Reactive oxygen species is an aspect of oxidative stress
Reactive oxygen species is an aspect of oxidative stress which are associated with many diseases such as rheumatoid arthritis, diabetes, inflammation, atherosclerosis and cancers (Collins, 2005; Halliwell, 1994). Thus, the need for naturally occurring antioxidants is of crucial importance. Some biopeptides derived from natural proteins have significant therapeutic activities, including anti-oxidative, anti-inflammation, antimicrobial and antihypertensive actions (FitzGerald et al., 2004; Korhonen and Pihlanto, 2007; Lee et al., 2004; Silva and Malcata, 2005). The interesting feature of bioactive peptides as nutraceuticals or therapeutics is that they display little side effects in human. Some studies reported the antioxidant effects of camel milk; however, these studies were performed on fermented milk (Ayyash et al., 2017; Ayyash et al., 2018), whole milk (Al-Ayadhi and Elamin, 2013; Zhu et al., 2016), total protein hydrolysate (Al-Shamsi et al., 2018), or specific protein such as lactoferrin (Habib et al., 2013). There is little information available in the literature about the study on the antioxidant action of the isolated protein fractions of camel milk or their peptides. Thus, the assessment of the peptides from camel milk protein (CMP) fractions for their abilities to scavenge free radicals to prevent oxidative stress would be highly rewarding, has yet to be investigated and is crucially needed. Therefore, this study is to explore the antioxidant activities of the protein fractions, casein and whey, of camel milk as well as their peptides released by peptic digestion.
Materials and methods
Discussion Oxidative stress is a state characterized by increased levels of free radicals which cause damages to vital biomolecules, such as lipids, proteins and DNA. Oxidative stress is generally linked to numerous chronic diseases including atherosclerosis, cancer, diabetics, rheumatoid arthritis, cardiovascular diseases, chronic Balaglitazone sale and other degenerative diseases in human (Fridovich, 1999; Uttara, Singh, Zamboni, and Mahajan, 2009). Thus, antioxidants are vital in scavenging of radicals and prevention of the oxidative stress and the associated diseases. This study demonstrates that the peptic hydrolysates of CMP contain several peptides with antioxidant activities. A large number of peptides were obtained from caseins (Fig. 6B, P1 to P3), but the most active peptides were originated from whey proteins (Fig. 5B). The active peptides characterized by their hydrophilic nature as they eluted faster from C18 column in RP-HPLC. However, antioxidant peptides are characterized with high contents of the hydrophobic residues (Dávalos et al., 2004). The results of this study argue in favor of high contents of hydrophobic residues, such as Tyr, Leu, Phe and Ile, of the identified peptides, particularly peptides from camel caseins (Table 1). Peptides from whey proteins were rather amphiphilic in nature, due to the presence of hydrophilic residues, such as Lys, Asp, Glu and Ser, scattered along the peptides with the hydrophobic residues (Table 2). The presence of certain amino acid sequence makes contribution to the antioxidant activity of a peptide, particularly Glu-Tyr, Glu-Trp, Asn-Pro and Pro-Tyr sequence motifs (Zou et al., 2016). Peptides from camel milk whey proteins found in this study are rich in these motifs, such as Glu-Tyr, Glu-Trp, and Asp-Pro pairs in peptides with masses of 1,421.84, 1,544.61 and 1,758.37 m/z, respectively of P-CWP P2 fraction (Table 2). In caseins derived peptides, the sequences Pro-Try, Asn-Tyr, Asp-Pro and Tyr-Pro pairs were confirmed in peptides with masses of 913.12, 1,091.59, 1,059.76 and 1,092.70 m/z, respectively, of P-CCP P1 and P-CCP P2 (Table 1). The importance of these sequence for antioxidant activity has been documented that the carboxyls of acidic residues (Glu or Asp) possess excess electrons while the phenolic hydroxyls of Tyr can release hydrogen thus display high reducing abilities (Zou et al., 2016). It is noteworthy that the peptides from whey have generally Leu residue or Phe at the C-terminal (Table 2), which has been documented to be essential in many antioxidant peptides whereas hydrophobic residue at the C-termini of the peptide are essential for radical scavenging activity (Saito et al., 2003). In addition, there are evidences that intact peptides are absorbed in the upper part of human digestive tract, in the duodenum and jejunum through the tight junctions between epithelial cells, and the amphiphilic peptides and those containing Pro residues are readily absorbed (Warren et al., 2014). Thus, the use of pepsin as the hydrolyzing enzyme of CMP can be an effective approach to the production of potent antioxidant peptides and likely for the enhancement of their intestinal absorption. This should await further investigation.