Studies on the sialidoses: properties of human leucocyte neuraminidases

At least two components of neuraminidase (acylneuraminyl hydrolase, EC 3.2.1.18) can be distinguished in human leucocytes on the basis of pH optimum, thermolability at 30 degrees C and the effect of the detergent octyl-beta-D-glucoside. With 4-methylumbelliferyl-alpha-D-N-acetylneuraminate as substrate, the A component has a pH optimum of 5.0, is labile at 30 degrees C and is unaffected by 0.2 M octyl-beta-glucoside. The B component has a pH optimum of 4.0-4.2, is stable at 30 degrees C but loses most of its activity in the presence of 0.2 M octyl-beta-glucoside. Both A and B components are membrane-bound but only the A component is solubilized by octyl-beta-glucoside in an active form. Molecular weights of neuraminidases by gamma-ray radiation inactivation (a method that does not require solubilization of the enzyme) were found to be 240 000 +/- 19 000 for the B component, 203 000 +/- 17 000 for the A component and 238 000 +/- 8000 for the octyl-beta-glucoside-solubilized A component. Gel filtration of soluble A component on Sephacryl S-300, in the presence of octyl-beta-glucoside, showed a single peak of activity eluted at or near the void volume suggesting that the enzyme is still in an aggregated form. Profound deficiency of neuraminidase activity was found for both A and B components in leucocytes of patients affected with sialidoses type 1 and 2 (less than 15% normal) and intermediate activity in obligate heterozygotes. These results suggest that the A and B components of leucocyte neuraminidase are closely related from the genetic point of view and that rapid diagnosis of sialidoses can be done by fluorimetric assay of neuraminidase in leucocytes.