Biophysical model approaches to mechanistic transepithelial studies of peptides

Abstract

Systematic studies were conducted to understand the physicochemical and biophysical mechanisms governing the membrane transport of peptides. These studies focused primarily on the Peyer's patch of the small intestine, particularly the M cells, with respect to the vesicular transport mechanisms, and also the buccal, intestinal and Caco-2 (human-derived colon carcinoma cell) membrane systems with respect to the passive diffusional mechanism of peptides. The development of quantitatively sensitive experimental methodologies was required and, in conjunction with the specific membrane system, was initially assessed with non-peptide compounds of well-known active or passive transport properties. Using a miniature closed perfusion cell system positioned over the large Peyer's patch of the rabbit coupled with cannulation of the mesenteric blood and lymph vasculatures, adsorptive endocytosis uptake and concurrent appearance kinetics in the blood and lymph were followed with cationic poly(d-lysine) (PDL, 55 kDa) conjugated with [ 14C] formaldehyde. The influx of PDL by the apical membrane was 100-fold larger than the efflux into blood; none was detected in lymph. A mixture of metabolic inhibitors, 2-deoxyglucose and Na azide, caused partial inhibition of endocytosis. By using a fluorescein isothiocyanate-PDL conjugate and fluorescence microscopy, PDL was found to be localized predominantly on the apical membrane surface of all intestinal epithelia. It also was found to be accumulated intracellularly by epithelia, most likely M-cells, that occupy the dome region of the Peyer's patch and by cells within the lymphoid follicles. The results suggested that PDL was trapped by the cells and lymphoid follicles and that the rate-determining step in the appearance of PDL in the mesenteric lymph is the migration of lymphocytes from the lymph space associated with M-cells. Transport studies were conducted to determine structure-passive absorption relationships of small model peptides using the buccal, intestinal mucosal and/or Caco-2 cell monolayer membranes. In the course of buccal absorption studies, amino acids and their BOC-derivatives, and ancillary non-peptide compounds were included to aid in data interpretation. Based on the amphoteric peptide series, ( d- Phe) nGly, and the neutral series, Ac( d- Phe) n NH 2, where n = 1, 2 or 3, terminal charges on zwitterionic peptides have a negative effect on membrane permeability even though the effective partition coefficient in n-octanol/water is relatively high. Although the partition coefficient is increased by incremental additions of d- Phe, the membrane permeability tends to decrease. This may be related not only to molecular size but also to the number ofsolvated amide bonds. Partition coefficients by the n-octanol/water scale appear to be poor predictors of membrane absorption of peptides.