The present invention relates to a method for manufacturing a potato protein extract enriched with one or more protease inhibitors, such as potato proteinase inhibitor II, potato proteinase inhibitor I, potato cysteine protease inhibitor, and potato carboxypeptidase inhibitor, that offer a significant therapeutic benefit in controlling appetite, food intake, and body weight in mammals. In particular, the present invention relates to a method of obtaining at least one proteinase protein inhibitor from a plant material comprising the steps of extracting plant proteins from the plant material using an aqueous solvent; heating the extract to denature the plant proteins; cooling the extract to precipitate the denatured plant proteins; performing a separation process on the cooled extract to remove the denatured plant proteins; and salting-out the remaining fraction of the plant proteinase inhibitors designated as PPI fraction, and desalting the PPI fraction.