Uptake of the components of phenylalanylphenylalanine and maltose by intestinal epithelium.

The observed rate of phenylalanine absorption into rat intestinal rings with 0.5 or 5.0 mM phenylalanine is greater than that for absorption of phenylalanine from 0.25 or 2.5 mM Phe-Phe, respectively. With the amino acid phenylalanine, V for absorption is the same whether Na+ is present (149 mM) or absent, but the concentration at which the half-maximal transport rate occurred (Kt) is greater in the absence of Na+. For Phe-Phe, the V decreases in the absence of Na+ whilst Kt is not influenced by the Na+ concentration. The different effect of Na+ on Phe and Phe-Phe transport indicates that the absorptive mechanism for Phe-Phe is different from that for phenylalanine. Absorption of a mixture of [U-14C]Phe-[he and Phe-[G-3H]Phe showed identical rates of uptake of the carboxyl and amino terminal amino acids. Studies of transport of radioactive maltose showed that the rates of uptake of the reducing and non-reducing glucosyl moieties are identical. Radioactive maltose absorption is not inhibited by glucose oxidase. These results provide evidence that in intestinal epithelium, hydrolysis of Phe-Phe and maltose does not occur on the cell surface with release of the hydrolyzed products to the medium. Rather, hydrolysis and release of the reaction products occur at a point on the cytosol side of a diffusion barrier located in the brush border membrane.