Differences in the modulations of the soluble of plasma-membrane-bound 5'-nucleotidase

The treatment of plasma membranes by a French pressure cell in sucrose medium devoid of detergents solubilized 20% of the total protein and 95--100% of 5'-nucleotidase activity. The soluble enzyme was 40--90-fold purified by centrifugation in a sucrose gradient with a 10--20% yield with respect to the orginal lysate. The purified fraction retained the same high specificity for 5'-AMP (Km = 20 micron) as in the plasma membranes and was enriched in sphingomyelin. Whereas 5'-AMP at high concentration inhibited the membrane-bound enzyme, it had no effect on the solubilized form. The soluble enzyme was stimulated by 2 X 10(-13)-2 X 10(-15) g concanavalin A without any inhibition with higher doses of lectin. The plasma-membrane bound stimulated and inhibited 5'-nucleotidase was modulated by concanavalin A concentrations higher than 0.1 microgram. Inhibition of the activity of the soluble enzyme by antiphosphorylcholine antibodies was not observed with membranes. The regulation of 5'-nucleotidase acitvity in plasma membranes might be associated with a supramolecular organization.