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

Abstract

Summary

1. L-lyxoflavin, D-galactoflavin and isoriboflavin markedly increase the growth response of Lactobacillus casei to suboptimal amounts of riboflavin, but do not promote growth in the absence of this vitamin. Appropriate differential assays indicated that lyxoflavin was not deposited in cells of L. casei under conditions where it stimulated growth, and actually decreased the concentration 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 concentrations, lyxoflavin and galactoflavin inhibit growth by acting as competitive antagonists of riboflavin. Dichlororiboflavin and isoriboflavin are ineffective as inhibitors of this organism. 2. L-lyxoflavin promotes maximum growth of Lactobacillus lactis in the absence of riboflavin, and such growth continues indefinitely upon subculture. Differential assay with L. casei and L. lactis showed 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. 3. In chicks, lyxoflavin stimulated 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 inhibited growth. Amounts of lyxoflavin that inhibited growth on diets of low riboflavin content were non-toxic when the amount of riboflavin was increased. 4. 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.