[The effect of alpha-tocopherol over a wide range of concentrations (10(-2)-10(-17) M) on protein kinase C activity. Connection with proliferation and tumor growth].

The effect of the natural antioxidant alpha-tocopherol (alpha-TL) on protein kinase C (PK-C) from rabbit heart has been studied. alpha-TL inhibits the PK-C activity; the concentration dependence curve has two maxima-at (10(-4)-10(-6) M) and at (10(-11)-10(-15) M). There is a "zone of silence" between these two regions. The shift of the first maximum towards higher alpha-TL concentrations (5 x 10(-3) M) and the decrease in the values of the both inhibition maxima (from 80-90% to 40-60%) was observed for the PK-C preactivated by 12-O-tetradecanoyl-phorbol-13-acetate. The putative role of alpha-TL and lipid hydroperoxides in the regulation of proliferation of normal and tumour cells is discussed.