Transport of neutral, cationic and anionic amino acids by systems B, b(o,+), X(AG), and ASC in swine small intestine

Amino acid influx across the brush border membrane of the intact pig ileal epithelium was studied. It was examine whether in addition to system B, systems ASC and b(o,+) were involved in transport of bipolar amino acids. The kinetics of interactions between lysine and leucine demonstrates that system b(o,+) is present and accessible also to l-glutamine. d-aspartate (K(1/2) 0.3 mM) and l-glutamate (K(i) 0.5 mM) share a high affinity transporter with a maximum rate of 1.3 μmol cm^-2 h^-1, while only l-glutamate with a K(1/2) of 14.4 mM uses a low affinity transporter with a maximum rate of 2.7 μmol cm^-2 h^-1, system ASC, against which serine has a K(i) of 1.6 mM. In the presence of 100 mM lysine, l-glutamine (A), leucine (B), and methionine (C) fulfilled the criteria of the ABC test for transport by one and the same transporter. However, serine inhibits not only transport of l-glutamate but also of glutamine (K(i) 0.5 mM), and l-glutamate inhibits part of the transport of glutamine. The test does, therefore, only indicate that the three bipolar amino acids have similar affinities for transport by systems B and ASC. Further study of the function of system B must be carried out under full inhibition by lysine and glutamate. (C) 2000 Elsevier Science Inc.