A membrane-associated neuroaminidase in Entamoeba histolytica trophozoites

Abstract

Trophozoites of the parasitic amoeba Entamoeba histolytica HM-1:IMSS possess a surface neuraminidase capable of liberating N-acetylneuraminic acid (NANA) from N-acetylneuramin-lactose (α2→3 or α2→6) or mucin in their medium. The neuraminidase was found to be membrane associated, with more than 50% of the yield being recovered in the plasma membrane fraction. The neuraminidase specific activity of the plasma membrane fraction was six times that of internal membrane fraction enzyme. The optimum pH and temperature for this enzyme were 6.7 and 37°C, respectively. Neuraminidase activity was inhibited by ethylene glycol-bis(β-aminoethyl ether)-N,N,N'N'-tetraacetic acid, and the optimum Ca2+ concentration was 2 mM. The microfilament disruptor cytochalasin D (30 μg/ml) inhibited motility and neuraminidase activity of intact Entamoeba trophozoites. The cytochalasin D-induced loss of surface neuraminidase activity was explained in part by a redistribution of enzyme with a loss of plasma membrane enzyme and an increase in intracellular membrane enzyme. A qualitatively similar cytochalasin D effect was observed with two other membrane-associated enzymes, calcium-regulated ATPase and acid phosphatase. Membrane-associated enzyme was minimally affected by Triton X-100 and saponin. An N-acetylneuraminic acid aldolase, optimum pH, 7.4, was found in trophozoite homogenate supernatant fractions. NANA and NANA-containing compounds stimulated trophozoite-directed motility. This motility stimulation by NANA-containing compounds did not apparently require prior release of free NANA by the trophozoite surface neuraminidase. Entamoeba neuraminidase is one of a series of enzymes that may modify the mucus blanket and target cell surface and thereby play a role in the pathogenesis of amebiasis.