200 PRACTICAL PHYSIOLOGY 



(2) The xanthoproteic reaction : only a slight coloration is 

 produced. (3) The Millon's test : only a slight reddening of the 

 precipitate occurs on boiling. (4) The glyoxylic test : absent or 

 very faint. (5) Half saturation with (NH 4 ) 2 S0 4 : salted out. It 

 is also precipitated on full saturation with magnesium sulphate, 

 also on full saturation with sodium chloride in an acid medium. 



The reason why the second, third and fourth tests are not very 

 distinct, is because gelatin does not yield aromatic bodies on decom- 

 position, and both these tests depend on the presence of aromatic 

 bodies. Some varieties of gelatin give these reactions more distinctly 

 than others, but absolutely pure gelatin 1 is said not to give them 

 at all, so that their presence is held to depend on contamination of 

 the gelatin with native protein. 



Elastin.- This is the chief protein constituent of elastic fibrous 

 tissue. It is quite insoluble in water and neutral salt solutions. 

 By prolonged treatment with dilute acids or alkalis it goes slowly 

 into solution, with stronger solutions it is more rapidly dissolved, 

 but it also undergoes decomposition. 



Keratins. These are horny materials found in hair, nails, hoofs 

 and horns, etc. These substances are characterised by their high 

 sulphur content. They are quite insoluble in water and neutral salt 

 solutions. They are also insoluble in dilute acids and alkalis, 

 but on treatment with stronger solutions they go into solution and 

 at the same time are decomposed. They give the ordinary protein 

 colour reactions. 



EXPERIMENT I. Using small pieces of yellow elastic tissue try its 

 solubility in water, hot and cold, and in strong alkalis and acids. 

 Do the colour reactions. 



EXPERIMENT II. Using hair try its solubility in water and in 

 strong acids and alkalis. The presence of loosely combined sulphur 

 can readily be shown by heating nails or hair with caustic soda in a 

 test tube. On the cautious addition of strong acid to the cooled 

 alkaline solution H 2 S is given off and can be detected by holding 

 paper moistened with lead acetate solution over the mouth of the 

 test tube. 



EXPERIMENT III. Using small pieces of finger-nail show that 

 keratin also gives the xanthoproteic and Millon tests. 



Conjugated Proteins. In this group we have proteins to which 

 groups (called prosthetic groups) other than protein are united to 

 form a complex molecule. The chief groups are : 



(i) The chromo -proteins, 

 (ii) The gluco -proteins, 

 (iii) The nucleo -proteins. 



Chromo-proteins. This group of substances is a combination of 

 protein with a coloured substance. The best known example is, of 

 course, haemoglobin, a compound of the iron containing substance 



1 Pure gelatin is said to lack Tyrosine, Tryptophan and Cystine. 



