Beyond this general indication nothing is known of the chemical nature of the co-enzyme. The intimate relation of phosphoric acid to the process of fermentation renders it not impossible that the co-enzyme may contain this group, but there is no definite evidence for such a belief. Purely negative results have been obtained with all the substances of known composition which have yet been tested, among these being soluble phosphates, hexosephosphates and a number of oxidisable and reducible substances, such as quinol, p-phenylenediamine, methylene blue, peptone beef broth, etc. (Harden and Young; Harden and Norris [1914]; see also Euler and Bäckström [1912]), glycero-phosphates (Buchner and Klatte). The precise function of the co-enzyme is even more obscure than its chemical nature. The system of reacting substances consisting of fermentable material, enzyme and co-enzyme, bears, however, an obvious superficial resemblance to many of the systems required for the accomplishment of chemical changes in the animal or vegetable organism. Such a triad of substances is, for example, requisite for the process by which the red blood corpuscles of an animal are broken up by the serum of a different animal into the blood of which the red corpuscles of the first animal have been injected. This effect is only produced when two substances are present, the amboceptor or immune body and the complement. The analogy may be carried to a further stage since the amboceptor is, like the co-enzyme, more thermostable than the complement, which therefore corresponds with the enzyme. Immune serum can, in fact, be freed from complement by being heated at 57-60° for half an hour, whereas the amboceptor is unaffected by this treatment. On the other hand, the complement and amboceptor do not [p069] appear to act like enzymes but rather like ordinary chemical reagents, remaining in combination even after the blood corpuscle has been broken up, whereas the enzyme and co-enzyme of yeast-juice are again liberated when the reaction between sugar and phosphate has been completed. [Pg 070] CHAPTER V. ACTION OF SOME INHIBITING AND ACCELERATING AGENTS ON THE ENZYMES OF YEAST-JUICE.