Competition between ligands of glycosyltransferases and horseradish peroxidase for binding sites on intracellular and plasma membranes of HeLa cells. Application of a micro-method for the semi-quantitation of surface-bound HRP

A micro-method for the semi-quantitation of surface-bound horseradish peroxidase (HRP) was developed and was applied to study the competition between ligands of glycosyltransferases and HRP for binding sites on the surface of HeLa cells. Dried coverslip cultures of HeLa cells, fixed in methanol, were placed on 0.3 ml of the incubation medium on parafilm and were incubated for 45 min at 37 degrees C. The incubation medium contained HRP, lysozyme and Ca2+ in HEPES buffer, pH 7.2. After washing, the cells were incubated for 60 min at 37 degrees C in HEPES buffer containing 20 mM Ca2+. After this treatment, the plasma membranes showed a strong cytochemical reaction for HRP. Most of the HRP was released into buffer solution during a 5 h incubation at 37 degrees C in the absence of Ca2+, and was measured by spectrophotometry. The addition of 20 mM Ca2+ to the buffer solution prevented the release of most of the HRP from the plasma membranes thus showing that the binding of HRP required Ca2+. Ligands of glycosyltransferases were added to the incubation medium with HRP. The amount of HRP released from the cells decreased in relation to the competing potency and concentration of these ligands. The method was applied to estimate the concentration of some ligands of galactosyltransferase and sialyltransferase that caused a 50% decrease in the release of previously-bound HRP. CMP-neuraminic acid and gangliosides showed a higher competing potency to the surface binding of HRP than UDP-galactose and chitotriose. The spectrophotometric analysis was correlated (on duplicate samples) with cytochemical observations.(ABSTRACT TRUNCATED AT 250 WORDS)