Molecular characteristics of thiamin-binding protein from sunflower seeds

The molecular characteristics of a thiamin-binding protein (TBP) from sunflower (Helianthus annuus cv. Taiyo) seeds were investigated. The protein was composed of six subunits consisting of some combination of 29-, 25-, 24-, 22-, and 21-kDa polypeptides linked by disulfide bond(s). The amino acid composition of the polypeptides was similar: all contained a high percentage of glycine and glutamine or glutamic acid. The level of tryptophan in the polypeptides was low. The N-terminal amino acid sequences of the polypeptides had homology. This suggested that the constituent subunits of sunflower seed TBP had similar structural and biochemical properties. The apparent dissociation constant of the thiamin–protein complex was 0.04 μM. Thiamin bound to the protein with a capacity of 0.268 n mol mg–1 protein. Hydroxyethylthiamin, 2-northiamin, and thiamin phosphates did not bind to the protein. The binding to thiamin of sunflower seed TBP was inhibited by modification of the carboxyl and histidine residues of the protein as was that of the TBPs from buckwheat, rice, and sesame seeds. However, pretreatment with thiamin did not protect the protein from inactivation by the modification. Hydroxylamine did not restore the thiamin-binding activity of the sunflower seed TBP inactivated by diethylprocarbonate.