A method of preparation of x-casein and some observations on its nature

Action of rennet and other proteolytic enzymes on casein in casein-agar gels

A series of precipitation zones appears when proteolytic enzymes diffuse through an agar gel containing 1% whole casein and 10–20 mM-calcium chloride. As the enzyme diffuses through the gel the casein stability changes giving first a precipitate which subsequently redissolves followed by a second precipitate which also redissolves. The first or outermost zone of precipitation occurs at the periphery of the area of the diffused enzyme. Using a high enzyme concentration all four zones, two of precipitation and two of subsequent clearing, are visible in about 24 h. The proportion of the total nitrogen which occurs as non-protein nitrogen (N.P.N.) in the first or outermost precipitation zone is similar to that found after the first stage of rennet action on casein solutions.

The action of calf rennet and other proteolytic enzymes on κ-casein

The hydrolysis of κ-casein by a number of rennets and other proteolytic enzymes has been followed by measuring the increase in opacity due to the formation of insoluble aggregates of para-κ-caseins. The stability of these precipitates varied markedly, some being solubilized rapidly by the further action of the enzyme. The turbidity obtained with certain enzymes was dependent upon the calcium ion concentration, indicating that the para-κ-caseins produced were not identical for all enzymes.

For high concentrations of calf rennet, the rate of aggregation was linear with respect to time. With low concentrations of enzyme, increase in turbidity was preceded by a lag period which was lengthened by decreasing the enzyme concentration or increasing the κ-casein concentration. This increase in lag is favoured by a high κ-casein/para-κ-casein ratio, suggesting that the aggregation of newly formed para-κ-casein is prevented by the unchanged κ-casein. In addition, small amounts of αs1- or β-caseins present in the κ-casein also markedly affected the aggregation of para-κ-casein, indicating that all 3 major casein components can inhibit the aggregation of para-κ-casein in the absence of calcium ions. In the light of these observations the possible role of protein-protein interactions in casein coagulation by calf rennet is discussed.

The preparation of k -casein

A method is described for the preparation of k -casein and for its purification by chromatography on DEAE cellulose. The conditions of preparation have been chosen so as to avoid drastic alteration of the k -casein and permit the isolation in relatively pure form of one-half to two-thirds of the amount theoretically available.

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

The biochemical properties of rennin purified from calf stomach,Mucor pusillusandEndothia parasiticahave 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 andM. pusillusshowed some similarity although several bands were found which did not correspond to one another. The products formed with rennin fromE. parasiticadiffered 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 theKmvalue for the aggregation of κ-casein by the enzymes is about 100 times smaller thanKmfor 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 lowKmvalues for the peptide bond modifications involved in the formation of para-κ-casein, the first enzymic step in the clotting of milk.