Characterization of neuraminidase activity of cultured human fibroblasts

Investigations have been carried out to establish the enzymatic properties and specifications of the neuraminidase of cultured human fibroblasts. Homogenates of these cells cleaved the actylated derivative of neuraminic acid from fetuin, N-acetylneuraminyllactose and 2-(3′ methoxyphenyl)-N-acetyl-α-neuraminic acid. Maximum activity occurred between pH 4.2 and 4.6 in sodium acetate buffer. The K_m values were 3.6 · 10^-4 M, 3.0 · 10^-3 M and 1.1 · 10^-3 M, respectively, against fetuin, N-acetylneuraminyllactose and 2-(3′ methoxyphenyl)-N-acetyl-α-neuraminic acid. Against the first two substrates, the rate of hydrolysis fell below the expected value as the cell homogenate was diluted with water or 10 mN NaCl. Dilution with 8 mg/ml bovine serum albumin prevented the deviation and yielded the expected linear decrease. After the first 2-h incubation, the rate of hydrolysis decreased from the initial linear rate. The enzyme(s) was partially or completely inactivated by sonication at 20 kHz, freeze-thaw treatment, incubation at 52°C or storage for 48 h at -70°C. Suspension of the fibroblasts in water for 10 min at room temperature, followed by homogenization with a tissue grinder, yielded preparations that were suitable for the assay of the neuraminidase activity.