Solubilities of Uric Acid
In the older conceptions of the pathology of gout the hypothesis that found most vogue was that the separation of uric acid from the blood into the tissues was due to diminished alkalinity of the blood and tissue juices; but, as before pointed out, it has been established that the alkalinity of the blood is not reduced, and the theory has consequently been abandoned.
But, with the advent of Gudzent’s findings, viz., that uric acid existed in two forms—one soluble and unstable, and the other insoluble and stable, and that the former is constantly changing into the latter—another conception of the origin of tophi arose. It was supposed that, by reason of the disparity in solubility of these tautomeric types of uric acid, the blood in gouty subjects must at times be in a state of super-saturation with uric acid; and, moreover, that equilibrium could only be restored through abstraction of the urates by crystallisation.
Unfortunately for this theory, it has been shown that the blood of gouty subjects is not super-saturated therewith; indeed, over and above the highest increments hitherto met with in gouty blood, a considerable margin of solubility for uric acid is still available. In truth, the problem is by no means so simple; for the conditions governing the solvency of uric acid in the blood are bewilderingly complex, subject as they are to the manifold variations in solubility exhibited by crystalloids in the presence of the many divers colloids.
But, to resume, Minkowski, it will be recalled, noted that from a mixed solution of uric and nucleinic acids the former cannot be precipitated by either acetic acid or alkaline ammonio-silver-magnesia mixture. Accordingly he advanced the view that uric acid “primarily exists in the blood and the tissue juices in combination with nucleinic acid, and that, not only the conversion of the purin bases into uric acid, but also the solubility and transportation, as well as the further changes of the uric acid in the living body, is regulated by this linking with a nucleinic acid rest.”
But, unfortunately for the value of this hypothesis, there is no proof that nucleinic acid is actually present in the blood; for, as Fürth remarks, is this inhibition of the precipitation of uric acid, in the presence of nucleinic acid, “necessarily indicative of a true acid combination with nucleinic acid,” “but such inhibition of precipitation is rather to be referred to the general group of variations of solubility which are manifested by crystalloid substances in the presence of all sorts of colloids.”
Complex phenomena of solubility of this nature must be considered in connection with the circulating uric acid. Nucleinic acid is not the only important substance, but “the general mass of the blood proteins must be particularly thought of.”
Continuing, Fürth reminds us that uric acid is much more soluble in blood serum than in water, and forthwith envisages this disparity in light of the factors that affect solubility of uric acid in the urine. The latter is markedly influenced by the presence of urea and di-sodium phosphate, and the relation of this to mono-sodium phosphate. Nor, he reflects, is there any doubt “of the importance of such inter-relations, too, in the formation of uric acid deposits in the tissues.” But he adds, “Although the importance of these complex conditions of solubility as they prevail among colloid and crystalloid substances in the animal juices may be accepted in relation to the formation of uric acid concretions, there is no real reason for seeking the explanation of gout in this sphere.”