Abstract
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.
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More From: Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology
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