Studies of brain membrane-bound neuraminidase. II. Effect of detergents on the kinetics of the enzyme prepared from calf brain

1. 1. The influence of various detergents on the kinetics of membrane-bound neuraminidase acting on gangliosides (amphipatic substances) was studied. The enzyme was prepared from calf brain and disialoganglioside GD1a was used as substrate. 2. 2. In the absence of detergents the enzymatic hydrolysis of GD1a follows hyperbolic kinetics with a maximum rate reached at a substrate concentration close to the critical micellar concentration, 0.1 mM. 3. 3. In the presence of sodium dodecylsulphate, cholate, deoxycholate and Triton X-100 the hyperbolic kinetics maintained with the maximum rate still reached at 0.1 mM GD1a. These detergents behave as non-competitive inhibitors although Triton X-100, at low concentrations, exhibits an activating effect. 4. 4. Triton QS-31 and Lubrol WX cause a specific change of the enzyme kinetics. With Triton QS-31 the v/[S] relationship shifts to a double-shouldered curve with a transition around 0.07-0.08 mM GD1a. With Lubrol WX the v/[S] curve is sigmoidal with transition around 0.3 mM and a maximum rate at 1.0 mM GD1a. In the case of Triton QS-31 the addition of albumin restores the hyperbolic kinetics. 5. 5. Under our experimental conditions, brain membrane-bound neuraminidase appears to act on the monomeric form of the gangliosidic substrate. In the presence of Triton QS-31 the enzyme apparently gains the capacity to work on the micellar form of substrate, while the Lubrol WX it recognizes and affects only the micellar form of substrate.