ENZYMATIC-HYDROLYSIS OF WHEY PROTEINS - INFLUENCE OF HEAT-TREATMENT OF ALPHA-LACTALBUMIN AND BETA-LACTOGLOBULIN ON THEIR PROTEOLYSIS BY PEPSIN AND PAPAIN

Proteolysis at 40-degrees-C of purified samples of alpha-lactalbumin ( alphaLa) and of beta-lactoglobulin A and B (betaLgA, betaLgB) by pepsi n at pH 2.5 and papain at pH 8.0 was studied by pH-stat experiments an d gel electrophoresis. AlphaLa is hydrolysed by pepsin but not by papa in; preheating for 8 min at 82-degrees-C hardly affects hydrolysis by either enzyme. Native betaLg is not hydrolysed by pepsin; but it is af ter preheating for 8 min at 82-degrees-C. Native betaLg is rapidly hyd rolysed by papain, the A variant more rapidly than the B variant. The betaLg molecule consists of a compact barrel-like structure from which an elongated flexible loop protrudes (M.Z. Papiz et al., Nature 1986 324:383-385). If the first peptide bond cleaved by the papain is situa ted in this loop, the different rates of hydrolysis of betaLg A and B might be ascribed to the genetic substitution of Asp64 in betaLgA for Gly64 in betaLgB in the elongated loop. The hydrolysis curve of native betaLg with papain is sigmoidal but, after preheating for 8 min at 82 -degrees-C, it becomes hyperbolic; this effect was ascribed to unfoldi ng of the polypeptide chain.