Membrane‐associated Sialidase of Rat Liver and Its Decrease in Hepatomas

Using the participate fraction of tissue homogenate, plasma membrane‐associated sialidase was assayed at pH 4.5 with bovine brain mixed gangliosides as the substrate. The activity was lower in rat hepatoma induced by 3′‐methyl‐4‐dimethylaminoazobenzene (MeDAB) and transplantable AH‐109A rat hepatoma than in normal rat liver. The enzyme was almost quantitatively solubilized from liver particulate fraction by using 0.5% (w/v) sodium deoxycholate plus 0.2% (w/v) Triton X‐100, When chromatographed on DEAE‐cellulose, the solubilized activity emerged as a single peak. The enzyme thus obtained was maximally active at pH 4.5, and readily hydrolyzed mixed gangliosides but was less active toward 4‐methylumbelliferyl‐α‐N‐acetylneuraminic acid, 3′‐sialyllactose and fetuin. The corresponding enzyme from MeDAB‐induced hepatoma was indistinguishable from the liver enzyme in terms of ease of solubilization, pH‐activity relationship, chromatographic behavior and substrate preference. It therefore appears that the plasma membrane‐associated sialidase of hepatomas differs from that of liver only in the tissue level of activity.