Growth-Promotion by Lyxoflavin. II. Relationship to Riboflavin in Bacteria and Chicks.

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Abstract
L-lyxoflavin, D-galactoflavin and isoriboflavin markedly increase the growth response of Lactobacillus casei to subopt. amts. of riboflavin, but do not promote growth in the absence of this vit. Appropriate differential assays indicate that lyxoflavin is not deposited in cells of L. casei under conditions where it stimulates growth, and actually decreases the concn. of riboflavin deposited in such cells. Thus, it does not serve as a partial substitute for riboflavin in L. casei, but appears to enhance the efficiency with which limited supplies of riboflavin are utilized. When riboflavin is present in excess, lyxoflavin does not increase the cell yield or the growth-rate. At higher concns., lyxoflavin and galactoflavin inhibit growth by acting as competitive antagonists of riboflavin. Dichlororiboflavin and isoriboflavin are ineffective as inhibitors of this organism. L-lyxoflavin promotes max. growth of L. lactis in the absence of riboflavin, and such growth continues indefinitely upon subculture. Differential assay with L. casei and L. lactis show that cells of the latter organism grown with lyxoflavin contain no riboflavin, but do contain lyxoflavin; i. e., in this organism lyxoflavin can fill the essential metabolic roles normally played by riboflavin. Similar assays of tissues from rats grown on natural rations indicate that lyxoflavin does not occur naturally in significant quantities. In chicks, lyxoflavin stimulates growth when fed at levels 2.5 to 10 times the amount of riboflavin in the diet. At ratios of lyxoflavin to riboflavin higher than 40 to 1, lyxoflavin inhibits growth. Amts. of lyxoflavin that inhibit growth on diets of low riboflavin content are non-toxic when the amt. of riboflavin is increased. Although the growth-responses observed in chicks when L-lyxoflavin is added to diets low in riboflavin may result in part from a sparing action similar to that found with L. casei, such a sparing action does not explain the growth effects of the analog observed in rations of high riboflavin content. Relatively large amts. of lyxoflavin are required for such growth stimulation, and since available evidence indicates that the compound does not occur naturally, it cannot be considered a vitamin.