ISOLATION, CHARACTERIZATION AND CELL GROWTH-REGULATORY PROPERTIES OF KUMARA (SWEET-POTATO) TRYPSIN-INHIBITORS

Locally grown kumara (sweet potato; Ipomea batatis) was used as the st arting point for the purification of a proteinase inhibitor. The purif ied inhibitor was highly specific for trypsin, and much less effective as an inhibitor of chymotrypsin. Two 22kDa variants were present, clo sely homologous to each other and to sporamin A, but with a single ami no acid substitution (proline in place of serine, the second residue i n mature sporamin A). One variant had the same N-terminus as sporamin A, whereas the other had a tripeptide N-terminal extension, which may represent an intermediate in the proteolytic processing of the precurs or protein. A larger variant was apparently a disulphide linked dimer of the monomeric inhibitor. A rabbit polyclonal antiserum prepared aga inst the trypsin inhibitor reacted with all of these variants, but did not cross-react with commercially-available soybean proteinase inhibi tors. The purified inhibitor did resemble other proteinase inhibitors in having a biphasic effect upon the proliferation of human fibroblast s, with a mitogenic action at low concentrations, and an inhibitory ef fect at higher concentrations.