[Absorption and malabsorption of protein digestion products]

Oral versus intravenous L-phenylalanine loading compared by simultaneous application of L-[2H5] and L-[15N]phenylalanine

Oral loading with 1.5 g of L-[15N]phenylalanine was performed simultaneously with an intravenous infusion of 1.5 g of L-[2H5]phenylalanine in two healthy volunteers with normal phenylalanine-hydroxylase activity. For both volunteers peak levels of oral L-[15N]phenylalanine were about 20 micrograms/ml compared to peak levels of around 50 micrograms/ml for intravenous L-[2H5]phenylalanine. Throughout the four hours following application, the plasma levels of the intravenously administered phenylalanine were higher than the plasma levels of the phenylalanine administered orally. In contrast, similar plasma levels of L-[15N]tyrosine and of L-[2H4]tyrosine formed in vivo by hydroxylation of the corresponding stable isotope labelled L-phenylalanine precursors were observed during the test, indicating that about equal fractions of both the oral and of the intravenous L-phenylalanine are converted into L-tyrosine.

Kinetics of the absorption of amino acids by the rat intestine in vivo

The kinetics of L-phenylalanine and L-lysine absorption by the rat small intestine in vivo have been studied by perfusing intestinal segments and monitoring simultaneously the uptake of the substrate into the intestinal tissue and its disappearance from the perfusate. The rate of phenylalanine disappearance is a linear function of the substrate concentration. Its uptake into the tissue is rapid and obeys saturation kinetics, but is not concentrative. Both tissue uptake and disappearance rate can be inhibited by leucine or methionine, but are not influenced by hydrophilic neutral or dibasic amino acids. Lysine disappearance from the perfusate and its uptake into the tissue both display saturation kinetics. Lysine transport is quantitatively smaller than that of phenylalanine. Both uptake and disappearance are inhibited by arginine and leucine, but are unaffected by other neutral amino acids or sugars. To analyse the kinetic results, integrated equations were developed to express the final concentration in the perfusate in terms of the original concentration. The disappearance rate was considered as a mixed process (saturable and non-saturable in parallel) in a one-compartment system, and the uptake by the tissue was treated as a two-compartment system in which the amino acid entered the cells by a mixed process but left them by a pure non-saturable mechanism. The results concerning disappearance from the lumen are compatible with the one-compartment model. Phenylalanine absorption can be described by a major non-saturable component and a minor saturable one, while lysine absorption occurs almost entirely by a saturable process. The two-compartment model does not adequately describe the tissue uptake results.