Analysis of direct transmembrane permeability of mono-amino acids by the planar membrane system.

Abstract

Amino acids are delivered to living cells by direct permeation, endocytosis, or assisted diffusion. Direct permeation does not require carriers or energy to deliver amino acids into the cytoplasm. Revealing how mono-amino acids permeate through the membrane will help to understand the permeation mechanism of cell-penetrating peptides and elucidate the way protocells take up amino acids. The relationship between the direct permeability of amino acids and their physicochemical properties is poorly understood, and their interactions with membranes remain controversial. In this study, we used the electrophysiological method, a label-free permeation measurement that avoids the influence of label molecules on the permeation, to analyze the transmembrane permeation through the planar lipid bilayer system. Considering the relationship between the shape of the current signal and the permeation models in the membrane, we attempt to estimate the membrane direct permeability of all twenty L-amino acids. As the result, the current signals were detected for amino acids at 20 mM. To confirm the transmembrane permeation of amino acids through the membrane, UV spectroscopy and chromogenic reaction were also measured. Three kinds of signals corresponding to the direct permeation (spike-like current) and the formation of membrane defects (erratic and multiple currents) were observed and assigned to each model. We analyzed the frequency and ratio of signal occurrence the spike-like signals and assessed the direct permeation of amino acids. The results show that hydrophobic amino acids exhibited higher current frequency and shorter duration, suggesting higher permeation. Through the relationship of current value and duration, we found differences in pore-forming ability and membrane permeation rate between amino acids. We will discuss factors that contribute to differences in membrane permeation.