Takeya A, Hosomi O, Yazawa S, Kogure T
Department of Legal Medicine, Gunma University School of Medicine, Japan.
Jpn J Med Sci Biol. 1993 Feb;46(1):1-15.
Previously we reported the occurrence of UDP-GalNAc:Gal beta 1-->4GlcNAc/Glc beta-1,3-N-acetylgalactosaminyltransferase activity in human plasma. Here, the donor substrate specificity of the enzyme partially purified from blood group O plasma was investigated by means of competition experiments with analogs of donor. The enzyme activity was found to be inhibited most strongly by UDP-GlcNAc among the nucleotide sugars tested, and UDP was the best inhibitor among test nucleotides. UDP-GlcNAc was much more inhibitory than UDP, whereas UDP-GalNAc was much less inhibitory than UDP. These results show that the donor-binding site of enzyme has a high affinity for UDP-GlcNAc, but not UDP-GalNAc, suggesting that the enzyme essentially functions in the transfer of N-acetylglucosamine. Indeed, UDP-GlcNAc:Gal beta 1-->4GlcNAc/Glc beta-1,3-N-acetylglucosaminyltransferase activity is known to occur in human plasma. The ratio of N-acetylgalactosamine to N-acetylglucosamine transferred to lactose with O or B plasma was 1:20 when assayed separately at 1 microM of donor, but increased to 1.2:1 at 5 mM. The nearly identical ratio was obtained with the partially purified enzyme preparation.
此前我们报道了人血浆中存在UDP - N - 乙酰半乳糖胺：半乳糖β1→4N - 乙酰葡糖胺/葡萄糖β - 1,3 - N - 乙酰半乳糖胺基转移酶活性。在此，通过与供体类似物的竞争实验研究了从O型血血浆中部分纯化的该酶的供体底物特异性。在所测试的核苷酸糖中，发现UDP - N - 乙酰葡糖胺对该酶活性的抑制作用最强，而在测试的核苷酸中UDP是最佳抑制剂。UDP - N - 乙酰葡糖胺的抑制作用比UDP强得多，而UDP - N - 乙酰半乳糖胺的抑制作用比UDP弱得多。这些结果表明，该酶的供体结合位点对UDP - N - 乙酰葡糖胺具有高亲和力，而对UDP - N - 乙酰半乳糖胺没有，这表明该酶主要在N - 乙酰葡糖胺的转移中起作用。实际上，已知人血浆中存在UDP - N - 乙酰葡糖胺：半乳糖β1→4N - 乙酰葡糖胺/葡萄糖β - 1,3 - N - 乙酰葡糖胺基转移酶活性。当在1μM供体浓度下分别测定时，用O型或B型血浆转移到乳糖上的N - 乙酰半乳糖胺与N - 乙酰葡糖胺的比例为1:20，但在5mM时增加到1.2:1。用部分纯化的酶制剂获得了几乎相同的比例。
