Riboflavin, overproduced during sporulation of Ashbya gossypii, protects its hyaline spores against ultraviolet light

Riboflavin (vitamin B2), essential in tiny amounts as a precursor for oxidoreductase coenzymes, is a yellow pigment. Although it causes cytotoxicity via photoinduced damage of macromolecules, several microorganisms are striking overproducers. A question, unanswered for decades, is whether riboflavin overproducers can benefit from this property. Here, we report an ultraviolet (UV) protective effect of riboflavin. The spores of Ashbya gossypii, a riboflavin-overproducing fungus, are more sensitive to UV than those of Aspergillus nidulans. The addition of riboflavin to suspensions improves the UV resistance of both spore types. Interestingly, we show that regulation of sporulation and riboflavin overproduction in A. gossypii are linked. In batch culture, both were elevated when growth ceased. At constant growth rates, obtained in a chemostat culture, neither was elevated. Supplementation of cultures by cAMP, a known stress signal, negatively affected sporulation as well as riboflavin overproduction, establishing a second, independent argument for the linkage.

Hypoxia potentiates ultraviolet A-induced riboflavin cytotoxicity

Flavins are thought to be important chromophores for chronic photo-induced skin injury, but the mechanism is not well known. We have reported that the primary cytotoxicity remaining in ultraviolet A-irradiated riboflavin solution is attributable to hydrogen peroxide. Because the dermis is more hypoxic than the atmosphere, we investigated the cytotoxicity of riboflavin solution during and after ultraviolet A irradiation under hypoxia. Riboflavin solution showed stronger cytotoxicity during irradiation under hypoxia than under air. Riboflavin solution that had been irradiated under hypoxia at lower ultraviolet A doses showed stronger cytotoxicity and contained more hydrogen peroxide than solution irradiated under air at the same doses. At higher ultraviolet A doses, however, the cytotoxicity and hydrogen peroxide quantity were similar in riboflavin solutions irradiated under different oxygen conditions. The effect of a singlet oxygen quencher, sodium azide, on the induction of cytotoxicity and production of hydrogen peroxide by ultraviolet A irradiation of riboflavin solution was examined. The presence of sodium azide in the solution during ultraviolet A irradiation suppressed the cytotoxicity and hydrogen peroxide production to similar levels at various ultraviolet A doses regardless of oxygen conditions. At the maximum suppression by sodium azide, hydrogen peroxide production decreased to 10% of the unsuppressed production. About 40% of the oxygen molecules of hydrogen peroxide produced was thought to be derived from oxygen dissolved in the riboflavin solution.

The primary cytotoxicity in ultraviolet-a-irradiated riboflavin solution is derived from hydrogen peroxide

The cytotoxic action of near-ultraviolet (UVA) radiation on cultured mammalian cells is dependent upon oxygen, suggesting that reactive oxygen species are involved in the cellular action of the radiation. Flavins are thought to be an important chromophore for photo-induced skin injury. Irradiation of riboflavin with UVA radiation is known to produce singlet oxygen, superoxide anions, and triplet-state riboflavin radicals, which, however, are immediately quenched by many constituents of the human skin. If the chemical produces a long-lived reactive oxygen species, hydrogen peroxide (H2O2), after UVA radiation, its deleterious effect is not limited to its generation site. Thus, we investigated whether H2O2, is produced in UVA-irradiated riboflavin solution and whether it plays an important role in the cytotoxic action of the solution. The solution showed a marked cytotoxic effect when placed on human fibroblasts, and cytotoxicity was retained in the solution for at least 40 min after radiation. Most of the toxicity appeared to be derived from H2O2 produced in the solution, because the solution lost its cytotoxicity as a result of catalase treatment, and the resultant restoration of survival was almost complete. Under our conditions, two molecules of riboflavin were calculated to produce one molecule of H2O2 after UVA radiation.

Conformational changes of beta H-crystallin in riboflavin-sensitized photooxidation

The effect of riboflavin-sensitized photooxidation on calf lens beta H-crystallin has been investigated by using fluorescence and circular dichroism techniques. beta H-Crystallin showed a pronounced change in its tertiary structure (conformation) as manifested in the near-u.v. circular dichroism spectra and fluorescence yield of tryptophan residues. The rate of tryptophan photolysis was significantly diminished under anaerobic conditions, but was not affected appreciably when D2O was used in the reaction mixture instead of H2O. Ferricyanide and ferricytochrome c added to the solution prior to irradiation inhibited the rate of photolysis of tryptophan, suggesting the involvement of O2- anion in the photoreactions. Quantitative assays of O2- and H2O2 in the irradiated protein solution strongly suggest that the Type I photosensitization pathway is involved in the RF-sensitized photooxidation of beta H-crystallin. The effect of photolysis on the cysteine residues of the protein was also studied. The sulfhydryl specific fluorophore N-iodoacetyl-N'-(5-sulfonaphthyl) ethylenediamine (1,5-IAEDANS) was used to study the change in the microenvironment of the cysteine (sulfhydryl) residues of the protein by photolysis. The results indicate that there is a quantitative loss of IAEDANS labeling sites due to photooxidation as well as structural changes of the protein. Fluorescence lifetime measurements indicate that the probe is bound in two environments--the major one (95%) is exposed and the minor one (5%) hydrophobic. A decrease in the lifetimes of the bound label occurs after photooxidation. However, the relative proportion of the hydrophobic IAEDANS-labeling sites increases in the photooxidized beta H-crystallin, probably due to the formation of supra-aggregated protein by photolysis.

Spectroscopic studies on the riboflavin-sensitized conformational changes of calf lens alpha-crystallin

The change in conformation of calf lens alpha-crystallin by oxidation in the presence of the photosensitizer riboflavin and light has been investigated. Near-UV circular dichroism (CD) spectrum, absorption spectrum, tryptophan fluorescence yield and fluorescence lifetime of the SH-specific fluorescent probe, N-iodoacetyl-N'-(5-sulfo-1-naphtyl) ethylenediamine (1,5-IAEDANS), were significantly altered by irradiation in the presence of RF. In the initial stages of photolysis (1-2 hr), a slight degradation of the protein to lower molecular weight peptides was observed. Upon increased photolysis, intersubunit cross-linking to dimers and other high molecular weight species was observed. To determine the effects of cross-linking on the accessibility of the cysteine residues of the protein, lifetime quenching studies on the IAEDANS-labeled alpha-crystallin were performed. A decrease in the quenching constant (kappa q) in the photolysed sample indicates that the labeled SH groups are less susceptible to collisional quenching, which requires contact between the quencher and the excited state of the fluorophore, due to steric inhibition in the cross-linked protein. Cross-linking and the rate of loss of tryptophan fluorescence of alpha-crystallin diminished under anaerobic conditions and increased when D2O was used in the medium for irradiation. Use of inhibitors and quenchers of active species of oxygen suggests that photo-oxidation probably occurs via the action of singlet oxygen as well as substrate-sensitizer complexation.

Metabolism of monoterpenes: demonstration of the hydroxylation of (+)-sabinene to (+)-cis-sabinol by an enzyme preparation from sage (Salvia officinalis) leaves

A microsomal preparation from the epidermis of Salvia officinalis leaves catalyzed the NADPH- and O2-dependent hydroxylation of the monoterpene olefin (+)-sabinene to (+)-cis-sabinol. The reaction catalyzed is a key step in the biosynthesis of C3-oxygenated thujane monoterpenes, and the hydroxylase is highly specific for (+)-sabinene as substrate. The hydroxylase from leaf homogenates was solubilized and characterized with regard to reaction conditions, inhibitors, and activators. Activity was partially inhibited by rabbit anti-rat cytochrome P-450 and by CO, and the latter inhibition was reversed by 450 nm light. A CO-difference spectrum and type I substrate binding spectrum were obtained. The hydroxylase meets most of the established criteria for a cytochrome P-450-dependent mixed function oxygenase and represents one of very few enzyme systems of this type to be isolated from leaves of a higher plant.

Photoinactivation of δ-aminolevulinic acid dehydratase from maize by flavin mononuclotide

The δ-aminolevulinic acid dehydratase activity was irreversibly inactivated by irradiation of the enzyme in presence of flavin mononucleotide. The loss of enzyme activity was dependent on time of irradiation, concentration of FMN and intensity of irradiance. It required oxygen and was markedly enhanced in heavy water. The presence of levulinic acid (a competitive inhibitor of δ-ALAD) during irradiation prevented the inactivation considerably indicating photooxidative damage at or near the active site. Superoxide dismutase, sodium benzoate and sodium formate offered no protection, but singlet oxygen quenchers like azide and tryptophan were effective. NADH, electron donor to excited flavins, also prevented the loss of enzyme activity. These results indicate that singlet oxygen produced by light absorption of FMN was responsible for the photooxidative inhibition of the enzyme.