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The widely accepted view that xanthine oxidase is a flavoprotein whose flavine prosthetic group is alternately reduced by the substrate and oxidized by the hydrogen acceptor has not been firmly established experimentally. It rests almost entirely on the observations of Ball (1939a) and Corran, Dewan, Gordon & Green (1939) that preparations of the enzyme from milk contain flavin-adenine dinucleotide (FAD). A number of investigators (Ball, 1939a; Corran et al. 1939; Horecker & Heppel, 1949; Kalckar, Kjeldgaard & Klenow, 1950) have found that the FAD in xanthine oxidase preparations is reduced when xanthine or hypoxanthine is added under anaerobic conditions, but the reduction is incomplete and, in the only study in which the rate was measured, the authors (Corran et al. 1939) concluded that the reduction was too slow to account for the catalytic activity of the enzyme. Ball (1939a, b) suggested that xanthine oxidase contained a second prosthetic group in addition to FAD. He claimed to have split the enzyme reversibly into its apo-enzyme and prosthetic group moieties by prolonged dialysis. The apo-enzyme was reactivated by the supernatant obtained by heating and centrifuging an enzyme preparation but not by pure FAD. Since the supernatant solution contained FAD, Ball suggested that the active enzyme contained as prosthetic groups both FAD and an additional component present in the heated enzyme preparation. However, he pointed out that it was possible that this additional component would be active on its own. The significance of Ball's findings has been questioned by Kalckar et al. (1950) who found that the reactivation obtained was due to the presence in the heated supernatant of sulphydryl compounds which removed metal ions acquired by the intact enzyme during dialysis. Thus, the reversible splitting of xanthine oxidase into apoenzyme and prosthetic group has not yet been achieved. Corran et al. (1939) believed that the typical reddish tinge of the enzyme was due to a second prosthetic group. This view, however, has since been criticized by Lowry, Bessey & Crawford (1949), who have suggested that since the red tinge disappears entirely when the enzyme is denatured, it may be

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