Three thiamin‐binding proteins (TBPs) from sesame (Sesamum indicum L.) seeds (STBP‐I, ‐II and ‐III) were characterized. Binding of thiamin to the three STBPs was inhibited by pyrithiamin, which did not inhibit the binding of thiamin to TBPs from other plant seeds. STBP‐I alone bound 2‐northiamin and hydroxyethylthiamin. Isoelectric points (pIs) of STBP‐I and ‐II both were 7.5. The pI of STBP‐III was 6.5. STBPs did not have immunological homology with TBPs from rice seeds and buckwheat seeds. On the other hand, the amino acid compositions of the small and large polypeptides isolated from STBP‐I, ‐II and ‐III resembled each other. Both the polypeptides contained large amounts of Glu (or Gln) and Arg. The small polypeptides contained more Ser than the large polypeptides. The N‐terminal amino acid sequences (the first 29 residues) of the small polypeptides were identified. The N‐terminal amino acid sequences of the three STBPs were the same. The small polypeptides had homology to castor bean 2S albumin small subunit. These results showed that STBPs were part of a plant protein superfamily and that STBPs differed from the TBPs of other plant seeds as to the binding to thiamin‐related compounds and immunological properties, and further, that STBP‐I, ‐II and ‐III differed in the affinity for thiamin‐related compounds and pI, indicating that STBP‐I, ‐II and ‐III are isomers.
SummaryA 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 con sisted of polypeptides linked by disulfide bond(s). The protein contained a large amount of glutamine or glutamic acid (19.9 mol%) and aspa ragine 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, oxy thiamin, 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. Key Words seed, storage protein, sunflower, thiamin, thiamin-binding protein Plant seeds contain thiamin-binding proteins (TBPs) that specifically bind free thiamin, but not thiamin phosphates (1). The TBPs are assumed to retain thiamin, which is accumulated into dormant seeds for germination, and the proteins are degraded to supply thiamin and a nitrogen source during germination (2). TBPs having such dual functions are novel storage proteins because, in general, storage proteins have no functions except to provide nitrogen sources during germination. However, only TBPs from sesame seeds, rice seeds and buckwheat seeds have been characterized (1-5). The results indicate that these TBPs differ in molecular weight and subunit structure, amino acid composition, optimum pH for thiamin-binding activity, and binding activity for thiamin analogs, although the TBPs are found only in seeds and specifically bind free thiamin, but not thiamin phosphates. The
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