Milk coagulants

Purification and Characterization of a Substrate-Size-Recognizing Metalloendopeptidase from Streptococcus cremoris H61

During the ripening of Gouda-type cheese, two kinds of endopeptidases were found to participate in the degradation of αs1-CN(f1-23), a specific product from αs1-casein hydrolyzed by chymosin. One of the endopeptidases, lactic acid bacteria endopeptidase (LEP-II), which can recognize the size of its substrates, has already been purified and characterized (T. R. Yan, N. Azuma, S. Kaminogawa, and K. Yamauchi, Eur. J. Biochem. 163:259-265, 1987). The other endopeptidase, LEP-I, was purified to homogeneity by conventional chromatographic techniques from Streptococcus cremoris H61. The enzyme appeared to be monomeric, with an apparent molecular weight of 98,000, and its isoelectric point was 5.1. For the hydrolysis of αs1-CN(f1-23), the enzyme had an optimum pH and temperature of 7.0 to 7.5 and 40�C, respectively. Its activity was inhibited by such chelating agents as EDTA and 1,10-phenanthrolin, and it could be fully reactivated by Mn 2+ . Inhibitors specific for serine and thiol proteases had no effect on the protease activity. The enzyme showed a high affinity toward the Glu-Asn peptide bond of αs1-CN(f1-23) and αs1-CN(f91-100) but showed no hydrolysis activity toward αs1-CN(f1-52), αs1-CN(61-122), αs1-CN(136-196), αs1-casein, β-casein, κ-casein, α-lactalbumin, and β-lactoglobulin. The K m and V max of LEP-I for αs1-CN(f1-23) were 14.2 pM and 139 U, respectively.

Separation of chymosin and pepsin in calf rennet by dye-ligand affinity chromatography

When calf rennet containing approximately 15% pepsin was applied to a Cibacron Blue agarose column at pH 5.5 in a low salt medium, pepsin passed through unadsorbed while chymosin was bound to the gel in the column. After washing the column, the bound chymosin was eluted with 1.7 M NaCl or 50% (v/v) aqueous ethylene glycol. The salt eluate was analyzed and found to contain greater than 97% pure chymosin. The fraction that passed through unadsorbed was found to contain greater than 96% pure pepsin. Thus a complete separation of chymosin and pepsin was effected by this technique without having to destroy either enzyme. Both enzymes are highly negatively charged at pH 5.5 but the separation does not arise from anion exchange since the gel functions as a cation exchanger. The separation appears to result from a combination of hydrophobic and electrostatic interactions of chymosin with Blue agarose. It is suggested that the enhanced affinity of chymosin to the Blue gel over pepsin may arise from topographically specified interaction between chymosin and the blue chromophore. Differential surface hydrophobicity may also play a key role, since in the presence of 0.7 M Na2SO4 the same behavior as at low ionic strength is observed.

The influence of temperature on the flocculation rate of renneted casein micelles

The flocculation rate constant of completely renneted casein micelles in milk ultrafiltrate was measured by Rayleigh light scattering between 20 and 35 degrees C. In this temperature range an apparent energy of activation of 103 kJ mol (+/-11 kJ mol : n = 50) was measured. At 15 degrees C clotting was not longer perceptible. The activation of the flocculation between 20 and 35 degrees C is explained not so much by the height of the energy barrier separating the clotting micelles, as by the very negative temperature coefficient of that barrier. In line with this conclusion it is suggested that renneted micelles adhere through hydrophobic bonding. The flocculation rate constant of renneted casein micelles is independent of micelle size at the four temperature levels studied.