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 Dgalactose.
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.