1. The sodium-dependent amino acid transport systems responsible for prolin
e, glycine and glutamine transport, together with the sodium-independent sy
stems for leucine and tryptophan, have been investigated in isolated bovine
chondrocytes by inhibition studies and ion replacement. Each system was ch
aracterized kinetically.
2. Transport via system A was identified using the system-specific analogue
alpha-methylaminoisobutyric acid (MeAIB) as an inhibitor of proline, glyci
ne and glutamine transport.
3. Uptake of proline, glycine and glutamine via system ASC was identified b
y inhibition with alanine or serine.
4. System Gly was identified by the inhibition of glycine transport with ex
cess sarcosine (a substrate for system Gly) whilst systems A and ASC were i
nhibited. This system, having a very limited substrate specificity and tiss
ue distribution, was also shown to be Na+ and Cl- dependent. Evidence for e
xpression of the system Gly component GLYT-1 was obtained using the reverse
transcriptase-polymerase chain reaction (RT-PCR).
5. 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.
6. System L was identified using the system specific analogue 2-aminobicycl
o(2,2,1)heptane-2-carboxylic acid (BCH) and D-leucine as inhibitors of leuc
ine and tryptophan transport.
7. The presence of system T was tested by using leucine, tryptophan and tyr
osine inhibition. It was concluded that this system was absent in the chond
rocyte.
8. Kinetic analysis showed the Na+-independent chondrocyte L system to have
apparent affinities for leucine and tryptophan of 125 +/- 27 and 36 +/- 11
mu M, respectively.
9. Transport of the essential amino acids leucine and tryptophan into bovin
e chondrocytes occurs only by the Na+-independent system L, but with a high
er affinity than the conventional L system.