These two enzymes are typical hydrolyzing enzymes, but it is to be noted that there is not only a taking on of water with a retention of the oxygen, but there is also a giving off of ammonia, by which the transformation is made possible. Adenin is known as an amino-purin and guanin as an amino-oxypurin, while hypoxanthin is an oxypurin and xanthin a dioxypurin. In other words, the two intracellular enzymes are able to transform the two amino-purins into the corresponding oxypurins; _i. e._, the enzymes are deamidizing ferments, liberating the NH_{2} group of the adenin and guanin and thus forming two new compounds. These reactions, though more or less technical, are emphasized in this way not merely because they illustrate the action of intracellular enzymes in intermediary metabolism, thus affording a striking example of the gradual changes that take place in ordinary katabolic processes, but especially because they throw light upon the production of another substance common in body metabolism, viz., uric acid. It has long been known that nucleoproteids, nucleins, and other compounds containing these purin radicles, when taken as food, cause at once an increased output of uric acid, and it has been clearly recognized that in some way this latter substance, as a product of metabolism, must come from the transformation of nuclein bases. To-day, we understand that in many tissues, as in the liver, spleen, lungs, and muscle, there is present a peculiar oxidizing ferment, an oxidase, by the action of which hypoxanthin can be converted into xanthin, and the latter directly oxidized to uric acid. This conversion into uric acid is purely a process of oxidation, brought about by a typical intracellular oxidase, known specifically as “xanthin oxidase,” the reaction involved being as follows: HN--CO HN--CO | | | | CO C--NH + O = CO C--NH | || \ | || \ | || CH | || CO | || // | || / HN--C--N HN--C--NH Xanthin Uric acid From these several reactions, it is clear how various intracellular enzymes working one after the other are able gradually to evolve uric acid from tissue nucleoproteids. Further, it is to be noted that there is another tissue oxidase--contained principally in the kidneys, muscle, and liver--which has the power of oxidizing and thus destroying uric acid, with formation, among other substances, of urea. Remembering that urea has the following chemical constitution NH_{2} / C==O \ NH_{2} it is easy to see, by comparison of the formulæ, how uric acid might easily yield two molecules of urea through simple oxidation. In this way, excess of uric acid produced in the body can be converted into urea, and in this harmless form be excreted from the system.