SUBSTRATE-SPECIFICITY AND SUBSITE AFFINITIES OF RABBIT LIVER ACID ALPHA-GLUCOSIDASE

The substrate specificity of rabbit liver acid alpha-glucosidase was i nvestigated. The enzyme showed a wide specificity for various substrat es, and hydrolyzed alpha-glucans such as glycogen and soluble starch. The k(o) values (s(-1)) for maltose, kojibiose, nigerose, isomaltose, phenyl alpha-glucoside, panose, phenyl alpha-maltoside, soluble starch , beta-limit dextrin, amylopectin, shellfish glycogen, and rabbit live r glycogen were estimated to be 94.8, 18.8, 143, 3.6, 11.8, 27.8, 115, 99.2, 155, 83.5, 126, and 108, and the K-m values (concentration of n on-reducing terminal) for these substrates were 2.1, 1.8, 7.5, 36, 5.4 , 1.9, 1.2, 0.90, 9.1, 1.0, 16, and 13 mM, respectively. Isomaltose an d phenyl alpha-glucoside were unfavorable as substrates. The acid alph a-glucosidase is characterized by a relatively high activity toward gl ycogen. The k(o) values (s(-1)) for maltotriose, -tetraose, -pentaose, -hexaose, -heptaose, and -octaose, and maltodextrin (($) over bar n = 17) were 140, 140, 131, 132, 134, 132, and 74.3, and the K-m values, 2.1, 1.8, 1.9, 3.4, 5.0, 4.9, 4.9, and 2.6 mM, respectively. Based on the rate parameters for the series of maltooligosaccharides, the subsi te affinities (A(i)s) in the active site were evaluated as 0.54 (A(1)) , 5.34 (A(2)), and 0.34 (A(3)) kcal/mol for subsites 1, 2, and 3, resp ectively. These three subsites were considered to be predominantly res ponsible for the binding of substrates to the active site.