E. Insoluble in water, salt solutions, alcohol, dilute acids and alkalies; soluble in strong acids, alkalies, and in pepsin-hydrochloric acid and alkaline solutions of trypsin.—Coagulated proteids, fibrin.[53]
II. Compound Proteids.—A. Compounds of a proteid (globulin) with an iron-containing pigment, soluble in water and coagulable by heat and alcohol. Hæmoglobin, oxyhæmoglobin, methæmoglobin, etc.
B. Compounds of proteids with members of the carbohydrate group. Insoluble in water; soluble in very weak alkalies.—a. True mucins. b. Mucoids or mucinoids.
C. Compounds of proteids with nucleic acid. Phosphorized bodies yielding by decomposition metaphosphoric acid. Insoluble in water and in pepsin-hydrochloric acid, but more or less soluble in alkalies.—Nucleins.
D. Compounds of proteids with nucleins. Very soluble in dilute alkalies.—Nucleoalbumins, as casein of milk, and nucleoalbumins of cell-protoplasm and cell-nuclei, etc.
III. Albuminoids.—A. Soluble in boiling water with formation of gelatin and yielding by decomposition leucin and glycocoll.—Collagen (gelatin).
B. Insoluble in boiling water, and yielding by decomposition much leucin and some tyrosin, together with glycocoll and lysatin. Slowly hydrated by boiling dilute acids and by treatment with pepsin-hydrochloric acid.—Elastin.
C. Insoluble in water, dilute acids and alkalies, also in gastric and pancreatic juice. Yield leucin and tyrosin by decomposition.—Keratin, neurokeratin.
We may now advantageously consider the composition of a few of the more prominent representatives of the individual groups, taking for illustration those bodies which have been most thoroughly studied, and which we may have occasion to refer to in our discussion of proteolysis. I have not included in the table any of the alteration-products of the proteids formed by the action of pepsin-acid, trypsin, or boiling dilute acids, confining myself here simply to those bodies which occur ready-formed in nature.