Mass spectrometric characterisation of proteins in rennet and in chymosin-based milk-clotting preparations

A novel electrochemical assay for chymosin determination using a label-free peptide as a substrate

Chymosin is a predominant enzyme in rennet and is used in cheese production because of its excellent milk-clotting activity. Herein, we proposed a facile and label-free electrochemical method for determining chymosin activity based on a peptide-based enzyme substrate. The synthesized substrate peptide for chymosin was assembled onto the surface of the Au-deposited grassy carbon electrode. The current was proportional to chymosin activity, and thus chymosin activity could be determined. The detection ranges of chymosin activity were 2.5 to 25 U mL^-1. The detection limit of chymosin activity was 0.8 U mL^-1. The sensing platform was used to quantify chymosin activity in commercial rennet with high selectivity, excellent stability, and satisfactory reproducibility. We developed a facile, fast, and effective electrochemical assay for detecting chymosin activity, which has potential applications in cheesemaking.

Development and evaluation of a monoclonal antibody-based inhibition ELISA for the quantification of chymosin in solution

Chymosin is the major enzyme of natural rennet, traditionally used in cheese making for its high milk-clotting activity. For technical reasons, an accurate characterization of rennet should include its total clotting activity and also its enzymatic composition. Monoclonal antibodies specific to chymosin were obtained from mice immunized with purified bovine chymosin, and an inhibition enzyme-linked immunosorbent assay (ELISA) was developed for the quantification of chymosin in solution. No cross-reactivity was observed with other milk-clotting enzymes commonly used in cheese making. The limit of detection and limit of quantification were 125 and 400 ng/mL, respectively. The values of precision within and among runs were 7.23 and 7.39%, respectively, and satisfying recovery, from 92 to 119%, was found for spiked samples. The inhibition ELISA was successfully applied to commercial rennets, and the results were consistent with those obtained using the standard chromatographic method (IDF 110: A, 1987).

Validation of recombinant and bovine chymosin by mass spectrometry

Mass spectrometry has been used to map chymosin from a fermentative source. The copresence of the two known genetic variants A (Asp(244)) and B (Gly(244)) was ascertained in bovine chymosin. By contrast, either the A or the B genetic variant occurred in the three commercial samples of recombinant calf chymosin (RCC). Specific biomarker proteins were searched to identify the enzyme source, in both bovine chymosin and RCC samples. Analyzing the derived tryptic peptides, evidence was provided that RCC and bovine chymosin are mainly formed by (1-323), (3-323), and (40p-323) (suffix "p" denotes residues in the pro-segment region of chymosin), whereas the minor components, (4-323), (5-323), and (6-323), were only detected in bovine chymosin. Additionally, the three commercial RCC samples contained the protein species (1-323), (38p-323), (39p-323), and (40p-323) and the shorter form (3-323). Differentiation of the natural and bioengineered enzyme is based upon the detection of these unique minor components by mass spectrometry.