Membrane-bound sialidases in the mouse thymus are unique and mysterious because their activity at pH 6.5 is equal to or higher than that in the acidic region. The pH curve like this has never been reported in membrane-bound form. To clarify this enzyme, we studied the sialidase activities of crude membrane fractions from immature-T, mature-T and non-T cells from C57BL/6 mice and from SM/J mice, a strain with a defect in NEU1 activity. Non-T cells from C57BL/6 mice had high activity at pH 6.5, but those from SM/J mice did not. Neu1 and Neu3 mRNA was shown by real-time PCR to be expressed in T cells and also in non-T cells, whereas Neu2 was expressed mainly in non-T cells and Neu4 was scarcely expressed. However, the in situ hybridization study on the localization of four sialidases in the thymus showed that Neu4 was clearly expressed. We then focused on a sialidase on the thymocyte surface because the possibility of the existence of a sialidase on thymocytes was suggested by peanut agglutinin (PNA) staining after incubation of the cells alone in PBS. This activity was inhibited by NEU1-selective sialidase inhibitor C9-butyl-amide-2-deoxy-2,3-dehydro-N-acetylneuraminic acid. The natural substrate for the cell surface sialidase was identified as clustered differentiation 5 (CD5) by PNA-blot analysis of anti-CD5 immunoprecipitate. We conclude that NEU1 exists on the cell surface of mouse thymocytes and CD5 is a natural substrate for it. Although this is not the main reaction of the membrane-bound thymus-sialidases, it must be important for the thymus.

中文翻译：

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NEU1唾液酸酶在天然底物为CD5的小鼠胸腺细胞中的存在

小鼠胸腺中的膜结合唾液酸酶是独特而神秘的，因为它们在pH 6.5时的活性等于或高于酸性区域。这样的pH曲线从未以膜结合的形式报道过。为了阐明这种酶，我们研究了来自C57BL / 6小鼠和SM / J小鼠（具有NEU1活性缺陷的菌株）的未成熟T细胞，成熟T细胞和非T细胞的粗膜级分的唾液酸酶活性。C57BL / 6小鼠的非T细胞在pH 6.5时具有高活性，而SM / J小鼠的非T细胞则没有。实时PCR显示Neu1和Neu3 mRNA在T细胞和非T细胞中表达，而Neu2主要在非T细胞和Neu4中表达。几乎没有表达。但是，对胸腺中四个唾液酸酶的定位进行的原位杂交研究表明，Neu4可以清晰表达。然后我们将注意力集中在胸腺细胞表面的唾液酸酶上，因为在PBS中单独孵育细胞后，花生凝集素（PNA）染色提示胸腺细胞上存在唾液酸酶的可能性。这种活性由NEU1选择性抑制唾液酸酶抑制剂C9丁基酰胺-2-脱氧-2,3-脱氢Ñ-乙酰神经氨酸。细胞表面唾液酸酶的天然底物通过抗CD5免疫沉淀物的PNA印迹分析鉴定为聚集分化5（CD5）。我们得出结论，NEU1存在于小鼠胸腺细胞的细胞表面，而CD5是其天然底物。尽管这不是膜结合的胸腺唾液酸酶的主要反应，但对于胸腺必须很重要。