Spontaneous, non-enzymatic breakdown of peptides during enzymatic protein hydrolysis

• A-specific peptides are formed during enzymatic protein hydrolysis.
• A-specific peptides are formed from instable parental peptides.
• Synthetic parental peptides show spontaneous cleavage in absence of enzyme.
• Spontaneous cleavage of synthetic peptides is accelerated in the presence of enzyme.
• Proteases bind all peptide bonds in a protein independently of the specificity.

It is expected that during the hydrolysis of proteins with specific enzymes only peptides are formed that result from hydrolysis of the specific cleavage sites (i.e. specific peptides). It is, however, quite common to find a-specific peptides (i.e. resulting from a-specific cleavage), which are often ignored, or explained by impurities in the enzyme preparation. In recent work in a whey protein isolate (WPI) hydrolysate obtained with the specific Bacillus licheniformis protease (BLP), 13 peptides of 77 identified were found to be the result of a-specific cleavage. These were formed after degradation of 6 specific peptides, after 5 different types of amino acids. The fact that other peptides were not hydrolyzed after these 5 amino acids suggests that the cleavages were not the result of a contamination with a different enzyme. In other systems, certain peptide sequences have been described to degrade chemically, under relatively mild conditions. This process is referred to as spontaneous cleavage. To test if the a-specific peptides observed in the WPI hydrolysis are the results of spontaneous cleavages, the parental peptides were synthesized. Surprisingly, 4 of the 5 synthesized peptides were indeed spontaneously cleaved under the mild conditions used in this study (i.e. 40 °C and pH 8) showing that peptides are less stable than typically considered. The rate of cleavage on the a-specific bonds was found to be enhanced in the presence of BLP. This suggests that the formation of a-specific peptides is not due to side activity but rather an enhancement of intrinsic instability of the peptides.

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