Assays of cell surface protein kinase: importance of selecting cytophilic substrates.

The ability of phosvitin and histone to serve as substrates for possible protein kinase(s) at the cell surface was examined with regard to their suitability as indicators of such activity. While phosvitin did not interfere with the membrane barrier of HeLa cells, 3T3 and SV 3T3 cells, histone caused severe damage as indicated by both uptake of viability stains Trypan Blue and diamidino-phenylindol and by release of intracellular compounds such as lactate dehydrogenase, cAMP-dependent protein kinase(s), and metabolically prelabelled proteins. Where intactness of the cell membrane is prerequisite for verification of ecto-protein kinase, histone cannot be used as substrate. In contrast, we found that phosvitin is suitable for assays of cell surface located protein kinase activity.