Common Molecular Features among Amaranth Storage Proteins

The structure of albumin 2 protein fraction of amaranth was investigated. It was formed by several major polypeptide subunits of molecular masses of 52.3 ± 0.8, 54 ± 2, and 56 ± 1 kDa. The former and the latter subunits were composed of a peptide of molecular mass between 31 and 38 kDa linked by S−S bonds with another peptide of molecular mass between 19 and 23 kDa. The 54 kDa subunit together with the 31−38 and 19−23 kDa subunits formed S−S-linked aggregated polypeptides. Lyophilized albumin 2 was highly polymerized, having a complex monomer component with a molecular mass of 300 ± 10 kDa. The polymers were partially stabilized by SS linkages. Because of these structural characteristics, albumin 2 was very similar to amarantin except for the presence of the 54 kDa subunit and its tendency to polymerize. Two components were obtained by gel filtration of globulin fraction. The major one exhibited heterogeneity of species and showed some common features with albumin 2. The minor component eluted at a lower volume and also showed heterogeneity, with a main species of 7S and a minor one of 12S. Their major peptides had molecular masses of 78, 72, 39, 30, and 20 kDa similar to the 7S type globulin. Its size, larger than that of amarantin, is different from a 7S type globulin, but the possibility of becoming polymerized or having a shape quite different from that of a sphere cannot be dismissed. Keywords: Amaranth; globulin; albumin 2; protein structure; gel filtration; electrophoresis