Serological studies on heat-induced interactions of α-lactalbumin and milk proteins

Abstract

The heat denaturation of a-lactalbumin (a-la) in NaCl and KC1 solutions, milk ultrafiltrate and milk was studied using the method of micro complement fixation. It was established that this protein was very resistant to heat denaturation and that it was more stable in milk ultrafiltrate than in the other media studied at temperatures up to 70 °C. Of the various milk proteins added to a-la, only /?-lactoglobulin (/?-lg) formed a heat-induced complex with this protein. This complex was identical in milk ultrafiltrate or in milk and depended on the molar ratio between both proteins; it was not modified by any other milk proteins. The binding of a-la to yff-lg changed the ability of the latter protein to bind /c-casein.The heat-induced interactions between /Mactoglobulin (/?-lg) and casein have been one of the predominant subjects investigated in dairy chemistry during the last 20 years. In contrast, only a limited number of studies has been devoted to heatinduced complex formation between a-lactalbumin (a-la) and other milk proteins. Hunziker & Tarassuk (1965), however, were able to demonstrate that an interaction occurred between a-la and /?-lg when the proteins were heated together in a buffer at pH 6-7. It should be emphasized that most of the physico-chemical analyses performed on heat-treated milk proteins have been done in buffers, at protein concentrations often different from those found in milk; also it has been uncertain whether results so obtained could be compared to reactions in the original milk medium. Therefore, in the present study, we have compared the immunochemical behaviour of a-la in milk and milk ultrafiltrate, as well as in isolated systems after heat treatment. MATERIALS AND METHODS Reagentsa-Ladalbumin. a-Lactalbumin was prepared from bulk raw milk by gel filtration and ion-exchange chromatography by the method of Armstrong et al. (1970). Only the highest of the 3 peaks eluted from DEAE-Sephadex was used. The homogeneity of the preparation was checked by polyacrylamide-gel (PAG) disc electrophoresis and by immunoelectrophoresis using a pool of antisera against the whey proteins.

Aptitude à la gélification thermique de la β-lactoglobuline : influence du pH, de l'environnement ionique et de la présence des autres protéines du lactosérum

“…La diminution de la Tm de la~-Lg en présence d'a-La peut s'expliquer par des interactions entre ces 2 protéines. Leur existence est d'ailleurs aujourd'hui bien établie (Hunziker et Tarassuk, 1965;Baer et al, 1976;Elfagm et Wheelock, 1978;Melo et Hansen, 1978). Bien que la nature de ces interactions soit loin d'être élucidée, la participation de ponts disulfures résultant d'échanges SH (de la~-Lg) et SS (de l'a-La) à pH alcalin est très probable.…”

Characterization of heat-induced aggregates of β-lactoglobulin, α-lactalbumin and bovine serum albumin in a whey protein concentrate environment

“…Since whey proteins are a heterogenous group of proteins, it is likely that the gelation behaviour of any individual protein would be altered by the presence of the others. A number of studies have shown that soluble aggregates of whey proteins are formed during the early stages of heat-induced gel formation, and that subsequent polymerization results in the formation of a rigid gel network (Baer et al 1976). It is possible that gel properties may be influenced by the formation of specific soluble aggregates between different whey proteins in the early stages of heating.…”

Irreversible heat denaturation of bovine α-lactalbumin

“…In the first, the extent of denaturation is measured at the temperature of the experiment, as in the optical rotation studies of Hiraoka et al (1980) or the differential scanning calorimetry of Ruegg et al (1977a, b) and Pfeil (1981). In the second type, the protein solution is heated to a set temperature for a known length of time, cooled, and the extent of denaturation then measured in various ways, (Lyster, 1970;Baer et al 1976;Hillier, 1976;Levieux, 1980). Experiments of the first type have shown that heat denaturation is 80-90% reversible (Riiegg et al 1977a), whereas those of the second type are capable of measuring only the extent of irreversible denaturation.…”