The effect of wheat germ agglutinin on sialyl and galactosyltransferases of rat liver Golgi membranes.

The sialyltransferase and galactosyltransferase activities of the Golgi-rich fraction from rat liver were enhanced by the binding of wheat germ agglutinin (WGA). The sialytransferase was more sensitive than the galactosyltransferase to the WGA. Maximal stimulation of the galactosyltransferase activity resulted from the binding of 60--80 micrograms WGA to the Golgi membrane, while only 40 micrograms of WGA produced a maximal enhancement in the sialyltransferase activity. Within 5 min of WGA binding, the Golgi sialytransferase activity was doubled. After the initial binding of WGA to the Golgi fraction, the galactosyltransferase activity was decreased by 30%. However, in 15 min the activity was doubled by the binding of WGA. The activities of both enzymes were further enhanced by incubation for up to 90 min. The stimulation of both sialyltransferase and galactosyltransferase activities by WGA was reversed by N-acetyl-D-glucosamine (GlcNAc), the specific inhibitor of agglutination by WGA. Complete reversal of the enhanced activity was observed after 20--30 min in the presence of 1 micromol GlcNAc. The association constant for the binding of WGA to the Golgi membranes was calculated to be 4.16 X 10(-6) M from a Steck-Wallach plot. The 'n' value or mean binding sites was calculated as 5.26 X 10(-5) M/mg of Golgi membrane protein.