The first product of the _oxidation_ of purin is _hypoxanthine_, long recognised as a constituent of meat extracts. _Adenine_, the amino derivative of hypoxanthine, is met with in combination with other substances in _nuclear_ material. The second oxidation product of purin is _xanthine_, and its amino derivative _guanine_, both of which are found in the same substances as hypoxanthine and adenine. Further oxidation of purin gives rise to _uric acid_. We have to recognise, also, that in addition to the purins of animal origin there are some also derived from _vegetables_, viz., the _methyl_ purins, caffeine, theobromine, and theine. Now, as will be seen later, certain compounds, containing nitrogen and phosphorus, constitute the chief, if not the exclusive, source of _uric acid_. These substances, long known as _nucleins_ or _nucleo-proteins_, exist in the animal tissues, and in special abundance in those largely made up of cell nuclei, viz., thymus, lymph-glands, etc. The important and, indeed, the distinguishing component of the nucleins or nucleo-proteins is _nucleic acid_. This, in that through the action of ferments, it is from the nucleic acids that _uric acid_ and the _purin bases_ are derived. But, apart from this, we have to recollect that nucleic acids yield constituents other than purin bases, viz., the _pyrimidine bases_, phosphoric acid, and a carbohydrate group. From a study of the structural formulæ of the pyrimidine bases it will be seen that they are closely related to the purin bases, lacking, however, one of the _urea_ radicles. Moreover, it is believed that, though included in the makeup of nucleic acid, they are not derived from purin but are _primary_ products. { NH—CO { N==C—NH₂ { NH—CO Thymine { CO C CH₃ Cytocine { CO CH Uracil { CO CH { NH—CH { NH—CH { NH—CH To sum up, the characteristic constituents of _nucleic acid_ are the purin bases (adenine, guanine, hypoxanthine, and xanthine), pyrimidine bases (uracil, cytosine, thymine), phosphoric acid and a carbohydrate group. We have now discussed the _chemical structure_ of uric acid and its relationship to the _purin bases_; but before proceeding to consider the various sources from which uric acid is derived, it will I think be convenient to consider (1) the physical properties of uric acid and (2) the condition in which it circulates in the blood. PROPERTIES OF URIC ACID When pure, uric acid is white in colour and crystallises in rhombic form. In contrast to _urea_ it is very insoluble, but much less so in _blood serum_ than in distilled water, viz., ⅟₄₀₀₀₀ of water as opposed to ⅟₁₀₀₀ parts of plasma. It yields with alkalies two series of salts, viz., the biurate or mono-basic, and the so-called neutral or bi-basic urate, the latter of which is much more soluble. In water the mono-basic urate forms a colloidal solution from which the crystalline salt gradually precipitates.