MECHANISM OF PROTON PERMEATION THROUGH CHLOROPLAST LIPID-MEMBRANES

Electrical measurements were carried out to investigate the contributi on of chloroplast lipids to the passive proton permeability of both th e thylakoid and inner-envelope membranes. Permeability coefficient and conductance to protons were measured for solvent-free bilayers made f rom id:sulfoquinovosyldiglyceride:phosphatidylglycerol (2:1:0.5:0.5, w /w) in the presence of a pH gradient of 7.4/8.1. The permeability coef ficient for protons in glycolipids was 5.5 +/- 1.1 x 10(-4) cm s(-1) ( n = 14). To determine whether this high H+ permeability could be expla ined by the presence of lipid contaminants such as weak acids, we inve stigated the effects of (a) bovine serum albumin, which can remove som e amphiphilic molecules such as free fatty acids, (b) 6-ketocholestano l, which increases the membrane dipole potential, (c) oleic acid, and (d) chlorodecane, which increases the dielectric constant of the lipid bilayer. Our results show that free fatty acids are inefficient proto nophores, as compared with carbonylcyanide-m-chlorphenylhydrazone, and that the hypothesis of a weak acid mechanism is not valid with glycol ipid bilayers. In the presence of deuterium oxide the H+ conductance w as reduced significantly, indicating that proton transport through the glycolipid matrix could occur directly by a hydrogen bond process. Th e passive transport of H+ through the glycolipid matrix is discussed w ith regard to the activity of the thylakoid ATP synthase and the inner -envelope H+-ATPase.