Golgi vacuolization and immunotoxin enhancement by monensin and perhexiline depend on a serum protein. Implications for intracellular trafficking.

Vacuole formation around the Golgi and immunotoxin enhancement induced by low doses of the ionophore monensin were inhibited by 50% human plasma (final concentration), whereas the lysosomal pH increase remained unaffected. Immunotoxin enhancement by the Ca2+ antagonist perhexiline was also inhibited by plasma. The inhibiting factor was present in different species and highly concentration-dependent. After purification on DEAE- and CM-Sepharose it showed a heterogeneous distribution between 45 and 50 kDa, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, an extreme isoelectric point near 3.5, and binding to wheat germ agglutinin-Sepharose. Maximum inhibition was found in the lower molecular mass fraction of 45 kDa. The 50-kDa fraction, although showing immunological identity reactions, remained almost inactive. The simultaneous inhibition of morphological alterations and the enhancement of immunotoxin activity by the highly enriched protein provides a first direct link between both events. Apart from a role of this serum glycoprotein on in vivo inhibition of immunotoxin enhancement, its ability to maintain normal intracellular trafficking in the presence of blocking agents, such as monensin and perhexiline, suggests a more fundamental role in the regulation of these mechanisms.