Gout, with a section on ocular disease in the gouty

by Llewellyn J. Llewellyn

Sources of Fallacy in Uric Acid Estimation.—With Folin’s findings as his basis, Walker Hall estimates that, excluding the lymphatics and lymph spaces, the entire blood-stream contains normally 70 mg. of uric acid, i.e., 2 mg. of uric acid per 100 grams of blood, 3,500 c.c. (total quantity of blood).

Thence he argues that, inasmuch as about 1 litre of blood traverses the kidney per minute, the total content thereof of uric acid would gain access to the renal organs in three and a half minutes. Now the average total output of the kidneys is 500 mg. per twenty-four hours. Accordingly, assuming that the blood arriving at the kidneys contains as a constant the above 70 mg. uric acid, the total daily output would pass through these organs in twenty-five minutes.

Now, given immediate extraction of all the uric acid by the renal cells, then the blood in the renal veins will become free of uric acid. If so, the estimates of the uric acid content of the blood will reflect exactly the measure of the endogenous or exogenous nuclein metabolism. But, “if the renal vein blood is not purin-free, then the estimations will fail to yield a true picture of the activities of nuclein exchange.”

Again, as to the precise import of isolated estimates of the uric acid blood content, we must recollect that the excretion of purins is not distributed evenly over the twenty-four hours, varying as it does under the influence of food, exercise, sleep, and other factors. A propos of this, Pratt’s observations clearly show that both in gouty and non-gouty subjects fluctuations in the uric acid blood content also occur, and this independently of diet. To what, then, may these variations be referred? Obviously a question of great moment, especially when we recall the eccentric behaviour of the blood uric acid in relation to the incidence of acute gouty attacks. For, until the inward meaning of these vagaries is revealed, the value of recorded estimates must necessarily be discounted considerably.

We must recall, too, that a certain moiety of the purins derived from nuclein metabolism lags in the lymph spaces and lymphatics, and this, as Walker Hall reminds us, must reduce the quantity present in the blood-stream at any one time. Also, as the same authority reflects, the lymph stream being probably richer in sodium ions than the blood, the entry of the nucleins therein may be retarded and so lead to a still further reduction of the blood content.

There is yet another possibility, he reminds us, viz., “that the purin content of the blood varies in the peripheral pulmonary hepatic and osseous streams, and that, while in some parts the purins are being carried to the kidneys for excretion, in others they are being transported from one organ to another for further metabolism.”

For, as before pointed out, the enzymes responsible for the ultimate disruption of the nucleosides are scattered in different organs, and Walker Hall suggests that “a transport of half metabolised nucleotides from one organ to another may form a part of the normal processes of nuclein metabolism.” This may well lead us on to consider the limitations that beset even the most modern tests in use for uric acid determination.

Disabilities of Modern Tests.—With all its outstanding advantages, even the Folin method of uric acid estimation has its drawbacks. As Curtman and Lehrman have pointed out, different workers have, even on identical blood samples, arrived at results which vary widely. Nor, disconcerting though it be, do the limitations of this mode of hæmo-analysis cease here.

Thus we know from Gudzent and Apolant that the soluble but unstable biurate is constantly being transmuted into an insoluble stable type, in other words, metamorphosis from one isomer into another. But, unhappily, the tests to hand fail of differentiation of the several tautomeric forms of uric acid. Also, as isomers of uric acid actually exist, then quâ Walker Hall, why not isomers of purins and pyrimidins also? But here again our tests are insufficiently eclectic. They give us no clue as to the affinities or blends of purins or pyrimidins for or with other substances.