Further Characterization of a Myelin‐Associated Neuraminidase: Properties and Substrate Specificity

A neuraminidase activity in myelin isolated from adult rat brains was examined. The enzyme activity in myelin was first compared with that in microsomes using N ‐acetylneuramin(α2 → 3)lactitol (NL) as a substrate. In contrast to the microsomal neuraminidase which exhibited a sharp pH dependency for its activity, the myelin enzyme gave a very shallow pH activity curve over a range between 3.6 and 5.9. The myelin enzyme was more stable to heat denaturation (65°C) than the microsomal enzyme. Inhibition studies with a competitive inhibitor, 2,3‐dehydro‐2‐deoxy‐ N ‐acetylneuraminic acid, showed the K i value for the myelin neuraminidase to be about one‐fifth of that for the microsomal enzyme (1.3 × 10 −6 M versus 6.3 × 10 −6 M ). The apparent K m values for the myelin and the microsomal enzyme were 1.3 × 10 −4 M and 4.3 × 10 −4 M , respectively. An enzyme preparation that was practically devoid of myelin lipids was then prepared and its substrate specificity examined. The “delipidated enzyme” could hydrolyze fetuin, NL, and ganglioside substrates, including G M1 , and G M2 . When the delipidated enzyme was exposed to high temperature (55°C) or low pH (pH 2.54), the neuraminidase activities toward NL and G M3 decreased at nearly the same rate. Both fetuin and 2,3‐dehydro‐2‐deoxy‐ N ‐acetylneuraminic acid inhibited NL and G M3 hydrolysis. With 2,3‐dehydro −2‐deoxy‐ N ‐acetylneuraminic acid, inhibition of NL was greater than that of G M3 ; however, the K i values for each substrate were almost identical. G M3 and G M1 , also competitively inhibited the hydrolysis of NL and NL similarly inhibited G M3 hydrolysis by the enzyme. These results indicate that rat brain myelin has intrinsic neuraminidase activities toward nonganglioside as well as ganglioside substrates, and that these two enzyme activities are likely catalyzed by a single enzyme entity.