12 ESSENTIALS OF PHYSIOLOGY. 



acid, the precipitate being insoluble in excess of the acid. The mucoids, 

 for example ovomucoid, are also glucoproteins. 



The best example of a chromoprotein is the blood pigment, haemo- 

 globin, in which a protein, globin, is combined with an iron-containing 

 body, hsematin. Haemoglobin crystallises with comparative ease, and 

 is freely soluble in water. 



The Derivatives of Proteins. 



When a protein molecule is broken up, either by prolonged boiling 

 with a mineral acid, or by the action of certain enzymes, the resulting 

 products are found to belong to the class of amino-acids that is, they are 

 fatty acids in which one or more atoms of hydrogen have been replaced 

 by NH 2 groups. Some of these acids are combined with the benzene 

 ring, e.g. tyrosine and tryptophane ; two, proline and oxyproline, contain 

 the pyrrol ring ; one, histidine, the iminazol ring ; and one, cystine, 

 contains sulphur, that is, it is a thioamino-acid. Whereas the fats of 

 the body belong to the upper end of the fatty acid series, it is to be 

 noted that the amino-acids belong to the lower end of the same series. 

 The simplest amino-acid found in the body is glycine, or amino- 

 acetic acid, CH 2 .NH 2 .COOH. This is one of the most abundant deriva- 

 tives of gelatin. The next in the series, alanine or amino-propionic 

 acid, occurs in abundance, both in its simple form, CH 3 .CH.NH 2 .COOH, 

 and also combined, with phenyl as phenylalanine, with oxyphenyl as 

 tyrosine (C 6 H 4 .OH.CH 2 CH.NH 2 COOH), with indol as tryptophane, 

 with iminazol as histidine, and with sulphur as cystine. Although 

 analysis of any one protein will yield all or most of the amino-acids 

 which have so far been isolated, nevertheless the relative proportions 

 of amino-acids contained in different proteins vary widely, and to this 

 variation the proteins probably owe their distinctive characteristics. 

 Gelatin is remarkable in containing no tyrosine, tryptophane, or cystine. 



The following tables from Starling's Principles of Human Physiology 

 give (1) a list of amino-acids, and (2) the proportion of the different 

 amino-acids contained in various proteins. 



Mono -amino-acids. 



Glycine (amino-acetic acid) . 



Alanine (amino-propionic acid) 



Serine or oxyalanine (oxyamino-propionic 



acid) 



Amino-valerianic acid 



Monobasic acids of 

 fatty series. 



Leucine (amino-isobutylacetic acid) 



Isoleucine (amino-caproic acid) 



Aspartic acid 1 Dibagic aci(is 



Glutamic acid .... 



