Multiple thiamine-binding proteins of legume seeds. Thiamine-binding vicilin of Vicia faba versus thiamine-binding albumin of Pisum sativum

Thiamine-binding proteins ubiquitously occur in plant seeds but they have not been characterised in legume seeds where, on the other hand, the general storage proteins are known in structural and functional detail. In this work, we show that thiamine-binding activity in seeds of several leguminous species is distributed between both globulin and albumin fractions. A thiamine-binding protein (TBP) with molecular properties typical of 7S storage globulin (vicilin) is isolated from broad bean (Vicia faba L.) seeds. It has a molecular mass of 150–170 kDa and is an oligomer containing 50 and 27 kDa subunits. It binds thiamine with a dissociation constant of 1.3 μM comparable to that of other angiosperm thiamine-binding globulins. Garden pea (Pisum sativum L.) seeds contain, in addition to a similar thiamine-binding vicilin, a smaller TBP which is a dimer of 24 kDa subunits. Its molecular characteristics are very similar to those of a well-characterised major pea albumin designated PA2. Specifically, the sequence of the first 35 amino acids from the N-terminus of TBP exactly matches that of PA2 albumin. This thiamine-binding protein binds thiamine with a low affinity (dissociation constant of 35 μM). Thiamine-binding vicilins of legume seeds probably represent a thiamine-storage function but a significance of the albumin-attributable thiamine binding remains to be determined.