Greene W C, Parker C M, Parker C W
J Biol Chem. 1976 Jul 10;251(13):4017-25.
In an effort to clarify the mechanism by which certain plant lectins induce lymphocyte activation, we have studied amino acid (amino[14C]isobutyric acid) uptake in purified human lymphocytes exposed to mitogenic and nonmitogenic lectins. In confirmation of earlier work, mitogenic lectins (concanavalin A and two phytohemagglutinins) produced a dose-related, 2- to 10-fold increase in aminoisobutyric acid transport. Changes occurred as early as 2 hours and reached a maximum after 18 hours. The stimulation by concanavalin A was inhibited by alpha-methyl-D-mannoside but not by other selected monosaccharides, indicating that the effect is modulated through specific carbohydrate receptors. In contrast to the stimulation with concanavalin A and phytohemagglutinin, the nonmitogen wheat germ agglutinin inhibited aminoisobutyric acid transport, both in the presence and absence of the mitogenic lectins. The inhibition was seen over a broad wheat germ agglutinin dose range, was prevented by N-acetylglucosamine, a known inhibitor of wheat germ agglutinin binding, and did not appear to be associated with cytotoxicity. Comparative binding studies with radiolabeled concanavalin A and wheat germ agglutinin demonstrated that differences in transport occurred in cells containing comparable numbers of bound lectin molecules indicating that the failure of wheat germ agglutinin to stimulate a response was not a result of ineffective binding. The lack of stimulation by wheat germ agglutinin was not due to its inability to interact multivalently with membrane receptors since this lectin is divalent, produces capping and agglutination, and continues to inhibit aminoisobutyric acid transport even after the minimum valence was increased to 4 by cross-linking with glutaraldehyde. In contrast both divalent and tetravalent concanavalin A produced stimulation. Competitive binding studies with soluble wheat germ agglutinin or lectin attached to 300 A latex spheres revealed little or no competition for binding sites with radiolabeled concanavalin A, suggesting the two lectins are interacting with different receptors. This was further suggested by kinetic studies of aminoisobutyric acid transport which indicated that wheat germ agglutinin was probably affecting both the Vmax and Km of transport, whereas concanavalin A affected only the Vmax. While the mechanism by which concanavalin A and wheat germ agglutinin produce opposing effects on amino acid transport is not clear, since the two lectins appear to be interacting with different surface receptors we would suggest that they are perturbing microanatomically and functionally different domains on the lymphocyte plasma membrane.
为了阐明某些植物凝集素诱导淋巴细胞活化的机制，我们研究了暴露于促有丝分裂和非促有丝分裂凝集素的纯化人淋巴细胞中氨基酸（氨基[14C]异丁酸）的摄取。正如早期研究证实的那样，促有丝分裂凝集素（伴刀豆球蛋白A和两种植物血凝素）使异丁酸转运增加了2至10倍，且呈剂量相关。变化最早在2小时出现，18小时后达到最大值。伴刀豆球蛋白A的刺激作用被α-甲基-D-甘露糖苷抑制，但未被其他选定的单糖抑制，这表明该效应是通过特定的碳水化合物受体调节的。与伴刀豆球蛋白A和植物血凝素的刺激作用相反，非促有丝分裂的麦胚凝集素在有或没有促有丝分裂凝集素存在的情况下均抑制异丁酸转运。在较宽的麦胚凝集素剂量范围内均可见到抑制作用，已知的麦胚凝集素结合抑制剂N-乙酰葡糖胺可阻止这种抑制作用，且这种抑制作用似乎与细胞毒性无关。用放射性标记的伴刀豆球蛋白A和麦胚凝集素进行的比较结合研究表明，在含有相当数量结合凝集素分子数量的细胞中发生了转运差异，这表明麦胚凝集素未能刺激反应不是结合无效的结果。麦胚凝集素缺乏刺激作用不是由于其无法与膜受体多价相互作用，因为这种凝集素是二价的，可产生帽化和凝集作用，并且即使在通过与戊二醛交联将最小价态增加到4之后仍继续抑制异丁酸转运。相比之下，二价和四价的伴刀豆球蛋白A均产生刺激作用。用可溶性麦胚凝集素或附着于300 A乳胶球的凝集素进行的竞争性结合研究表明，与放射性标记的伴刀豆球蛋白A几乎没有或没有竞争结合位点，这表明这两种凝集素与不同的受体相互作用。异丁酸转运的动力学研究进一步表明了这一点，该研究表明麦胚凝集素可能同时影响转运的Vmax和Km，而伴刀豆球蛋白A仅影响Vmax。虽然伴刀豆球蛋白A和麦胚凝集素对氨基酸转运产生相反作用的机制尚不清楚，但由于这两种凝集素似乎与不同的表面受体相互作用，我们认为它们正在扰乱淋巴细胞质膜上微观解剖学和功能上不同的区域。
