Takeya A, Hosomi O, Kogure T
Department of Legal Medicine, St. Marianna University School of Medicine, Kanagawa, Japan.
Biochim Biophys Acta. 1998 Sep 16;1425(1):215-23. doi: 10.1016/s0304-4165(98)00074-9.
Plant seed lectins play a defense role against plant-eating animals. Here, GalNAc-specific Vicia villosa B4 lectin was found to inhibit hydrolysis of UDP-GalNAc by animal nucleotide pyrophosphatases, which are suggested to regulate local levels of nucleotide sugars in cells. Inhibition was marked at low concentrations of UDP-GalNAc, and was reversed largely by the addition of GalNAc to the reaction mixture. In contrast, lectin inhibited enzymatic hydrolysis of other nucleotide sugars, such as UDP-Gal and UDP-GlcNAc, only to a small extent, and GalNAc did not affect such an inhibition. The binding constant of the lectin for UDP-GalNAc was as high as 2.8 x 10(5) M-1 at 4 degrees C, whereas that for GalNAcalpha-1-phosphate was 1.3 x 10(5) M-1. These findings indicate that lectin inhibition of pyrophosphatase activity toward low concentrations of UDP-GalNAc arises mainly from competition between lectin and enzyme molecules for UDP-GalNAc. This type of inhibition was also observed to a lesser extent with GalNAc-specific Wistaria floribunda lectin, but not apparently with GalNAc-specific soybean or Dolichos biflorus lectin. Thus, V. villosa B4 lectin shows unique binding specificity for UDP-GalNAc and has the capacity to modulate UDP-GalNAc metabolism in animal cells.
植物种子凝集素对植食性动物起到防御作用。在此，发现GalNAc特异性的蚕豆B4凝集素可抑制动物核苷酸焦磷酸酶对UDP-GalNAc的水解，而核苷酸焦磷酸酶被认为可调节细胞中核苷酸糖的局部水平。在低浓度的UDP-GalNAc时抑制作用明显，并且通过向反应混合物中添加GalNAc可在很大程度上逆转这种抑制作用。相比之下，凝集素仅在较小程度上抑制其他核苷酸糖如UDP-Gal和UDP-GlcNAc的酶促水解，并且GalNAc并不影响这种抑制作用。在4℃时，凝集素对UDP-GalNAc的结合常数高达2.8×10⁵ M⁻¹，而对GalNAcα-1-磷酸的结合常数为1.3×10⁵ M⁻¹。这些发现表明，凝集素对低浓度UDP-GalNAc的焦磷酸酶活性的抑制主要源于凝集素与酶分子对UDP-GalNAc的竞争。在GalNAc特异性的紫藤凝集素中也观察到了这种类型的抑制作用，但程度较小，而在GalNAc特异性的大豆或双花扁豆凝集素中显然未观察到。因此，蚕豆B4凝集素对UDP-GalNAc表现出独特的结合特异性，并具有调节动物细胞中UDP-GalNAc代谢的能力。
