The unique enzymatic function of field bean (Dolichos lablab) D-galactose specific lectin: a polyphenol oxidase.

The polyphenol oxidase (PPO) of field bean (Dolichos lablab) is a tetramer made up of two subunits of mass 29,000 and 31,000 Da. The amino acid sequence of the tryptic peptides showed approximately 90% sequence identity to the D: -galactose specific legume lectins. The haemagglutinating activity of a pure and homogenous preparation of PPO measured using human erythrocytes was 1261 HAU mg(-1) protein and was inhibited by D: -galactose. Purification by galactose-sepharose chromatography also indicated that the PPO and haemagglutinating activities were associated with a single protein. Crude extracts of other legumes did not exhibit PPO activity, yet cross reacted with anti-PPO antibodies. This dual function protein with PPO and haemagglutinating activity is unique to field bean. The two activities are independent of each other occurring at different loci on the protein. These observations further evidence and strengthen the assumption that galactose specific legume lectins have enzymatic function. Both PPO and lectins are proteins that play a vital role in the defense mechanism of plants. The complementarity of these two simultaneous and independent powerful defense mechanisms exhibited by a single protein renders it a candidate gene for the development of inbuilt plant protection.