Kinetic studies on the aggregation of kappa-casein by Mucor pusillus protease

Selected achievements of the Central Food Research Institute, Budapest, from its establishment until the turn of the 21 st century

The treatise provides selected examples on the activities and main results of the research and development work during the first forty years of history of the Central Food Research Institute, Budapest.

Influence of calcium chloride on the chymosin-initiated coagulation of casein micelles

Coagulation of para-casein micelles was monitored using a Platelet Aggregometer at 37·8 °C and pH 6·7. The macropeptide released by chymosin was determined quantitatively with fluorescamine. The aggregatability of para-casein micelles integrated over the complete hydrolysis of κ-casein was calculated from the amount of κ-casein hydrolysed during the clotting time (Tc). Low concentrations of CaCl2 enhanced the rate of κ-casein hydrolysis but increases in CaCl2 concentration above 8 mM caused marked decreases in this rate. Calcium chloride enhanced the ability of para-casein micelles to aggregate. Little aggregation of para-casein micelles occurred at 0·6 mM-CaCl2 even after all of the κ-casein was hydrolysed. As the concentration of CaCl2 was increased from 3 to 60 mM, aggregation of para-casein micelles took place at progressively lower levels of κ-casein hydrolysis and the percentage hydrolysis at the Tc decreased markedly from 71±7% to 26±8%. The combined influence of CaCl2 on the velocity of the chymosin-catalysed reaction and on para-casein micelle aggregatability accounted for its effect on the average rate of coagulation (calculated by the reciprocal of the Tc). Results are consistent with the hypothesis that electrostatic repulsions and ionic bonding are involved in the interaction between chymosin and κ-casein.

Characterization of bovine kappa-casein fractions and the kinetics of chymosin-induced macropeptide release from carbohydrate-free and carbohydrate-containing fractions determined by high-performance gel-permeation chromatography

Bovine kappa-casein was fractionated at pH 8.0 on DEAE-Sepharose with an NaCl gradient, followed by DEAE-cellulose chromatography using a decreasing pH gradient from pH 6.0 to 4.5. At least ten components could be identified, each differing in N-acetylneuraminic acid (NeuAc) and/or phosphorus content. Two components appeared to be multiply-phosphorylated, but did not contain NeuAc. The possible significance of this finding in relation to the mode of phosphorylation and glycosylation in vivo is discussed. A carbohydrate-free fraction as well as two NeuAc-containing fractions were compared in their substrate behaviour towards the action of the milk-clotting enzyme chymosin at pH 6.6 and 30 degrees C. To this end the trichloroacetic acid-soluble reaction products were analysed by high-performance gel-permeation chromatography. In order of increasing carbohydrate content the kcat. values found ranged from 40 to 25 s-1 and the Km values from 9 to 3 microM; the overall substrate properties of these components as reflected by the kinetic parameter kcat./Km ranged from 5 to 8 microM-1 X S-1. Irreversible polymerization of the carbohydrate-free fraction brought about a more-than-2-fold increase in Km, the kcat. value remaining virtually constant. The kcat./Km found for the cleavage of whole kappa-casein at pH 6.6 was of the same magnitude as the kcat./Km found for the polymerized carbohydrate-free fraction (i.e. about 3 microM-1 X S-1). No indication of substrate inhibition was found for the carbohydrate-free fraction.

On enzymic clotting processes V. rate equations for the case of arbitrary rate of production of the clotting species

The rate theory for enzyme-triggered coagulation reactions, such as the clotting of fibrin or casein, is extended to the case of an arbitrary rate of production of the clotting species. It is shown that the general expression for the growth of the weight-average molecular weight of the clotting product, MW, is given by MW = M1 [1 + ks[integral of 0tP(t)2dt]/P(t)], where M1 is the "monomer" molecular weigt, ks the smoluchowskian flocculation rate constant and P(t) the total number of monomers produced by the enzyme in t. In the purely smoluchowskian case P(t) stands for the total number of monomers at the beginning of the clotting process. Numerical examples in which the rate of enzymic production is governed by complete Michaelis-Menten kinetics, are compared to cases in which this rate equals Vmax. It is shown that after exhaustion of the substrate the system continues to coagulate in a purely smoluchowskian way. Turbidimetric experiments on the clotting of micelles of whole and kappa-casein are presented which suggest inactivation of the enzyme by non-productive binding in the flocs formed.

Kinetics of pepsin-initiated coagulation of kappa-casein

The kinetics of pepsin initiated coagulation of kappa-casein have been studied at pH 5.8. The primary and secondary phases of coagulation are shown to proceed simultaneously. The theory of enzymatically initiated clotting reactions proposed by Payens (Payens, T.A.J. (1976) Neth. Milk Dairy J. 30, 55--59) has been applied to this clotting system and has been used to obtain rate constants for the secondary phase of coagulation. As expected, clotting rate constants for kappa-casein increase with pepsin concentration. An activation energy of 30.6 kcal/mol has been obtained for the secondary phase of coagulation. Turbidity measurements are a convenient means for studying the secondary phase of coagulation but do not provide an unambiguous means for studying the primary phase of the reaction.

On enzymatic clotting processes. I. Kinetics of enzyme-triggered coagulation reactions

The kinetics of enzymatic clotting reactions such as the clotting of blood or milk, is analyzed. The appearance of a lag phase in the clotting is shown to be due to the difference in reaction order of enzymatic production and flocculation. The weight-average particle weight of the product formed is found to be a quadratic function of the reaction time. The condition for the clotting time is t square root of ksV/2 = C, where t is the clotting time, ks the flocculation rate constant, V the maximum rate of enzymatic product formation and C a constant. In agreement with this result double-logarithmic plots of t versus enzyme dilution are always observed to be linear over a wide range of enzyme concentrations. No statistical evidence could be produced for the widely held belief that the clotting time should be inversely proportional to the enzyme concentration. Varying exponents of the latter in its relation to the clotting time are discussed in terms of von Smoluchowski's theory of the slow coagulation of colloidal particles.

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.