Calcium-activated, phospholipid-dependent protein kinase activity in calf platelets.

Protein kinase C (PKC) has been widely studied from different tissues of mammals. Human platelets display higher levels of PKC activity, if compared with other sources. The PKC activity from calf platelets crude extract was determined in the presence of various protease inhibitors such as PMSF, Leupeptin or Trypsin inhibitro, and the Ca(2+)-chelators EGTA and EDTA. The free calcium requirement was 0.25 mM, calculated with the help of the Solgas-water computer program, which represents 1 mM CaCl2, in these assay conditions. Optimum PKC activity was obtained at 4 min in the presence of PS plus DAG or TPA, using H1 type III-S histone as substrate. Phospholipid-interacting drugs, such as trifluoperazine, chlorpromazine and tetracaine, inhibited the PKC activity in a dose-dependent manner. Triton X-100, a non-ionic detergent, which is usually employed to solubilize the membrane fraction, in different translocation assays, inhibited PKC activity at concentrations higher than 0.01%. In these conditions, non-proteolytic PKC activity from calf platelets was easily determined, and shares similar activity levels with those described in human platelets.