Proton/hydroxide conductance and permeability through phospholipid bilayer membranes

Area of Science:

• Biophysics
• Membrane Transport
• Biochemistry

Background:

• Proton/hydroxide (H+/OH-) permeability through phospholipid bilayers significantly exceeds that of alkali or halide ions at neutral pH.
• Understanding the mechanisms of H+/OH- transport across biological membranes is crucial for cellular function.

Purpose of the Study:

• To elucidate the mechanisms governing proton/hydroxide conductance and permeability in planar phospholipid bilayer membranes.
• To identify the nature and pathways of H+/OH- charge carriers.

Main Methods:

• Formation of planar phospholipid bilayer membranes using bacterial phosphatidylethanolamine, diphytanoyl phosphatidylcholine, or egg phosphatidylcholine with cholesterol in decane.
• Measurement of H+/OH- conductance (GH/OH) and net permeability at pH 7.
• Investigation of the effects of inhibitors (serum albumin, phloretin), fatty acids, pH, and voltage on GH/OH.

Main Results:

• At pH 7, H+/OH- conductance ranged from 2 to 6 nS.cm-2, with net permeabilities of (0.4-1.6) X 10(-5) cm.sec-1.
• GH/OH was inhibited by serum albumin, phloretin, and low pH, but increased by chlorodecane, fatty acids, and voltages > 80 mV.
• Water permeability was not correlated with GH/OH, suggesting distinct transport mechanisms.

Conclusions:

• Results indicate the H+/OH- charge carrier is primarily anionic and traverses the membrane via nonpolar pathways.
• Serum albumin effectively removes the charge carrier from the membrane.
• Weakly acidic phospholipid contaminants likely act as primary proton carriers at neutral pH, with additional background mechanisms present at low pH or with inhibitors.