Sugar chains of lysosomal enzymes and sphingolipid activator proteins, and the relationship with their transport mechanism

溶酶体中许多鞘脂的酶促降解受到称为鞘脂激活蛋白SAP-1、SAP-2和SAP-3的小（8至13 kD）糖蛋白的刺激。为了阐明SAP进入溶酶体的运输机制以及酸性糖苷酶在溶酶体降解糖蛋白糖链中的作用，我们研究了从正常人肝和GM 1神经节苷脂沉积症中纯化的SAR-1释放的糖链的结构，1型，肝脏，豚鼠肾脏SAP-3和戈谢病脾脏SAP-2。从GM 1神经节苷脂沉积症肝中纯化的SAP-1以外的SAP的糖链在性质上彼此相同，并且它们的结构是以下八种组分：男性1 6（男性1 3）男性1 4GlcNAc 1 4（Fuc 1 6）GlcNAc，Man 1 6 Man 1 4 GlcNAc 1 4（Fuc 1 6）GlcNAc，Man 1 4 Man 1 4 GlcNAc 1 4（Fuc 16）GlcNAc、GlcNAc 14 GlcNAc和GlcNAc，表明天然SAP的糖链被溶酶体中的外切糖苷酶依次降解。从正常器官中纯化的SAP-1等天然蛋白的糖链在转运机制中的作用似乎难以推测，而从GM 1型神经节苷脂沉积症1型肝脏中纯化的SAP-1的糖链中，15%含有唾液酸化的单-至四触角寡糖，其余85%为中性的单-、双-、三-、四-触角寡糖，和四触角复合型糖链，其含有岩藻糖基化和非岩藻糖基化的三甘露糖基核心，以及外链上的N-乙酰基乳糖胺基团，尽管它们被轻微降解。这些结果表明，一些复杂类型的糖链，包括N-乙酰乳糖胺基团，岩藻糖残基连接到近端的N-乙酰葡萄糖胺，唾液酸等可能作为交通信号，运输SAP-1从高尔基体到溶酶体。

The enzymatic degradation of a number of sphingolipids in the lysosomes is stimulated by small (8 to 13 kD) glycoproteins named sphingolipid activator proteins SAP-1, SAP-2, and SAP-3. Since these activators are translocated into lysosomes after biosynthesis, their asparagine-linked sugar chains should act as a traffic signal to lysosomes.In order to elucidate the traffic mechanism of SAPs to the lysosome and the role of acid glycosidases in lysosomal degradation of sugar chains of glycoproteins, we have investigated the structures of sugar chains released from SAR-1 purified from normal human liver and GM1 gangliosidosis, Type 1, liver, guinea pig kidnney SAP-3, and Gaucher disease spleen SAP-2. The sugar chains of SAPs except SAP-1 purified from GM1 gangliosidosis liver were equal qualitatively with one another and their structures were the following eight components: Man 1 6(Man 1 3)Man 1 4GlcNAc 1 4( Fuc 1 6)GlcNAc, Man 1 6Man 1 4GlcNAc 1 4( Fuc 1 6) GlcNAc, Man 1 4Man 1 4GlcNAc 1 4( Fuc 1 6)GlcNAc, GlcNAc 1 4GlcNAc, and GlcNAc, indicating that sugar chains of native SAPs were degraded sequencially by exoglycosidases in lysosoms. It seems difficult to speculate the role in the transport mechanism of those suger chains of the native protein like SAP-1 purified from the normal organs.While,fifteen percent of the sugar chains of SAP-1 purified from liver of GM1 gangliosi-dosis, Type 1, contained sialylated mono- to tetra-antennary oligosaccharides and the remaining 85 % was neutral mono-, bi-, tri-, and tetra- antennary complex type sugar chains containing fucosylated and nonfucosylated trimannosyl cores, and N-acetyllactosamine groups at outer chains, although they are slightly degraded. These results indicate that some complex type sugar chains including N-acetyllactosamine group, the fucosyl residue linked to the proximal N-acetylglucosamine, and the sialic acid et al. may act as a traffic signal to transport SAP-1 from the golgi apparatus to the lysosome.