Why Is Uric Acid Called an Acid?

In the field of chemistry, there are a number of ways in which a substance is defined as an acid. The best-known of those ways is the Brønsted-Lowry Theory, in which a proton donor (a positively charged hydrogen ion) is called an acid. Uric acid (Fig. 1) is declared an acid under this theory.

The value used for relative strength of a weak acid is pKa. This is a dissociation constant, measuring proton release. Any substance with a pKa between about minus 2 and 12 is considered a weak acid. Anything below the minus 2 level is considered a strong acid. Uric acid has pKa 5.75.

Atypical Acid Structure and the Example of Phenol

The chemical structure most closely associated with organic acids is the carboxylic group. There are other groups, such as the sulfonic group. Nevertheless, there are other structures that allow the abstraction of a proton, and hence acidity. For instance, phenol is simply benzene with one ring hydrogen replaced by a hydroxy group (-OH). The hydrogen or proton can depart from that group. For that reason that one hydrogen (and none of the others) is called an acidic hydrogen.

This is possible because of the stabilization of the negatively charged remainder of the specie (Fig. 2). It is a fundamental principle of chemistry, that the greater the number of valid equivalent structures one can draw to disperse a charge, the more stable that species will be.

Uric Acid

For uric acid, there are no carboxylic or sulfonic groups. Nevertheless, through the process of tautomerism, in which a molecule can readily slip from one form into another (Fig. 3), uric acid can produce structures from which a hydrogen can more readily depart. This results in the formation of urate (Fig. 4).

Importance of Uric Acid

Primarily uric acid is important in matters pertaining to health. Uric acid results from the metabolizing of purines in the body. Purines are prevalent in beans, peas, liver, mackerel and anchovies. Uric acid is excreted by the kidneys. If uric acid becomes elevated and incompletely eliminated, a dangerous buildup can occur, called hyperuricemia. Gouty arthritis, sometimes inaccurately considered a rich man's disease, is associated with the deposition of uric acid crystals in joints.

Causes of Elevated Uric Acid

Excess uric acid can be caused by a combination of factors, including diet (especially the use of alcohol), certain medications, and a high fructose intake. Certain health conditions can contribute, such as diabetes, obesity and hypertension. Genetics also can be a factor.

Uric Acid as Seed Crystals

Under some conditions, uric acid can serve as seed material for the deposition of calcium oxalate monohydrate. Such deposition can lead to stone formation.