PURIFICATION, INHIBITORY PROPERTIES, AMINO-ACID-SEQUENCE AND IDENTIFICATION OF THE REACTIVE-SITE OF A NEW SERINE PROTEINASE-INHIBITOR FROM OIL-RAPE (BRASSICA-NAPUS) SEED

A new serine proteinase inhibitor, rapeseed trypsin inhibitor (RTI), h as been isolated from rapeseed (Brassica napus var. oleifera) seed. Th e protein inhibits the catalytic activity of bovine beta-trypsin and b ovine alpha-chymotrypsin with apparent dissociation constants of 3.0 x 10(-10) M and 4.1 x 10(-7) M, at pH 8.0 and 21 degrees C, respectivel y. The stoichiometry of both proteinase-inhibitor complexes is 1:1. Th e amino acid sequence of RTI consists of 60 amino acid residues, corre sponding to an M(t) of about 6.7 kDa. The P-1-P-1' reactive site bond has been tentatively identified at position Arg(20)-Ile(21). RTI shows no similarity to other serine proteinase inhibitors except the low mo lecular weight mustard trypsin inhibitor (MTI-2). RTI and MTI-2 could be members of a new class of plant serine proteinase inhibitors.