Riboflavin-binding sites associated with flagella of Euglena: A candidate for blue-light photoreceptor?

The hypothesis was tested that reversible riboflavin (RF)-binding sites are part of the photoreceptor in Euglena gracilis. Published evidence shows that the phototactic stimulus - with a flavin-type action spectrum - is perceived at the paraflagellar body (PFB). Flagella with PFBs were isolated from Euglena gracilis by a combined cold and Ca(2+) shock. Saturable binding of [(14)C]RF was demonstrated with such preparations, in the oxidized state as well as under reducing conditions in the presence of dithionite. Affinities for RF were high: K D (oxidized)=0.08 μM, and K D (reduced)=0.7 μM. Flavin mononucleotide and flavin adenine dinucleotide showed lower binding affinities. The in vitro RF binding per unit of protein was enriched approximately tenfold in the flagellar preparations when compared with homogenates of whole cells. The number of (reduced) binding sites per entire flagellum was determined to be in the order of 10(6). This number is in line with published estimates of chromophores bound in or at the PFB.