Webb D J, Crookston K P, Figler N L, Lamarre J, Gonias S L
Department of Biochemistry, University of Virginia Health Sciences Center, Charlottesville 22908, USA.
Biochem J. 1995 Dec 1;312 ( Pt 2)(Pt 2):579-86. doi: 10.1042/bj3120579.
Human alpha 2-macroglobulin (alpha 2M) is a proteinase inhibitor and carrier of certain growth factors, including transforming growth factor beta 1 (TGF-beta 1). The constitutively synthesized homologue of human alpha 2M in the adult rat is alpha 1M. Rat alpha 2M is an acute-phase reactant, expressed at high levels in experimental trauma, pregnancy and in certain pathological conditions. The physiological role of rat alpha 2M is not known. In this investigation, we demonstrated that rat alpha 1M and rat alpha 2M bind TGF-beta 1. The equilibrium dissociation constants (KD) for the binding of TGF-beta 1 to the native forms of alpha 1M and alpha 2M were 257 and 109 nM respectively. alpha 1M underwent conformational change when it reacted with methylamine. The resulting product bound TGF-beta 1 with higher affinity (32 nM). Methylamine-treated rat alpha 2M did not undergo conformational change and did not bind TGF-beta 1 with increased affinity. Previous studies suggest that the native conformation may be the principal form responsible for the cytokine-carrier activity of alpha 2M in plasma and serum-supplemented cell culture medium. To confirm that native rat alpha 2M is a more efficient TGF-beta 1 carrier than native alpha 1M, fetal bovine heart endothelial cell (FBHE) proliferation assays were performed. TGF-beta 1 (5 pM) inhibited FBHE proliferation, and native alpha 2M (0.3 microM) counteracted this activity whereas alpha 1M (0.3 microM) had almost no effect. Rat alpha 2M underwent conformational change when it reacted with plasmin incorporating 1.1 mol of plasmin/mol. alpha 2M-plasmin bound TGF-beta 1; the KD (61 nM) was lower (P < 0.01) than that determined for the native alpha 2M-TGF-beta 1 interaction. These studies demonstrate that both rat alpha-macroglobulins are carriers of TGF-beta 1. The native form of rat alpha 2M probably has a predominant role, compared with native alpha 1M, as a TGF-beta 1 carrier in the plasma during the acute-phase response.
人α2-巨球蛋白（α2M）是一种蛋白酶抑制剂，也是某些生长因子的载体，包括转化生长因子β1（TGF-β1）。成年大鼠中组成性合成的人α2M的同源物是α1M。大鼠α2M是一种急性期反应物，在实验性创伤、妊娠及某些病理条件下高水平表达。大鼠α2M的生理作用尚不清楚。在本研究中，我们证明大鼠α1M和大鼠α2M可结合TGF-β1。TGF-β1与天然形式的α1M和α2M结合的平衡解离常数（KD）分别为257和109 nM。α1M与甲胺反应时发生构象变化。生成的产物与TGF-β1结合的亲和力更高（32 nM）。经甲胺处理的大鼠α2M未发生构象变化，与TGF-β1结合的亲和力也未增加。先前的研究表明，天然构象可能是α2M在血浆和血清补充的细胞培养基中作为细胞因子载体发挥活性的主要形式。为了证实天然大鼠α2M比天然α1M是更有效的TGF-β1载体，我们进行了胎牛心脏内皮细胞（FBHE）增殖试验。TGF-β1（5 pM）抑制FBHE增殖，而天然α2M（0.3 μM）可抵消这种活性，而α1M（0.3 μM）几乎没有作用。大鼠α2M与每摩尔含1.1摩尔纤溶酶的纤溶酶反应时发生构象变化。α2M-纤溶酶结合TGF-β1；其KD（61 nM）低于（P < 0.01）天然α2M-TGF-β1相互作用所测定的值。这些研究表明，两种大鼠α-巨球蛋白都是TGF-β1的载体。在急性期反应期间，与天然α1M相比，天然大鼠α2M在血浆中作为TGF-β1载体可能起主要作用。
