Thiamin-binding protein from sunflower seeds.

A thiamin-binding protein was isolated from sunflower seeds. Its molecular mass was estimated to be 230 kDa by gel filtration. The protein was suggested to be composed of six subunits, which consisted of polypeptides linked by disulfide bond(s). The protein contained a large amount of glutamine or glutamic acid (19.9 mol%) and asparagine or aspartic acid (11.1 mol%). The levels of tryptophan and valine in the protein were low. These properties of the thiamin-binding protein were similar to those of helianthinin. Optimum pH for the thiamin-binding activity of the protein was 8.0 to 9.0. The thiamin-binding activity was not inhibited by thiamin monophosphate, thiamin pyrophosphate, oxythiamin, or pyrithiamin. These properties of the thiamin-binding protein from sunflower seeds were similar to those from buckwheat seeds, but not to those from rice seeds and sesame seeds.