Multiple Molecular Forms of α-Glucosidase from Spinach Seeds, Spinacia oleracea L

Four molecular forms of α-glucosidase were isolated from spinach seeds by several kinds of chromatography. The molecular masses of α-glucosidases I, II, III, and IV were 78, 78, 82, and 82kDa by SDS-PAGE, and 62, 62, 190, and 70kDa by gel filtration, respectively. α-Glucosidases I and II showed similar enzymatic properties, in which the K_m for soluble starch was about 10 times lower than that for maltose, and they had higher activity not only toward malto-oliosaccharides but also toward α-glucans. The optimum pH was 4.5-5.5 and about 50% of the activity remained after incubation at 70°C for 20min. On the other hand, α-glucosidases III and IV showed similar enzymatic properties, in which the K_m for maltose was 3-4 times lower than that for soluble starch, and they had high activity toward malto-oligosaccharides but faint activity toward α-glucans. The optimum pH was 4.5-5.0 and no activity was found after incubation at 70°C for 20min. However, anti-α-glucosidase III serum formed precipitation specifically with α-glucosidase III.