Abstract
Sheep cheese whey (SCW) is a by-product of the cheese making process that is often considered waste and treated as an environmental pollutant. This research project stemmed from a recent change in view that rather than being a by-product by some companies, SCW is a potential source of bioactive components that can be used in the formulation of various functional foods. Beta-lactoglobulin (B-Lg) is the main constituent of the soluble whey protein fraction. Bioactive peptides are short amino acid sequences that become active upon cleavage from a precursor protein, in this case B-Lg, and can exhibit various functions including antioxidant and antihypertensive activity in a health promoting role. Bioactive peptides can be considered nutraceuticals and used as additives in functional foods. An aim of this project was to evaluate whether proteases of non-gut origin could hydrolyse ovine B-Lg, and whether novel peptides could be generated. Another aim was to use the novel peptides and design similar sequences to correlate specific amino acid sequence with function in terms of bioactivity displayed by the peptide.
β-Lg from SCW was enriched using ion exchange chromatography. Enriched B‑Lg was subjected to hydrolysis using commercially available non-gut fungal-, plant- and bacterial-derived food-grade proteases. Fungal protease II (FPII) produced the most complete hydrolysis of β‑Lg following extended incubation, whereas papain achieved limited hydrolysis, generating two fragments of β-Lg. Using the results of FPII hydrolysis of B-Lg, peptide sequences were designed, synthesised and then the antioxidant capacity and antihypertensive activity (angiotensin-1 converting enzyme (ACE) inhibitor activity) of the peptides were established. After that the susceptibility of the synthetic peptides to simulated gastrointestinal hydrolysis was established, insights into the potential health benefits conveyed by the peptides was evaluated. The ACE inhibitor IC50 values for the peptides GQCH, LKP, SAAPLRVY, VEELKP and LDAQSAPLRVY that were determined, were comparable with those previously reported in the literature and the lowest IC50 was 0.044 M as exhibited by LKP. Tripeptides with a hydrophobic, a positively charged residue and a proline moiety have been found to be effective ACE inhibitors. Synthetic peptides DTDYKK, DTDYKKYLLF and SAPLRVY were found to display the highest antioxidant activity of all the peptides assayed, indicating the possible importance of arginine and tyrosine residues in the sequence. Results highlighted the correlation between the amino acid sequence of the peptide and the bioactivity displayed by the peptides. The results of this thesis showed that B‑Lg is a good source of bioactive peptides and that these peptides can be used to evaluate the functional importance of particular amino acids.