Two series of combinatorial tripeptide libraries were constructed, based on an antioxidative peptide isolated from a soybean protein hydrolysate. One was a library of 108 peptides containing either His or Tyr residues. Another was a library of 114 peptides related to Pro-His-His, which had been identified as an active core of the antioxidative peptide. The antioxidative properties of these libraries were examined by several methods, such as the antioxidative activity against the peroxidation of linoleic acid, the reducing activity, the radical scavenging activity, and the peroxynitrite scavenging activity. Two Tyr-containg tripeptides showed higher activities than those of two His-containing tripeptides in the peroxidation of linoleic acid. Tyr-His-Tyr showed a strong synergistic effects with phenolic antioxidants. However, the tripeptide had only marginal reducing activity and a moderate peroxynitrite scavenging activity. Cysteine-containing tripeptides showed the strong peroxynitrite scavenging activity. Change of either the N-terminus or C-terminus of Pro-His-His to other amino acid residues did not significantly alter their antioxidative activity. Tripeptides containing Trp or Tyr residues at the C-terminus had strong radical scavenging activities, but very weak peroxynitrite scavenging activity. The present results allow us to understand why protein digests have such a variety of antioxidative properties.
The development of degradable and edible films from protein sources has drawn significant attention for the utilisation of natural resources as well as for the alleviation of the environmental burden. Rice bran protein (RBP) was applied to protein film preparation in this study. The protein solutions were casted on plastic tissue culture dishes with glycerol as a plasticiser after heat treatment. Functional properties of the films were then measured. The puncture strength (PS) of RBP films increased up to pH 8.0 and then decreased. PS of protein films depends on the degree of protein purity, quality and composition. Higher concentration of glycerol weakened the films. The pH affected the water solubility of RBP films and the films showed least solubility at pH 3.0. RBP could be utilised in the preparation of degradable protein-based films. The RBPbased film had functional properties comparable to those of the soy protein-based ones.
The acid hydrolysis of proteins was miniaturized and simplified by employing microcapillary tubes (100 microl in volume) with 6 M HCl containing 1% 2-mercaptoethanol and 3% phenol for an amino acid compositional analysis. The method not only eliminated the laborious evacuation step for the hydrolysis tube but also decreased the destruction of tryptophan during hydrolysis. The recovery of tryptophan was 79% by acid hydrolysis at 145 degrees C for 4 h. Since the acid mixture could be removed under vacuum, the hydrolysate was subjected to an amino acid analysis without neutralization or dilution.
Protein hydrolysates, prepared by enzymatic digestion of soybean protein and egg white albumin using several proteases, inhibited the crystal growth of calcium carbonate. Each hydrolysate showed different inhibitory activities, suggesting the key role of peptide structures in the inhibition. The deamidation of protein hydrolysates by glutaminase increased not only the inhibitory activity toward the crystal growth of calcium carbonate but also the resistance of the hydrolysates against peptic digestion. Furthermore, the addition of sodium chloride, citric acid, or lactose into the reaction mixture enhanced the inhibitory activity. The protein hydrolysates inhibited both nucleation and crystal growth of calcium carbonate and also affected the crystal morphology.
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