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Neutral amino acid transport in bovine articular chondrocytes

G. A. Barker, R. J. Wilkins, S. Golding and J. C. Ellory

The sodium-dependent amino acid transport systems responsible for proline, glycine and glutamine transport, together with the sodium-independent systems for leucine and tryptophan, have been investigated in isolated bovine chondrocytes by inhibition studies and ion replacement. Each system was characterized kinetically.

Transport via system A was identified using the system-specific analogue -methylaminoisobutyric acid (MeAIB) as an inhibitor of proline, glycine and glutamine transport.

Uptake of proline, glycine and glutamine via system ASC was identified by inhibition with alanine or serine.

System Gly was identified by the inhibition of glycine transport with excess sarcosine (a substrate for system Gly) whilst systems A and ASC were inhibited. This system, having a very limited substrate specificity and tissue distribution, was also shown to be Na⁺ and Cl^- dependent. Evidence for expression of the system Gly component GLYT-1 was obtained using the reverse transcriptase-polymerase chain reaction (RT-PCR).

System N, also of narrow substrate specificity and tissue distribution, was shown to be present in chondrocytes. Na⁺-dependent glutamine uptake was inhibited by high concentrations of histidine (a substrate of system N) in the presence of excess MeAIB and serine.

System L was identified using the system specific analogue 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH) and D-leucine as inhibitors of leucine and tryptophan transport.

The presence of system T was tested by using leucine, tryptophan and tyrosine inhibition. It was concluded that this system was absent in the chondrocyte.

Kinetic analysis showed the Na⁺-independent chondrocyte L system to have apparent affinities for leucine and tryptophan of 125 ± 27 and 36 ± 11 µM, respectively.

Transport of the essential amino acids leucine and tryptophan into bovine chondrocytes occurs only by the Na⁺-independent system L, but with a higher affinity than the conventional L system.

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