Some anabolic aspects of protein metabolism in riboflavin deficiency in the rat.

Flavin nucleotides are components of certain enzyme systems which are involved in the oxidation of nitrogenous compounds, including at least some of the amino acids. The oldest and best-known evidence for this relationship was derived from studies of biochemical systems in vitro, but it is also supported by the results of studies on intact animals. For example, Burch, Lowry, Padilla & Combs (1956) showed that when rats were deprived of riboflavin for 6 weeks the activities of xanthine oxidase and of several amino-acid oxidases in their livers were substantially reduced. Kaunitz, Wiesinger, Blodi, Johnson & Slanetz (1954) observed that the deleterious effects of riboflavin deficiency on rats were more pronounced the higher the intake of protein, and concluded that riboflavin and protein are mutually limiting factors in nutrition. It has also been shown that on a diet low in protein the riboflavin content of the organs of rats decreases (McQuarrie & Venosa, 1945 ; Czaczkes & Guggenheim, 1946), regardless of the intake of the vitamin, and that the urinary excretion of it increases (Czaczkes & Guggenheim, 1946). With higher intakes of protein, however, the amount of the vitamin in organs was found to increase as its urinary excretion decreased (Czaczkes & Guggenheim, 1946). There have been other reports (Sarett & Perlzweig, 1943 ; Sherman & Ragan, 1947) that rats retain riboflavin in proportion to the intake of protein, and Sarett, Klein & Perlzweig (1942) showed that when positive nitrogen balance was induced in dogs and rats less riboflavin was excreted from test doses than when the nitrogen balance was negative. None of these findings are necessarily at variance with the role of riboflavin in katabolic processes in protein metabolism. On the other hand, they do not contradict the concept that it is involved in anabolic processes, for which there is also direct evidence. KIeiber & Jukes (1942) observed that chicks deficient in riboflavin stored about 30% less of their dietary nitrogen than pair-fed controls, and the difference was reflected in their body-weight, which would indicate that the nitrogen storage was