Several types of membrane-localized flavin binding sites were investigated in sporangiophores (spph) and mycelia of Phycomyces blakesleeanus. In-vitro binding of riboflavin, riboflavin-5′-phosphate, and flavin-adenine-dinucleotide was demonstrated with unfractionated membrane preparations by means of competition of [14C]riboflavin binding. Saturation of binding was only obtained with the highly water-soluble riboflavin-5′-phosphate, but by extrapolation it was shown that riboflavin showed the highest affinity towards the binding sites (KD about 4·10-6M). The number of binding sites was estimated to be 0.7 nmol g-1 fresh-weight equivalent. Analysis of endogenous soluble flavin revealed that only riboflavin, riboflavin-5′-phosphate, and flavin-adenine-dinucleotide occurred in Phycomyces, and at a concentration of at least 1 nmol g-1 fresh-weight equivalent in entire spph. Thus, the measured binding sites could reach saturation in-vivo. In the apical part of spph to which blue-light sensitivity is restricted, the amount of soluble flavin was three-fold higher. Exclusively in this zone, heat-labile riboflavin proteins were measured at a concentration of about 3 nmol g-1 fresh-weight equivalent. The amount of covalently bound flavin was higher in spph tips than in intact spph (8 nmol and 3 nmol g-1 fresh-weight equivalent, respectively). In either case, the concentrations of the flavin-membrane complexes were higher than the theoretical calculated concentration of (anisotropic) blue-light photoreceptor in Phycomyces (Bergman et al. 1969), and their involvement in blue-light photoreception is considered.
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Dohrmann, U. In-vitro riboflavin binding and endogenous flavins in Phycomyces blakesleeanus . Planta 159, 357–365 (1983). https://doi.org/10.1007/BF00393175
- Blue light photoreceptor