428 PROTEINS 



glucoprotein such as mucin yields a protein and a carbo- 

 hydrate, glucosamine.* 



The conjugated proteins appear to be rarely found in 

 plants ; they may be divided into three sub-groups : — 



1. Gluco-proteins, represented by mucin and possibly 

 by the mucilage of the roots of Dioscorea japonica which 

 in many of its characters resembles mucin obtained from 

 animal sources. 



2. Chromo-proteins represented by haemoglobin and 

 possibly by phycoerythrin (p. 353) together with some 

 of the chromoproteins described by Osborne, Wakemann, 

 and Leavenworth f as occurring in alfalfa. 



3. Nucleo-proteins. — With regard to the occurrence of 

 nucleo-proteins among plants, it is undoubtedly true that 

 nucleic acid has been repeatedly found in plants, and 

 compounds of proteins with nucleic acid have been isolated 

 by Osborne, but it is not certain whether these substances 

 actually occurred pre-formed in the seed, or were pro- 

 duced during the process of their isolation. Osborne % is 

 of the opinion that the small quantities of nucleo-protein 

 which occur in the seed are chiefly in those parts of the 

 embryo which are rich in nuclei, rather than in the places 

 of food storage, such as the cotyledons and endosperm. 



A nucleo-protein, when subjected to peptic digestion, 

 or treated with dilute acid, gives a protein and a nuclein ; 

 this latter with caustic alkali breaks up still further into 

 a second protein and a nucleic acid ; the nucleic acids 

 on further hydrolysis yield phosphoric acid, a carbo- 

 hydrate residue, either a pentose or glucose, and purine 



* Glucosamine is a peculiar nitrogen containing sugar of the formula 



CH2OHCHOHCHOHCHOHCHNH2CHO or CH2OHCHOHCH • CHOH ■ CHNH.CHOH 

 It has all the ordinary reactions of sugars as regards reduction of Fehling's 

 solution, reaction with phenylhydrazine, etc., but is not fermentable by 

 yeast. Owing to the presence of the amino group, it is also able to form 

 salts with acids such as hydrochloric acid. It was first obtained by the 

 hydrolysis of chitin contained in the shell of lobsters, and has since been 

 obtained by the hydrolysis of several gluco-proteins such as serum mucoid, 

 etc. 



f Osborne, Wakeman, and Leavenworth : " J. Biol. Chem.," 1921, 



49> 63- 



\ Osborne : " The Vegetable Proteins," London, 1924. 



