Characterization of alpha-glucosidases from rainbow trout liver

Three forms of alpha-glucosidase were separable in trout liver by DE-52 chromatography, isoelectrofocusing, and gel filtration. Two forms showed acid pH optima, hydrolyzed glycogen, maltose, and 4-methylumbelliferyl alpha-glucoside, and were associated with the lysosomes. The third enzyme form was largely associated with microsomes and was present in the highest activity; it showed a neutral pH optimum and did not hydrolyze glycogen. Molecular weights were 181 +/- 2, 130 +/- 1.5, and 365 +/- 3 kDa for the acid types I and II and the neutral enzyme, respectively. Maximal activities and kinetic and physical properties of the three enzymes were compared in liver samples from control, resting fish versus fish that underwent exhaustive swimming exercise. The properties of liver acid alpha-glucosidase type I changed significantly in response to exercise; maximal activity increased by 80% and Km values for both glycogen and maltose dropped by 50% in exercised, versus control, fish. Under the same exercise condition, liver glycogen phosphorylase a activity also increased 4.4-fold. These changes in alpha-glucosidase type I are consistent with an activation of the enzyme, in parallel with phosphorylase activation, under physiological stress conditions that promote glycogenolysis and glucose export from liver. These results are, we believe, the first demonstration of the activation of the glucosidic route of glycogenolysis in response to a physiological stress and suggest that the glucosidic route has a significant role to play in complementing the phosphorolytic pathway in the metabolic response by liver to the fuel demands of working muscle.