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Improved methods have been devised for the isolation in μmole quantities of a pure flavin pentapeptide and its acid-hydrolysis product (SD-flavin) from inner-membrane preparations of heart mitochondria and from soluble, purified succinate dehydrogenase. SD-flavin differs from riboflavin in still having an amino acid covalently linked to the isoalloxazine ring system. SD-flavin may be compared with riboflavin and with various 8α-substituted synthetic flavins by optical spectrophotometry in the neutral and cationic states and by ESR and ENDOR spectrometry in the cationic radical state. On the basis of these experiments is was concluded that the FAD prosthetic group of mitochondrial succinate dehydrogenase is covalently linked through the 8α-position to the peptide backbone of the protein. This conclusion is in accord with the acid stability of the natural product and its tendency to yield riboflavin under reductive conditions. The unusual ph-fluorescence spectrum of the flavin strongly suggests that the 8α-methylene group is linked to an amino acid through a tertiary nitrogen group. [ABSTRACT FROM AUTHOR]

Succinate dehydrogenase flavocoenzyme (‘SD-flavin’), previously shown to be an 8α-substituted riboflavin derivative containing a tertiary nitrogen homoconjugated to the flavin nucleus, was subjected to further hydrolysis and to reduction under acid conditions. Both conditions resulted in the liberation of 1 mole of histidine per mole of flavin. This proves histidine to be the covalent link between flavin and peptide backbone in succinate dehydrogenase and imidazole to be the tertiary nitrogen function homoconjugated to the flavin. 8α-Histidyl-riboflavin has been synthesized starting from riboflavin chemically and shown to be completely identical with the natural product in optical, ESR and NMR spectra, pH-fluorescence curve and behavior on thin-layer and paper chromatography, as well as paper electrophoresis. [This equation cannot be represented into ASCII Text] 3. Both the natural compound isolated by acid hydrolysis of flavin peptide and the synthetic one contain two isomers, which may be separated by high voltage electrophoresis. The isomers appear to be the N(1)- and N(3)-imidazole substituted compounds. Digestion of the flavin peptide with aminopeptidase M yields only one isomer but on treatment with 6-N HCl this is gradually converted to a mixture of the two isomers. The absolute assignment of the natural isomer is suggested as 8α-[N(3)-histidyl]-riboflavin on the basis of imidazole quaternization with CH3I, reductive cleavage of the flavin-imidazole bond and identification of the methyl-histidine liberated as 1-methyl-histidine. [ABSTRACT FROM AUTHOR]

In the previous paper in this series it was shown that a pure flavin pentapeptide isolated from hepatic monoamine oxidase contains I mole each of serine and tyrosine and 2 moles of glycine, as well as an additional amino acid which is covalently linked to the 8α carbon of riboflavin. This amino acid has been identified as cysteine and the linkage to the flavin as a thioether on the basis of positive chloroplatinic and negative iodine-azide tests, performic acid oxidation or reduction with zinc, followed by acid hydrolysis, which yield cysteic acid or cysteine, respectively. The fluorescence properties of the flavin from monoamine oxidase agree with those expected for a thioether and its oxidation products. In regard to optical and electron spin resonance spectra, chemical stability, and fluorescence characteristics the flavin peptide isolated from the enzyme agrees excellently with the properties of synthetic cysteinyl 8α-riboflavin. [ABSTRACT FROM AUTHOR]

A postulated capability of riboflavin and flavin mononucleotide to protect DNA from damage by far-ultraviolet irradiation was investigated in purified reaction mixtures and in cultured mammalian cells. Purified calf thymus DNA, admixed with flavin at approximately physiological ratios, was protected from undergoing irradiation-induced changes in spectral absorbance and viscosity compatible with those accompanying pyrimidine dimerization. Irradiation-induced changes in unprotected DNA were reversed by a subsequent treatment of the DNA with flavin in the presence of visible light. There was no apparent interaction between flavin and non-irradiated DNA. These results suggest that photoexcited flavin interacts directly with the ultraviolet photoproduct in DNA and does not function simply as an energy sink to dissipate photons absorbed by DNA. Cultured embryonic hamster cells were protected from ultraviolet-induced inhibition of growth by a preliminary exposure to relatively large amounts of flavin. This effect was not due to a vitamin- like promotion of cell growth by the flavin. The observation that flavin concentrated transiently in the cytoplasm of exponentially growing hamster cells during a 6 hr incubation period suggested that a shielding action may cooperate with the protective photochemical mechanism in vivo. It was concluded that flavins may function physiologically to protect the genetic material of tissues exposed to actinic radiations, and that when flavin levels in these tissues are inadequate, either because of nutritional deficiency or excessive intensity of irradiation, the damage which results may provide one pathogenetic avenue for the characteristic integumentary lesions of riboflavin deficiency disease. [ABSTRACT FROM AUTHOR]

The article presents information about the study "An Appraisal of the Therapeutic Effect of Riboflavin in Psoriasis," by Ashton L. Welsh and Mitchell Ede. In the study, riboflavin administered by the mouth or parenterally, in moderate or in massive dosages, to 348 patients with psoriasis failed to produce any significant response.