Enzymatic Modification of Proteins in Foodstuffs

Part IV. Bitter Dipeptides as Plastein-building Blocks Debittering of Peptic Proteolyzate with α-Chymotrypsin

Abstract

A bitter proteolyzate obtained by incubating soy protein with pepsin under the conventional condition contained 80.0μmole% glycyl-L-leucine (Gly-Leu) and 64.7μmole% L-leucyl-L-phenylalanine (Leu-Phe). On treating the bitter proteolyzate with α-chymotrypsin under the condition for plastein synthesis, the resulting product showed no bitterness and contained neither Gly-Leu nor Leu-Phe. Both Gly-Leu and Leu-Phe were insusceptible to the α-chymotryptic hydrolysis under this reaction condition. When the peptic proteolyzate to which either Gly-Leu or Leu-Phe was added (final content: 500μmole% each) was submitted to the plastein reaction, Gly-Leu and Leu-Phe disappeared within 12hr and 4hr, respectively, in spite of their insusceptibility to the α-chymotryptic hydrolysis. When the peptic proteolyzate to which a large amount of either Gly-Leu or Leu-Phe was added (final content: 500mmole% each) was submitted to the plastein reaction, the 50% aqueous ethanol insoluble core from each plastein-reaction product contained, as its constituents, significantly increased amounts of either glycine and leucine, or leucine and phenylalanine. Free glycine, leucine and phenylalanine each, however, did not participate the plastein synthesis. Accordingly it is concluded that, so far as the bitter peptides, Gly-Leu and Leu-Phe, are concerned, they play roles of plastein-building blocks, constitute the plastein in co-operation with other peptides in the peptic proteolyzate, and are hence deprived of their bitterness. In the case of Gly-Leu, its leucine terminus was mostly participant of constituting the plastein and, as a result, the glycyl-L-leucyl residues seemed to localize rather in the N-termini of the plastein chains. In the case of Leu-Phe, its both termini participated the plastein synthesis and, consequently, the L-leucyl-L-phenylalanyl residues seemed to be distributed almost uniformly in the plastein chains.