3 Results and discussion
1. Effect of NaCl on antioxidant activity of peptides
The DPPH radical scavenging activity increased slightly as the NaCl contents increased from 0 to 4%, and then decreased slightly (Fig. 1a). The highest DPPH radical scavenging activity was 92.2% when the NaCl content was 4%, being significantly higher than 84.1% at the NaCl content of 8% (p b 0.05). However, the blank was 85.4%, which was not significantly different from that containing 8% NaCl. With regard to Fe2+-chelating ability, there was a difference between NaCl contents particularly between 0 and 8% (p b 0.05) where the loss was up to 25%. This changemight be the result fromthe disruption caused by high NaCl content on specific peptide structures and amino acid side chain groups responsible for chelating transition metal ions. Our study showed that below6% NaCl therewere no significant changes in antioxidant activity.
2. Effect of temperature on antioxidant activity of peptides
As the temperature increased from 25 to 60 °C, the DPPH radical scavenging activity increased and then showed a sharp decline between 60 °C and 80 °C (Fig. 1b). A similar trend was also observed in the Fe2+-chelating ability. As the temperature increased from 60 to 80 °C, the Fe2+-chelating ability halved as it decreased from 62.7% to 31.5%. Short chain, low molecular weight peptides do not have the tertiary and quaternary structure (only those proteins having molecular weight ≧50 kDa can form the quaternary structure) (John, 1999), but they still can form secondary structures, which are the key factors affecting the antioxidant activity. The extremely high temperature used here would affect the secondary structure, which would lead to the instability of antioxidant activity.
3. Effect of pH on antioxidant activity of peptides
The antioxidant activity of peptides from Jinhua hams under different pH values is shown in Fig. 1c. Peptides at the neutral pH exhibited the strongest DPPH radical scavenging activity, and there was no significant decrease under acidic conditions. Even when the pH was reduced to 3, it stillmaintained 90% of its DPPH radical scavenging activity. However, the DPPH radical scavenging activity of the peptides was greatly affected by the alkaline condition. When the pH was increased to 9, the DPPH radical scavenging activity sharply declined and at pH 11, the activity was reduced by 40% compared with that under neutral pH condition. The Fe2+-chelating ability showed a fluctuated trend, but was not significantly affected by pH. There are several factors that could account for the loss of antioxidant activity under alkaline condition. One reason is the occurrence of racemization. Under alkaline conditions it is likely that racemization reactions will occur, forming a mixture of L- and D-isomers and it is known that differences in biological activities existed between isomers. It is likely that racemization reaction occurs when peptide was in alkaline condition (Mathews & Vam-Holde, 2000). Another reason for the loss of activity could be the result of a deamidation reaction. Deamination is promoted at higher pH values resulting in changes with structure and conformation and loss of antioxidant activity. The third possibility is that the activation energy of peptide degradation varies with changing pH. Different pH values will affect the actual degradation pathway used (Bell & Labuza, 1991; Patel & Borchardt, 1990). Generally speaking, each peptide has its proper pH range. During this pH range, the structure is relatively stable as well as the antioxidant activity. In addition, the side chains of some small peptides can be hydrolyzed by alkaline catalyzed (Skwierczynski & Conners, 1993; Tsoubeli & Labuza, 1991). Therefore, alkaline conditions are unfavorable to maintain the antioxidant activity of peptides from Jinhua ham.
4. Effect of light on antioxidant activity of peptides
As shown in Fig. 2, DPPH radical scavenging activity and Fe2+-chelating ability decreased with the storage time increased, irrespective of whether they were exposed to light or dark. However, it can be seen that the DPPH radical scavenging activity and Fe2+-chelating ability were lower after 4 weeks, being susceptible to stronger lighting levels. Exposure to sunshine reduced DPPH radical scavenging activity and Fe2+-chelating ability of peptides by 35.7% and 39.2%, respectively. However, the antioxidant activities of peptides stored under reduced lighting conditions fell by about 20% after 4 weeks (p b 0.05). This showed that these peptides are sensitive to light intensity. Therefore, during the preparation and for storage, peptides should be protected from light.
5. Effect of pepsin–trypsin digestion on amino acid compositions of peptides
The content of free amino acids in the sample peptides prior to digestion accounted for 37.7% of the total amino acids (Table 1). A two hour digestion with pepsin did not significantly alter the free amino acid contents. However, following 2 h incubation with trypsin, the content of free amino acids increased by 3.9% compared to the control without digestion. This is a proof that the majority of the peptides were not digested by pepsin; theywere just reduced to small-sized peptides. However, in the presence of trypsin, these peptides were more completely hydrolyzed, releasing higher amounts of free amino acids. After the digestion was completed, the percentage of free amino acids of the sample accounted for 41.6% of the total amino acids. Thus more than 50% of the amino acids in final were present in the form of peptides. Clemente (2000) found that protein hydrolysates were better absorbed than whole proteins. Maebuchi et al. (2007) also showed that the peptide transport system plays a major role in the absorption of protein and peptide digestive products. Studies reported that peptides containing one or several hydrophobic amino acids like Val or Leu can increase the presence of the peptides at thewater-lipid interface and therefore facilitate access to scavenge free radicals generated at the lipid phase (Anusha, Samaranayaka, Eunice, & Li, 2011). In addition, metal-chelating amino acid residues such as Met, Glu, Gln, Lys and Arg within the sequences of these peptides contributed to the superior Fe2+-chelating ability of the antioxidant peptides as well as their high radical scavenging potential. In this research, hydrophobic amino acids, including Val and Leu and metal-chelating amino acid residues such as Met, Glu, Gln, Lys and Arg take up nearly 50% of the total amino acids. These amino acids, containing non polar groups, have a high binding capacity for the poly unsaturated fatty acids, which contribute to the antioxidant activity of the peptides (Jung, Kim, & Kim, 1995).
6. Effect of pepsin–trypsin simulated gastro-intestinal digestion on surface hydrophobicity
As shown in Fig. 3, digestion with pepsin for 2.0 h did not result in any change to the surface hydrophobicity (p ≧ 0.05). However, further digestion with trypsin significantly decreased the surface hydrophobicity (p b 0.05). Protein oxidation can lead to the exposure of hydrophobic groups, as can thermal treatment and protein degradation (Pacifici, Kono, & Davies, 1993). Slow heating can result in the stretching of polypeptide chains, thus exposing more hydrophobic groups. In our work, pepsin splits peptides into smaller fragments, thereby exposing previously internal groups to the environment. While trypsin hydrolyzed peptides into smaller chains, it also produced more free amino acids due to its greater hydrolytic activities. These amino acids have a greater affinity with water.
3.7. Effect of pepsin–trypsin simulated gastro-intestinal digestion on antioxidant activity of peptides
3 Results and discussion
1. Effect of NaCl on antioxidant activity of peptides
The DPPH radical scavenging activity increased slightly as the NaCl contents increased from 0 to 4%, and then decreased slightly (Fig. 1a). The highest DPPH radical scavenging activity was 92.2% when the NaCl content was 4%, being significantly higher than 84.1% at the NaCl content of 8% (p b 0.05). However, the blank was 85.4%, which was not significantly different from that containing 8% NaCl. With regard to Fe2+-chelating ability, there was a difference between NaCl contents particularly between 0 and 8% (p b 0.05) where the loss was up to 25%. This changemight be the result fromthe disruption caused by high NaCl content on specific peptide structures and amino acid side chain groups responsible for chelating transition metal ions. Our study showed that below6% NaCl therewere no significant changes in antioxidant activity.
2. Effect of temperature on antioxidant activity of peptides
As the temperature increased from 25 to 60 °C, the DPPH radical scavenging activity increased and then showed a sharp decline between 60 °C and 80 °C (Fig. 1b). A similar trend was also observed in the Fe2+-chelating ability. As the temperature increased from 60 to 80 °C, the Fe2+-chelating ability halved as it decreased from 62.7% to 31.5%. Short chain, low molecular weight peptides do not have the tertiary and quaternary structure (only those proteins having molecular weight ≧50 kDa can form the quaternary structure) (John, 1999), but they still can form secondary structures, which are the key factors affecting the antioxidant activity. The extremely high temperature used here would affect the secondary structure, which would lead to the instability of antioxidant activity.
3. Effect of pH on antioxidant activity of peptides
The antioxidant activity of peptides from Jinhua hams under different pH values is shown in Fig. 1c. Peptides at the neutral pH exhibited the strongest DPPH radical scavenging activity, and there was no significant decrease under acidic conditions. Even when the pH was reduced to 3, it stillmaintained 90% of its DPPH radical scavenging activity. However, the DPPH radical scavenging activity of the peptides was greatly affected by the alkaline condition. When the pH was increased to 9, the DPPH radical scavenging activity sharply declined and at pH 11, the activity was reduced by 40% compared with that under neutral pH condition. The Fe2+-chelating ability showed a fluctuated trend, but was not significantly affected by pH. There are several factors that could account for the loss of antioxidant activity under alkaline condition. One reason is the occurrence of racemization. Under alkaline conditions it is likely that racemization reactions will occur, forming a mixture of L- and D-isomers and it is known that differences in biological activities existed between isomers. It is likely that racemization reaction occurs when peptide was in alkaline condition (Mathews & Vam-Holde, 2000). Another reason for the loss of activity could be the result of a deamidation reaction. Deamination is promoted at higher pH values resulting in changes with structure and conformation and loss of antioxidant activity. The third possibility is that the activation energy of peptide degradation varies with changing pH. Different pH values will affect the actual degradation pathway used (Bell & Labuza, 1991; Patel & Borchardt, 1990). Generally speaking, each peptide has its proper pH range. During this pH range, the structure is relatively stable as well as the antioxidant activity. In addition, the side chains of some small peptides can be hydrolyzed by alkaline catalyzed (Skwierczynski & Conners, 1993; Tsoubeli & Labuza, 1991). Therefore, alkaline conditions are unfavorable to maintain the antioxidant activity of peptides from Jinhua ham.
4. Effect of light on antioxidant activity of peptides
As shown in Fig. 2, DPPH radical scavenging activity and Fe2+-chelating ability decreased with the storage time increased, irrespective of whether they were exposed to light or dark. However, it can be seen that the DPPH radical scavenging activity and Fe2+-chelating ability were lower after 4 weeks, being susceptible to stronger lighting levels. Exposure to sunshine reduced DPPH radical scavenging activity and Fe2+-chelating ability of peptides by 35.7% and 39.2%, respectively. However, the antioxidant activities of peptides stored under reduced lighting conditions fell by about 20% after 4 weeks (p b 0.05). This showed that these peptides are sensitive to light intensity. Therefore, during the preparation and for storage, peptides should be protected from light.
5. Effect of pepsin–trypsin digestion on amino acid compositions of peptides
The content of free amino acids in the sample peptides prior to digestion accounted for 37.7% of the total amino acids (Table 1). A two hour digestion with pepsin did not significantly alter the free amino acid contents. However, following 2 h incubation with trypsin, the content of free amino acids increased by 3.9% compared to the control without digestion. This is a proof that the majority of the peptides were not digested by pepsin; theywere just reduced to small-sized peptides. However, in the presence of trypsin, these peptides were more completely hydrolyzed, releasing higher amounts of free amino acids. After the digestion was completed, the percentage of free amino acids of the sample accounted for 41.6% of the total amino acids. Thus more than 50% of the amino acids in final were present in the form of peptides. Clemente (2000) found that protein hydrolysates were better absorbed than whole proteins. Maebuchi et al. (2007) also showed that the peptide transport system plays a major role in the absorption of protein and peptide digestive products. Studies reported that peptides containing one or several hydrophobic amino acids like Val or Leu can increase the presence of the peptides at thewater-lipid interface and therefore facilitate access to scavenge free radicals generated at the lipid phase (Anusha, Samaranayaka, Eunice, & Li, 2011). In addition, metal-chelating amino acid residues such as Met, Glu, Gln, Lys and Arg within the sequences of these peptides contributed to the superior Fe2+-chelating ability of the antioxidant peptides as well as their high radical scavenging potential. In this research, hydrophobic amino acids, including Val and Leu and metal-chelating amino acid residues such as Met, Glu, Gln, Lys and Arg take up nearly 50% of the total amino acids. These amino acids, containing non polar groups, have a high binding capacity for the poly unsaturated fatty acids, which contribute to the antioxidant activity of the peptides (Jung, Kim, & Kim, 1995).
6. Effect of pepsin–trypsin simulated gastro-intestinal digestion on surface hydrophobicity
As shown in Fig. 3, digestion with pepsin for 2.0 h did not result in any change to the surface hydrophobicity (p ≧ 0.05). However, further digestion with trypsin significantly decreased the surface hydrophobicity (p b 0.05). Protein oxidation can lead to the exposure of hydrophobic groups, as can thermal treatment and protein degradation (Pacifici, Kono, & Davies, 1993). Slow heating can result in the stretching of polypeptide chains, thus exposing more hydrophobic groups. In our work, pepsin splits peptides into smaller fragments, thereby exposing previously internal groups to the environment. While trypsin hydrolyzed peptides into smaller chains, it also produced more free amino acids due to its greater hydrolytic activities. These amino acids have a greater affinity with water.
3.7. Effect of pepsin–trypsin simulated gastro-intestinal digestion on antioxidant activity of peptides
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