Uptake of glycosylated β-galactosidase into human brain synaptosomes: Occurrence of two different systems for specific glycosides

Abstract

Uptake of glycoproteins into brain nerve terminals was studied, using glycosylated β-galactosidase and human brain synaptosomes. A series of p-aminophenyl α- or β-glycosides were covalently bound to β-galactosidase purified from Aspergillus oryzae and uptake of the enzyme into human brain synaptosomes was measured. β-Galactosidase coupled with α- d-mannoside (α- d-Man β-gal) was the best to be taken up among glycoconjugates tested. The uptake was dependent on the amount of α- d-Man β-gal, the incubation temperature and the incubation time. Kinetic studies showed that the uptake was a saturable and receptor-mediated process, which followed Michaelis-Menten kinetics. The specificity of the uptake was confirmed by competition of the uptake of α- d-Man β-gal by α-methyl mannoside. The uptake was dependent on Ca 2+ and that of β-galactosidase coupled with α- l-fucoside (α- l-Fuc β-gal) was saturated by Ca 2+ at lower concentrations than that Of α- d-Man β-gal.Occurrence of two different uptake systems was also suggested by kinetic study; a low-affinity and high-capacity system for α- d-mannoside and a high-affinity and low-capacity system for α- l-fucoside. The former system may be mediated by a process coupled with a Ca 2+/calmodulin-dependent protein kinase, but the later may not be dependent on the protein kinase.