Naturally Occurring Protein Crystals in the Potato

Abstract

Protein crystals isolated from potato tubers consisted of a proteinase inhibitor active against the cysteine proteinases papain, chymopapain and ficin. The MW as determined by gel filtration pH 4.3 or by gel electrophoresis in the presence of dodecylsulfate was 80 kildaltons. When the inhibitor was evaluated at pH 8.4 in a linear concentration (4-30% polyacrylamide) under nondenaturing conditions, it appeared as 2 bands of approximately 320-350 kilodaltons, indicating that the inhibitor forms tetrameric aggregates in neutral or weakly alkaline media, while the monomeric form predominates under acidic conditions. Gel filtration in the presence of varying amounts of papain suggested that the monomer combines with 4 papain molecules. The inhibitor contains no cystine.