In its metabolism uric acid follows laws of its own, quite independent of other nitrogenous compounds. During the digestion of organs containing blood (e.g., spleen, liver, and other glands), xanthin bases originate if air be excluded, whereas if plenty of air be supplied their place is taken by uric acid. This occurs in varying degrees, e.g., from 3 grams of hypoxanthin swallowed by men 62 per cent. of uric acid has been found, while from 6 gram of guanin only traces of uric acid could be discovered in the urine. It is to be noted, however, that it is difficult to transform free guanin into uric acid, but it is easy to do so in the case of the combined guanin of the nucieo-proteins of the pancreas.

The methyl xanthins increase the purin bases of the urine, but are in no way dependent on nuclein metabolism, and do not influence the excretion of uric acid.

The excretion of uric acid bears no relationship to the extent of protein metabolism, for an excessive consumption of nitrogenous food (free from nuclein) does not increase the amount of uric acid excreted, even although the metabolism may be increased. Thus from 18 grams of protein food 433 mgr. uric acid were excreted, whereas from 80 grams of protein food not more than 442 mgr. uric acid were excreted.

Uric acid is, in fact, chiefly derived from the nucleins, and the proportion of total nitrogen to the uric acid nitrogen is not a fixed quantity, but fluctuates from 25: 1 to 126: 1.

Flesh foods contain more uric-acid-forming material than vegetable foods, not only in the form of purin bases but also as free xanthin bodies especially hypoxanthin and xanthin.

For physiological purposes the purins are subdivided into distinct classes, the endogenous and the exogenous. About one-half of the uric acid excreted in the urine comes from the purins set free in the tissues (endogenous) and the other half from the food ingested (exogenous).

(1) Endogenous Purins

These originate from the kata-bolism of the cell constituents, especially the nuclei. It is also stated that in all probability leucocytes, epithelial cells, etc, are eaten up by phagocytes and so katabolised into carbonic acid, water, and nitrogenous waste products of the nature of purins, chiefly uric acid. But this is hypothetical.

A small proportion of the endogenous purins is produced by muscle metabolism, and the katabolism of nuclein and nucleo-proteins. Mendel has shown that all purins can be resolved into uric acid in the liver, muscles, and to a less extent in the kidneys. Fifty per cent. of this is further oxidised into urea, for which a plentiful supply of oxygen is necessarily provided, of course, by the increased respiration of physical exercise.

Just how much endogenous uric acid is formed it is difficult to say, but each individual has a fairly constant excretion in similar conditions of exercise, etc. There is some reason to believe that the bacillus coli plays a part in the formation of uric acid, and if so, this suggests a possible relationship between a purin-free diet and the absence of auto-intoxication.

Even during starvation the excretion of uric acid does not cease, although it becomes rapidly diminished, and infants fed on milk containing only traces of purin bases may excrete as much as 100 mgr. uric acid daily.

(2) Exogenous Purins

These are simply the same purins as the endogenous, but introduced into the body in the form of food. They exist either in a free or bound condition, the former indicating those in solution, ready prepared or, as it were, mechanically mixed with the tissues; the latter, those in chemical combination with the tissues and unable to be dissolved until chemical decomposition takes place.

They are discovered almost as freely in vegetable as in animal foods, and the following is a fairly complete list of the food-stuffs into whose composition they enter. All animal foods, especially the glandular organs, liver, pancreas, kidney, thyroid, etc. All "beef-teas," meat soups, and gravies prepared from animal foods, beans, peas, lentils, oatmeal, asparagus, mushrooms, potatoes, sugar-beet; tea, coffee, cocoa (as methyl xanthins); and beer, all contain purin bodies, but in many cases only in minute amounts.

In all these substances they are simply waste products en route for excretion which have been arrested in the tissues before being discharged. In the case of vegetables, which possess no excretion except that of a gaseous character, the excretory products are removed from the neighbourhood of vital parts and deposited in such tissues as the bark of trees, the leaves, and the cell walls.

When partaken of by man as food, or rather in combination with food, as they possess no nutrient properties, they are katabolised into much simpler compounds, the terminal product in most instances being uric acid. In this last form or even as less katabolised, they are excreted at a fairly uniform rate, except for a distinct increase during the morning hours. After a meal containing purins, there is a decided increase in the excretion of uric acid, varying with the individual and his diathesis. Thus it is possible for a slight delay in the excretion to occur in the gouty, and this is perfectly certain to occur when any alcohol is consumed at meal-times.