Further Evidence for an Intrinsic Neuraminidase in CNS Myelin

Abstract: An intrinsic neuraminidase activity in rat brain CNS myelin has been demonstrated and compared with the neuraminidase activity in rat brain microsomes. With use of ganglioside G_M3 as a substrate, the myelin‐associ‐ated neuraminidase exhibited a shallow pH curve with an optimum at pH 4.8 whereas the microsomal activity had a marked optimum at pH 4‐4.3. Neuraminidase activity in both fractions was optimized in 0.3% Triton CF‐54 but activation was much greater in the microsomes. When the neuraminidase activities were examined at 60°C, the myelin neuraminidase activity was more than sevenfold of that observed at 37°C and was linear for at least 2 h; the microsomal activity increased only fivefold initially and exhibited a continual loss in activity. Addition of excess microsomes to the total homogenate prior to myelin isolation resulted in no change in myelin neuraminidase activity. When the two membrane fractions were examined at equivalent protein concentrations in the presence of additional cations or EDTA (1 mM), similar but not identical effects on neuraminidase activity were seen. The microsomal neuraminidase was considerably more susceptible to inhibition by divalent copper ion. Activity in both fractions was markedly inhibited by Hg²⁺ and Ag⁺ whereas EDTA had no effect on either activity. The myelin‐associated neuraminidase activity was the highest in cerebral hemispheres, followed by brainstem, cerebellum, and spinal cord and was extremely low in sciatic nerve. In fact, the myelin neuraminidase activity was higher than the microsomal enzyme activity in the cerebral hemispheres. Developmentally, the myelin‐associated neuraminidase activity in the cerebral hemispheres was also the highest at 20–30 days of age and declined to adult levels soon after. The combined evidence strongly indicates the presence of an intrinsic neuraminidase for gangliosides in CNS myelin and this neuraminidase may play an important role in determining the unique ganglioside pattern in the myelin sheath.