Riboflavin Antagonists
Galactoflavin is a potent riboflavin antagonist. Emerson and co-workers reported marked growth inhibition in rats on diets, containing this compound and reversal of the effect upon addition of sufficient riboflavin to the food. The diethyl derivative acts as a riboflavin antagonist in rats but can replace the vitamin for growth of L. casei. Egami and others showed that flavin mono sulfate inhibits D-amino acidI oxidase and acts as a competitive inhibitor of the vitamin in growth of L. casei. Fall and Petering synthesized a number of analogues of riboflavin in which the ribityl group is replaced by other groups. The formylmethyl analogue was found ;0 be a reversible riboflavin antagonist in the rat. The hydroxycthyl analogue proved to be a potent competitive antagonist of the vitamin in rats and with L. casei. The compound also exhibits antifungal and antibacterial activity.
Distribution of Ribofiavin
Riboflavin is one of the most widely distributed B vitamins, but there are very few common sources that contain high concentrations All cells of both plants and animals presumably contain the vitamin. Plant seeds synthesize the vitamin during germination. A few bacteria and the yeasts are rich sources. Muscle tissue is low in riboflavin, but certain internal organs, such as the liver and kidney, may contain appreciable amounts. . Note the high concentration in live, wheat germ, and yeast, and the very low content in celery, polished rice, and potatoes. It should be kept in mind that even though potatoes are low in the vitamin, the continued intake at one or even two meals a day by, the majority of many races makes the potato an important contributor to the total dietary intake It was pointed out previously that the thiamine oflive yeast is not available to the body. A similar situation exists with riboflavin.
Parsons and co-workers showed that the riboflavin in live bakers' yeast is only slightly available to humans but that the vitamin is completely available from samples of the same yeast after has been specially dried (dead cells). Other workers have reported similar findings. The methods used involve dearmnining the urinary and fecal riboflavin excretions after the oral administration of a known amount of pure riboflavin and comparing these with excretions after known amounts of the vitamin in yeast before and after special drying and heating treatments. Other foods do not yeld their total riboflavin content to the body. For instance, it was reported that in normal women the availability of riboflavin in ice cream was 90 per cent, whereas in green peas and almonds only 41 and 39 per cent, respectively, were available. The vitamin exists within many plant and animal cells, primarily in combination as one of :he two coenzyme forms. The retina and urine and milk contain principally the free vitamin. The nucieotides may in part be combined with specific proteins to form enzymes. In serum of 13 normal adults the sum of the free riboflavin and riboflavin phosphate was found to average 0.8 l!g per cent, and the riboflavin dinucleotide, 2.41 g per cent. The white cells plus platelets contained 252 fig per cent of total riboflavin (both derivatives plus the free vitamin) and the red cells only 22. In a later'study Bessey and others found somewhat similar values in plasma and cells, although the levels were lower in individuals on restricted vitamin intake.
