . 1-30 .. 2460 



Globulin . . . 1-38 .. 2320 



The greater part at any rate of the sulphur in the protein molecule occurs 

 as a constituent of a substance, cystine, each molecule of which contains two 

 atoms of sulphur. Each molecule of protein must also contain two atoms of 

 sulphur, and we must regard double the molecular weight given in this Table 

 as the minimum molecular weights of these various proteins. Some idea of 

 the molecular complexity represented by these weights may be gained by 

 writing out the empirical formulae of the various proteins, e.g., 



Egg albumin . . C 204 H 322 N 52 66 S 2 



Protein in haemoglobin (from horse) . . . C 680 H 1098 N 210 241 S 2 



Protein in haemoglobin (from dog) . . . C 725 H 1171 N 194 214 S 2 



Crystallised globulin (from pumpkin seeds) . C 292 H 481 N 20 83 S 2 



With some proteins we may make use of other elements to arrive at an 

 idea of the approximate molecular weight. Thus, oxyhaamoglobin contains 

 between 04 and 0-5 per cent. iron. If we assume that each molecule 

 of oxyhaamoglobin contains one atom of iron, its molecular weight must be 

 from 11,200 to 14,000. 



Attempts have been made to solve the same question by studying the 

 compounds of proteins with inorganic salts or oxides. Thus, the crystals of 

 globulin from pumpkin seeds prepared with magnesia contain 1 -4 per cent. 



