Spectroscopic characterization of specific phorbol ester binding to PKC-receptor sites in membranes in situ

Electron paramagnetic resonance (EPR) spectra were measured for the spin labelled phorbol-12,13-diester [5,6]PA bound to membranes of the particulate fraction of mouse brain tissue rich in PKC receptors. [5,6]PA is a bioactive derivative of the potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), carrying the spin label in a doxyl group in position 7' of the 12-O-tetradecanoyl residue. A mathematical model based on special algorithms (Griffith, O.H. and Jost, P.C. Spin labeling: theory and applications, 1976) allows a satisfactory reconstruction of the experimental spectrum. It reveals that in the experimental spectrum the signal from the [5,6]PA molecules bound non-specifically to the lipid bilayer of the membranes is superimposed by the signal of [5,6]PA molecules bound specifically, i.e. to the active site of PKC (approximately 10% of total EPR signal intensity). Moreover, interpretation of spectral parameters indicates that in [5,6]PA molecules bound specifically the tetradecanoyl chain exhibits a larger motional freedom compared to that in [5,6]PA bound non-specifically. These new findings are in accordance with views featured independently by two recent molecular models of interaction of PKC with cellular membranes (8,9).