Structural Formulæ
To describe the individual derivatives of purin we have to indicate to which particular atom of the purin nucleus the combining groups are attached. Thus, for example, adenine in structure is classed as a 6-amino-purin, and accordingly has the following formula:
Other important bodies built up round the purin nucleus C₅N₄, variously designated as xanthine, alloxuric and nuclein bodies:—
It will be seen that the purin bases stand in very close chemical relationship to uric acid in that the latter also is marked by the possession of a group called the purin nucleus; indeed, the relationship of uric acid to the purin bases is more intimate than to urea (CON₂H₄), close though this latter be as may be seen from the study of its constitutional formula. (For uric acid may be regarded as composed of two urea radicles, linked by a tricarbon chain. By oxidation and hydrolysis, two molecules of urea may be obtained from one of uric acid, and conversely uric acid is produced by the condensation of urea with hydroxy acids).
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.
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.