Abstract
The presence in milk of large amounts of the enzyme xanthine oxidase (XO) has long been known, although its physiological role has remained obscure. It was proposed, some 30 years ago, that ingestion of XO from milk products is implicated in cardiovascular disease, by way of either circulating active enzyme or of anti-XO antibodies. While both species undoubtedly occur in the bloodstream of all human subjects, it now seems likely that they are largely of endogenous origin, and that the antibodies, at least, are beneficial rather than hazardous. Recent knowledge of the ability of XO to generate reactive oxygen and nitrogen species has led to the suggestion that the role of the enzyme in milk is as an anti-microbial agent in the neonatal gut. In view of the fact that XO enzymic activity is absent from formula milk preparations, these suggestions inform the current debate on the relative merits of breast vs. bottle feeding of infants.