Mitranic M M, Pâquet M R, Moscarello M A
Department of Biochemistry, Hospital for Sick Children, Toronto, Canada.
Biochim Biophys Acta. 1988 Oct 12;956(3):277-84. doi: 10.1016/0167-4838(88)90144-6.
The activation of galactosyltransferase (UDPgalactose: N-acetyl-D-glucosaminyl-glycopeptide 4-beta-D-galactosyltransferase, EC 2.4.1.38) by alpha-lactalbumin has been studied at low concentrations of alpha-lactalbumin where the relationship is sigmoidal. The sigmoidal shape of the activation curve was eliminated by neutral lipids such as phosphatidylcholine and phosphatidylethanolamine, detergents such as Triton X-100 or by an aggregated form of alpha-lactalbumin generated by crosslinking alpha-lactalbumin with dithiobissuccinimidylpropionate. It is proposed that these different reagents present a hydrophobic surface to the enzyme which is necessary for lactose synthase activity. In competition experiments, large amounts of alpha-lactalbumin were able to displace lipid from the enzyme as suggested by the loss of the lipid-activating effect in the presence of an excess of alpha-lactalbumin. Optimal lactose synthase activity was obtained when the ratio of lipid/alpha-lactalbumin/enzyme was 60:6:1. The mechanism by which the lipid effect was obtained probably involved a phase transition in the enzyme which was detected as a sharp break in the Arrhenius curve. The presence of phosphatidylcholine abolished the break demonstrating that full activity of the enzyme required both alpha-lactalbumin and lipid.
在低浓度α-乳白蛋白的情况下，研究了α-乳白蛋白对半乳糖基转移酶（UDP-半乳糖：N-乙酰-D-葡糖胺基糖肽4-β-D-半乳糖基转移酶，EC 2.4.1.38）的激活作用，此时二者关系呈S形曲线。中性脂质如磷脂酰胆碱和磷脂酰乙醇胺、去污剂如Triton X-100或通过二硫代双琥珀酰亚胺丙酸酯交联α-乳白蛋白产生的α-乳白蛋白聚集形式可消除激活曲线的S形。有人提出，这些不同的试剂为该酶提供了一个疏水表面，这是乳糖合酶活性所必需的。在竞争实验中，大量的α-乳白蛋白能够从酶中取代脂质，这从过量α-乳白蛋白存在时脂质激活效应的丧失可以看出。当脂质/α-乳白蛋白/酶的比例为60:6:1时，可获得最佳乳糖合酶活性。获得脂质效应的机制可能涉及酶中的相变，这在Arrhenius曲线中表现为一个急剧的转折点。磷脂酰胆碱的存在消除了这个转折点，表明该酶的完全活性需要α-乳白蛋白和脂质两者。
