Effect of size and charge on the passive diffusion of peptides across Caco-2 cell monolayers via the paracellular pathway.

Purpose: To evaluate the effect of size and charge on the permeation characteristics of peptides across the intestinal mucosa.
Methods: The lipophilicities of neutral, positively and negatively charged capped amino acids (Asn, Lys, Asp), tripeptides (Ac-Gly-X-Ala-NH2; X = Asn, Lys, Asp) and hexapeptides (Ac-Trp-Ala-Gly-Gly-X-Ala-NH2; X = Asn, Lys, Asp) were estimated using an immobilized artificial membrane. The diffusion coefficients used to calculate the molecular radii were measured by NMR. The transport characteristics of the model peptides were determined across Caco-2 cell monolayers.
Results: When model compounds having the same charge were compared, permeation was highly size-dependent (capped amino acids > tripeptides > hexapeptides), suggesting transport predominantly via the paracellular route. For example, the flux of the negatively charged Asp amino acid (Papp = 10.04 +/- 0.43 x 10(-8) cm/s) was 3 times greater than that observed for the Asp-containing hexapeptide (Papp = 3.19 +/- 0.27 x 10(-8) cm/s). When model compounds of the same size were compared, permeation across the cell monolayer was charge-dependent (negative < positive < or = neutral). For example, the neutral, Asn-containing tripeptide (Papp = 25.79 +/- 4.86 x 10(-8) cm/s) was substantially more able to permeate the Caco-2 cell monolayer than the negatively charged Asp-containing tripeptide (Papp = 7.95 +/- 1.03 x 10(-8) cm/s) and the positively charged Lys-containing tripeptide (Papp = 9.86 +/- 0.18 x 10(-8) cm/s). The permeability of the cell monolayer to peptides became less sensitive to net charge as the size of the peptides increased.
Conclusions: A positive net charge of hydrophilic peptides enhances their permeation across the intestinal mucosa via the paracellular pathway. With increasing molecular size, molecular sieving of the epithelial barrier dominates the transport of peptides, and the effect of the net charge becomes less significant.