Turnover numbers of membrane carriers and the action of the polypeptide antibiotics [14]

CERTAIN polypeptide antibiotics may act as membrane "carriers" translocating alkali cations and protons by forming diffusible complexes with them¹. Alternatively, the antibiotics may form pores in the membrane, specific for those cations. To help to decide between these models, possible rates of penetration of polypeptide-cation complexes can be computed. The data on membrane permeability indicate, in general, a strong negative correlation between the permeability coefficient P and the number of hydrogen-bonding groups in the permeant². A cell membrane can be characterized by two factors: (PM)_max, expressing the maximum permeability that the membrane allows for a permeant bearing no hydrogen bonding groups and also by the average reduction in P, for the chemical structure of the permeant is varied on the addition of a hydrogen-bonding site. These two parameters (recorded in Table 1) permit permeabilities for a variety of substances and cell membranes to be predicted within a factor of five², if those with specialized permeability systems are excluded. If the polypeptide antibiotics permeate by simple diffusion, their permeabilities can then be computed from their molecular weights.