The Effect of pH on Partition Coefficient and Invitro Corneal Permeation of a (dMet2, Pro5) Enkephalin and Its Acetylgalactosylamine Derivative
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The aim of this study was to investigate the effect of pH on the partition coefficient of enkephalins, and to examine how this influences their ability to permeate corneal membranes. The partition coefficient, P, between n-octanol and water of (D-Met2-Pro5) enkephalin (Penta) and its derivative (D-Met2-Pro5)tetra-O-acetylgalactosylenkephalinamide (P-gal) at different pH values was determined by HPLC analysis of each phase. P-gal was more lipophilic than the parent compound Penta. The effect of pH on the ionization and passive transport through rabbit corneas of [14C-tyrosine]Penta and [14C-tyrosine]P-gal was also investigated. Penta has three pK(a) values, 3.57, 6.86 and 9.84. P-gal has two pK(a) values, 7.30 and 10.01. Autoradiophotography was used to detect the permeation and distribution of the two compounds in the corneal membrane at very low concentrations and at different pH values. At pH 7.6, P-gal penetrated while Penta did not. After adjusting the pH to 5, both Penta and P-gal penetrated into the corneal membranes.
