The mechanism of a step in the catabolism of tyrosine, named for the National Institutes of Health, where it was described. The sequence of events by which p-hydroxyphenylpyruvate hydroxylase forms homogentisic acid involves a-oxidative decarboxylation to form a peroxyacid, epoxidation of the aromatic ring by the peroxyacid, and migration of the side chain to the site on the ring adjacent to its original position.