Turnover Numbers of Membrane Carriers and the Action of the Polypeptide Antibiotics

Abstract

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: (PM½)_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.