In vitro binding of riboflavin to subcellular particles from maize coleoptiles and Cucurbita hypocotyls

Saturable and reversible in vitro binding of [(14)C]riboflavin was found to occur on subcellular, sedimentable particles from maize coleoptiles and Cucurbita hypocotyls. The KD was ca. 6 μM, the pH optimum was near 6.0, and the number of binding sites amounted to 0.1-0.5 μM on a fresh-weight basis. When the reducing agent dithionite was present, riboflavin binding increased-the KD was 2.5 μM, and the pH optimum above 8.0. The binding was specific: flavin mononucleotide (FMN) and flavin adenosine-dinucleotide (FAD) bound less tightly to these sites than riboflavin and another major soluble flavin, the previously described riboflavin-analog "FX", occurring in grass coleoptiles. These flavin-binding sites were localized on vesicles derived from plasmalemma and endoplasmic reticulum by analyzing sucrose and metrizamide density gradients and marker enzymes.