title: Spin-labeled phorbol esters and their interaction with cellular membranes.
abstract: A series of homologous spin-labeled fatty acid analogs of the type 12-O-FASL (n,m)-phorbol-13-acetate [(n,m)PA] of 12-O-tetradecanoylphorbol-13-acetate (TPA) with variable chain length N of the fatty acid moiety and various positions of the nitroxide group were synthesized. Their irritant doses 50 (ID50) on the ear and their initiation-promoting activities on the back skin of NMRI mice were determined. The irritancy 1/ID50 of the (n,m)PA follows the simple linear relation ln(1/ID50) = alpha N + beta. In an "equilibrium" approach, the electron paramagnetic resonance (EPR) spectra measured in absence and in presence of erythrocytes (E) were used to determine the partition coefficients KM,0 between the erythrocyte membrane (M) and the extracellular solution (o). The KM,0 depend on the chain length N of the fatty acid moieties following the equation ln KM,0 = aN + b and show that the (n,m)PA are accumulated preferentially in the erythrocyte membrane. In a "kinetic" approach, the decay rate J(t)/J(o) of the EPR signal intensity of (n,m)PA molecules in absence and in presence of equilibrated E was measured during oxidation with ferricyanide and reduction with ascorbate, respectively. From the data obtained by the "kinetic" approach, a "two-compartmental model" is shown to be the most simple approximation to describe the system, in which finite concentrations of the (n,m)PA are postulated for the extracellular and the membrane compartment only. EPR data indicate that generally the (n,m)PA are buried with their acyl chain in the E membrane with the phorbol moiety close to the outer membrane surface. Apparently only in case of (1,12)PA is the acyl chain long enough to be anchored in the lipid layer of the membrane in a way similar to the tetradecanoyl residue of TPA. A possible relationship with biological activities is discussed.
mesh_terms: D000818:Animals; D002273:Carcinogens / Q000494:pharmacology; D055598:Chemical Phenomena; D002621:Chemistry; D004578:Electron Spin Resonance Spectroscopy; D004910:Erythrocyte Membrane / Q000378:metabolism*; D004912:Erythrocytes / Q000378:metabolism*; D007509:Irritants / Q000494:pharmacology; D007700:Kinetics; D051379:Mice; D010704:Phorbols / Q000378:metabolism*; D013113:Spin Labels; D013329:Structure-Activity Relationship; D013755:Tetradecanoylphorbol Acetate / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D002273:Carcinogens; D007509:Irritants; D010704:Phorbols; D013113:Spin Labels; D013755:Tetradecanoylphorbol Acetate