Purification, complete amino acid sequence and structural characterization of the heat‐stable sweet protein, mabinlin II

A new sweet protein, named mabinlin II, was extracted with 0.5 M NaCl solution from the seeds of Capparis masaikai Lévl. and purified by ammonium sulfate fractionation, carboxymethylcellulose‐Sepharose ion‐exchange chromatography and gel filtration. The sweetness of mabinlin II was unchanged by at least 48 h incubation at nearly boiling temperature. Purified mabinlin II thus obtained gave a single band having a molecular mass of 14 kDa on SDS/PAGE. In the presence of dithiothreitol, mabinlin II gave two bands having molecular masses of 4.6 kDa and 5.2 kDa on SDS/PAGE. Two peptides (A chain and B chain) were separated from reduced and S ‐carboxamidomethylated mabinlin II by HPLC. The amino acid sequences of the A chain and B chain were determined by the automatic Edman‐degradation method. The A chain and B chain consist of 33‐amino‐acid and 72‐amino‐acid residues, respectively. The A chain is mostly composed of hydrophilic amino acid residues and the B chain also contains many hydrophilic residues. High similarity was found between the amino acid sequences of mabinlin II and 2S seed storage proteins, especially 2S albumin AT2S3 in Arabidopsis thaliana (mouse‐ear cress).