CHEMISTRY OF THE PROTOPLAST 65 



important proteids appear to be common both to animals and plants, and indeed 

 both animal and vegetable protoplasts seem to be composed of essentially similar 

 forms of proteid. The mode in which proteids are obtained, and their importance 

 in metabolism, will be dealt with in subsequent chapters. 



The group of proteid substances includes albumins, enzymes, mucin, chondrin, 

 chitin l , &c., bodies differing widely in character and properties, and from which 

 various more or less closely allied substances may be derived. All proteids 

 contain nitrogen, but not all of them contain sulphur or phosphorus. As a general 

 rule, hydrolytic decomposition results firstly in a formation of peptones and allied 

 substances, and finally amides are produced, while frequently, but not always, 

 carbohydrates, as well as leucin, tyrosin, and other aromatic substances 2 , may 

 appear. The latter need not, however, necessarily form an integral part of the 

 molecule of all proteids, although it is to the aromatic radicles that the results 

 given by Millon's test and indeed the principal colour reactions of proteids are due. 



Without assuming that these decomposition products form integral parts of 

 the molecule of albumin, it is nevertheless certain that the latter is built up of 

 numerous groups of atoms. Qualitative and quantitative differences in the latter, 

 together with substitution, polymerization, and different arrangements of the 

 component groups, render an endless variety of combinations possible. Certain 

 proteids, such as the vitellins found in aleurone grains, are even crystallizable. 

 Whether the molecule of living proteid is a polymerization product of aspartic 

 aldehyde, or is composed of a chain of cyan-alcohols, united to a benzene nucleus s , 

 or has a chemical constitution quite different from either of these, is impossible 

 at present to say 4 , and it is moreover doubtful whether any definite protoplasmic 

 molecule does actually exist. 



Among proteid substances three main classes may be recognized, namely, 

 albumins, globulins, and albuminoids 5 . Albumins may unite with various sub- 

 stances, and thus form a number of compound proteids ; for example, by adding 

 metaphosphoric acid to albumin a substance closely resembling nuclein is pro- 

 duced 6 , while by the combination of albumin with carbo-hydrates, glucosides, 



1 [According to Halliburton (Physiol. Chem.) the true vegetable proteids, like those of 

 animals, may be separated into seven classes albumins, globulins, albuminates, proteoses, peptones, 

 'coagulated' proteids, such as the gluten of flour, and enzymes. Nuclein, plastin, chitin, mucin, 

 chondrin, &c., are all albuminoids, differing in varying degrees from true proteids. Nuclein is 

 a highly phosphorized substance, as also is plastin. All mucins are glucosides, i.e. compounds 

 of a proteid with a gum, which latter, by treatment with dilute sulphuric acid, can be hydrated into 

 a reducing but non-fermentable sugar. Enzymes may possibly be nucleo-proteids.] 



8 Cf. Drechsel, I.e., p. 549; S. Schulze, Landwirthsch. Jahrbiicher, 1892, Bd. XXI, p. 121; 

 Malfatti, Bot. Centralbl., 1893, Bd. LV, p. 152 ; Cohn, Zeitschr. fur physiol. Chemie, 1896, Bd. XXII, 

 p. 153 ; Kossel, ibid., p. 176 ; Hedin, ibid., p. 191. 



3 [Lathom, Brit. Med. Journal, vol. i, 1886, p. 629.] 



4 Cf. Drechsel, 1. c., pp. 538 and 547. 



* See Neumeister, I.e., p. 33. Also Hammarsten, Zeitschr. fiir physiol. Chemie, 1894, Bd. XIX, 

 p. 19. On the synthetic production of certain proteids see the literature quoted by Neumeister, 

 Physiol. Chemie, 1893, Bd. I, p. 44. Also Kossel, Archiv f. Anat. u. Physiol., Physiol. Abtheilung, 

 1893, p. 157. 



6 For an account of nuclein and its compounds see the works already quoted, and also Milroy, 

 Zeitschr. f. physiol. Chemie, 1896, Bd. xxn, p. 307. 



PFEFFER F 



