Purification and partial characterization of a second cysteine proteinase inhibitor from ungerminated barley (Hordeum vulgare L.)

It was previously shown that ungerminated barley contains inhibitors that s uppress the activities of green malt cysteine proteinases. This paper repor ts the purification and partial characterization of a second barley cystein e endoproteinase inhibitor, a protein called lipid transfer protein 2 (LTP2 ). The chromatographically purified inhibitor had a molecular mass of 7112. The amino acid composition and sequence data of the purified inhibitor ind icated that it was a protein whose gene, but not the protein itself, was is olated earlier from barley aleurone tissue. The purified protein inhibited the activities of electrophoretically separated green malt cysteine protein ases but not the activities of the serine- or metalloproteinases. The purif ied LTP2 inhibited the same proteases as the LTP1 that was characterized pr eviously but was present in the mature seed in much smaller amounts. Neithe r LTP1 nor LTP2 has been proven to transport lipids in vivo, and it seems p ossible that both serve to keep cysteine endoproteinases that are synthesiz ed during barley seed development inactive until the plant needs them. The small amount of LTP2 in the seed made it impossible to determine whether it , like LTP1, is involved in beer foam formation. Because of its proteinase- inhibiting ability and its resistance to heat inactivation, some of the LTP 2 may persist in beer.