Characterization of peptides generated in proteolytic digest of steamed rice grains by sake koji enzymes

High-molecular-weight peptides (approximately 10-30 kDa) generated in a digest of steamed rice grains by sake koji enzymes were characterized. Among 13 major spots resolved by 2-D gel electrophoresis, 12 contained peptides having N-termini of rice glutelin as determined by mass fingerprinting analysis and/or MS/MS. The source of these peptides was presumed to be the acidic subunit of rice glutelin. An addition of up to 25% glucose in the digestion of an isolated rice protein body induced the accumulation of these peptides. The level of accumulation of these peptides in the digest of 70% polished rice samples correlated well with the crude protein content of the rice grains. The degree of accumulation of these peptides in Yamadanishiki and low-polish-rate rice was low, whereas that observed in 90% polished rice samples was extremely low.