Influence of pH on skin permeation of amino acids.

Skin permeation of amino acids through excised rat skin was measured at various pH values. The permeabilities varied with the donor pH and amino acid, indicating that each ionic species of amino acid may have a different permeability. The permeability coefficient of each ion was estimated from the permeability-pH profiles using the dissociation constants. The estimated values for mono-cation and uncharged zwitterion were not dependent on the lipophilicity but on the size of the amino acid, suggesting a porous mechanism of transport. The permeability coefficient was highest for di-cation, followed by mono-cation, positively charged, uncharged and negatively charged zwitterions. The electrical potential difference across the skin was too small to affect the permeation of ions. The permselective property of skin thus seems to be determined by the difference of diffusivity in aqueous pores of skin due to the hydration of ions and other factors.