Prieels J P, Maes E, Dolmans M, Leonis J
Eur J Biochem. 1975 Dec 15;60(2):525-31. doi: 10.1111/j.1432-1033.1975.tb21031.x.
beta(1-4)-Galactosyltransferase from human milk (the A protein of lactose synthase) has been found to be heterogeneous when fractionated by affinity chromatography against insolubilized alpha-lactalbumin, using a linear gradient of decreasing N-acetylglucosamine concentration. Three forms were isolated. Molecular weights of the different species, as determined by sodium dodecylsulphate gel electrophoresis, were found to be 38 000, 43 000 and 50 000. The 38 000 and 50 000 species were studied for their catalytic ability to synthesize either lactose in the presence of alpha-lactalbumin, or N-acetyllactosamine in the presence and absence of the 'specifier' protein. Appreciable difference was observed between the two enzyme forms with respect to their catalysis of lactose synthesis with alpha-lactalbumins from various sources. Differences in the rate of production of N-acetyllactosamine in the presence of alpha-lactalbumin were also observed. For the lowest-molecular-weight species it was found that the inhibitory effect of alpha-lactalbumin upon N-acetyllactosamine synthesis becomes an activating effect at higher alpha-lactalbumin concentrations, while no such inversion was observed for the other species. The results suggest that the conformation at the site of association of the enzyme with the acceptor saccharide or alpha-lactalbumin has been changed, presumably by a pratial enzymic hydrolysis.
人乳中的β(1-4)-半乳糖基转移酶（乳糖合酶的A蛋白），在用N-乙酰葡糖胺浓度递减的线性梯度，通过与不溶性α-乳白蛋白进行亲和层析分级分离时，被发现具有异质性。分离出了三种形式。通过十二烷基硫酸钠凝胶电泳测定，不同种类的分子量分别为38000、43000和50000。对分子量为38000和50000的种类，研究了它们在α-乳白蛋白存在时合成乳糖，以及在有和没有“特异性”蛋白存在时合成N-乙酰乳糖胺的催化能力。在这两种酶形式催化来自各种来源的α-乳白蛋白合成乳糖方面，观察到了明显差异。在α-乳白蛋白存在时，也观察到了N-乙酰乳糖胺产生速率的差异。对于分子量最低的种类，发现α-乳白蛋白对N-乙酰乳糖胺合成的抑制作用在较高α-乳白蛋白浓度时变成了激活作用，而其他种类未观察到这种反转。结果表明，酶与受体糖类或α-乳白蛋白结合位点的构象发生了变化，推测是通过部分酶促水解。
