Turning now to another phase of tissue metabolism, we may consider briefly the nucleoproteids and their characteristic decomposition products; bodies which are widely distributed as cleavage products formed in the disintegration of most cell protoplasm, and having special interest in nutrition because of their chemical relationship to that well-known substance, uric acid. Nucleoproteids of some type are found in all cells; consequently they are present in all tissues, in all glandular organs, and their widespread distribution constitutes evidence of their great physiological importance. Nucleoproteids are compound substances made up of some form of proteid and nucleic acid. By simple hydrolysis with dilute mineral acids they are broken down into proteid, phosphoric acid, and one or more bodies known as nuclein bases. Of these latter substances, there are four well-defined bodies, viz., adenin, hypoxanthin, guanin, and xanthin, which from their peculiar chemical constitution are known as “purin bases.” In the body, there is present in many cells a peculiar intracellular enzyme termed _nuclease_, which has the power of liberating these purin bases from their combination as a component part of tissue nucleoproteids, or of the contained nucleic acid. In autolysis or self-digestion of many glands, such as the spleen, thymus, etc., this chemical reaction is easily induced by action of the contained nuclease. Further, the liberated purin bases then undergo change because of the presence of certain deamidizing enzymes, and as a result guanin is transformed into xanthin, and adenin is converted into hypoxanthin. These ferments are true intracellular enzymes, and are termed respectively _guanase_ and _adenase_. The real essence of the reaction they accomplish is clearly indicated by the following formulæ, which likewise show the chemical nature and relationship of the four substances: HN--CO HN--CO | | | | H_{2}N·C C--NH + H_{2}O = CO C--NH + NH_{3} || || \ || || \ || || CH || || CH || || // || || // N--C--N HN--C--N Guanin Xanthin N==C·NH_{2} HN--CO | | | | HC C--NH + H_{2}O = HC C--NH + NH_{3} || || \ || || \ || || CH || || CH || || // || || // N--C--N N--C--N Adenin Hypoxanthin