Endogenous Purins
Even if we entirely eliminate all purin substances, by restricting the diet to purin-free foodstuffs (bread, milk, cheese, eggs and butter), purin in the form of uric acid is still excreted in the urine.
To this moiety the term endogenous purin is applied; for the continued excretion of purin on such a diet is explicable only on the view that they are derived from the waste of the tissues, the daily “wear and tear” of cells. In other words, it is the outcome of the katabolism of the nucleo-protein of the body tissues.