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Comparison of the specificity and kinetic properties of 3 milk-clotting enzymes

Published online by Cambridge University Press:  01 June 2009

Gizella Kovács-Proszt  and
T. Sanner
Gizella Kovács-Proszt
Affiliation:
Norsk Hydro's Institute for Cancer Research, The Norwegian Radium Hospital, Montebello, Oslo, Norway
T. Sanner
Affiliation:
Norsk Hydro's Institute for Cancer Research, The Norwegian Radium Hospital, Montebello, Oslo, Norway
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Summary

The biochemical properties of rennin purified from calf stomach, Mucor pusillus and Endothia parasitica have been compared.

The specificities of the enzymes have been studied by separating by disk-gel electrophoresis the macropeptides released on incubation with acid casein and κ-casein. The products formed on incubation with rennin from calf stomach and M. pusillus showed some similarity although several bands were found which did not correspond to one another. The products formed with rennin from E. parasitica differed considerably from those with the 2 other enzymes.

The kinetic properties of the enzymes have been investigated in stopped-flow experiments on the enzyme-induced aggregation of κ-casein. The aggregation has been characterized by 2 parameters—the lag time prior to the increase in extinction and the maximum rate of change in extinction during aggregation.

The specific activity measured from the inverse value of the lag time was under optimal conditions nearly 2·5 times higher for rennin from calf stomach than for rennin from the 2 micro-organisms. The fact that with increasing enzyme concentration the maximum rate of change of extinction during aggregation was identical for all 3 enzymes indicates that the same product is responsible for the formation of the aggregates. The data suggest that the Km value for the aggregation of κ-casein by the enzymes is about 100 times smaller than Km for the proteolytic activity of calfstomach rennin on κ-casein. The possibility is suggested that the high specificity of the enzymes in milk clotting is due to the very low Km values for the peptide bond modifications involved in the formation of para-κ-casein, the first enzymic step in the clotting of milk.

Type
Research Article
Information
Journal of Dairy Research , Volume 40 , Issue 2 , June 1973 , pp. 263 - 272
Copyright
Copyright © Proprietors of Journal of Dairy Research 1973

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References

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