Trinactin

The electrical properties predicted by a widely accepted model for carrier-mediated ion transport in lipid bilayers are described. The different steps leading to ion transport and their associated rate constants are reaction at the interface between an ion in the aqueous phase and a carrier in the membrane (kRi), followed by translocation of the ion-carrier complex across the membrane interior (kis) and its dissociation at the other interface (kDi) after which the free carrier crosses back the membrane interior (ks). Results on glyceryl monooleate (GMO) membranes for a family of homologue carriers, the macrotetralide actin antibiotics (nonactin, monactin, dinactin, trinactin, and tetranactin) and a variety of ions (Na+, Cs+, Rb+, K+, NH4+, and Tl+) are presented. Internally consistent data obtained from steady-state electrical measurements (zero-current potential and conductance, current-voltage relationship) allow us to obtain the equilibrium permeability ratios for the different ions and show that for a given carrier kRi is relatively invariant from one ion to the other, except for Tl+ (larger), which implies that the ionic selectivity is controlled by the dissociation of the complex. The values of the individual rate constants obtained from current relaxation experiments are also presented and confirm the findings from steady-state measurements, as well as the isostericity concept for complexes of different ions with the same carrier (kis invariant). These also allow us to determine the aqueous phase membrane and torus membrane partition coefficients. Finally, the observed increase in kis from nonactin to tetranactin and, for all homologues, from GMO-decane to solvent-free GMO membranes, together with the concomitant decrease in kDi, can be explained in terms of modifications of electrostatic energy profiles induced by variations in carrier size and membrane thickness.

Macrotetrolide antibiotic polynactins [dinactin, trinactin and tetranactin (1:4:5)] are hydrophobic cyclic esters produced by Streptomyces aureus. Polynactins (PN) and their major component tetranactin (TN) delayed or suppressed the onset of S-antigen-induced experimental autoimmune uveoretinitis (EAU) in Lewis rats. Termination of treatment with PN or TN before day 14 of immunization resulted in a delayed onset of EAU in many animals. Thus, the immunosuppressive effect of PN and TN was not lasting. PN and TN suppressed anti-S-antigen antibody formation. Skin hypersensitivity tests indicated suppression by PN of the delayed-type rather than Arthus type hypersensitivity to S-antigen. PN, TN and trinactin all inhibited 3H-thymidine incorporation into concanavalin A-treated lymphocytes at the early stage of cell activation. For each drug, 50% inhibition was obtained at about 0.1 ng/ml. Under the incubation condition that the cells were exposed to TN for 21 hours, cell viability remained unchanged up to 100 ng/ml of TN. It is evident that PN and TN suppress T-lymphocyte proliferation without cell injury. These results suggest that PN and TN inhibit the onset of EAU primarily through the suppression of cell-mediated immunity but also by affecting humoral immunity.

The manner in which molecular structure of the carrier and the lipid composition of the membrane modulate the membrane selectivity among monovalent cations has been investigated for nonactin, trinactin, and tetranactin, which differ only in their degrees of methylation, and for membranes made of two lipids, phosphatidyl ethanolamine and glyceryl dioleate, in which "equilibrium" and "kinetic" aspects of permeation, respectively, are emphasized. Bilayer permeability ratios for Li, Na, K, Rb, Cs, Tl,and NH4 have been characterized and resolved into "equilibrium" and "kinetic" components using a model for carrier-mediated membrane transport which includes both a trapezoid energy barrier for translocation of the complex across the membrane interior and a potential-dependence of the loading and unloading of ions at the membrane-solution interfaces. The bilayer permeability properties due to tetranactin have been characterized in each of these lipids and found not only to be regular but to be systematically related to those of the less methylated homologues, trinactin and nonactin. This analysis has led to the following conclusions: (1) The change in lipid composition alters the relative contributions of "kinetic" vs. "equilibrium" components to the observed carrier-mediated selectivity. (2) Increased methylation of the carrier increases the contribution of the "kinetic" component to the selectivity relative to that of the "equilibrium" component and additionally alters the "equilibrium component sufficiently that an inversion of Cs--Na selectivity occurs between trinactin and tetranactin. (3) For all ions and carriers examined, the "reaction plane" for ion-carrier complexation and the width for the "diffusion barrier can be represented by the same two parameters, independent of the ion or carrier, so that in all cases the complexation reaction senses 10% of the applied potential and the plateau of the "diffusion barrier" extends across 70% of the membrane interior.