Effect of tryptophan on the early tri-iodothyronine uptake in mouse thymocytes.
OBJECTIVE
We have studied the effect of tryptophan on cellular [(125)I]tri-iodothyronine (T3) uptake by mouse thymocytes.


METHODS
Mouse thymocytes (20 x 10(6 )cells/ml) were suspended in Krebs-Ringer solution buffered by Tris-HCl and incubation (23 degrees C at pH7.45+/-0.6), in the presence or absence of 1mM tryptophan, was started by adding 25 pM [(125)I]T3. At the end of incubation, samples were cooled in ice, centrifuged over a 30% sucrose cushion and the cell-associated radioactivity was measured in the pellet.


RESULTS
Tryptophan reduced both the total and the saturable fraction of [(125)I]T3 uptake by 44% (P=0.0009) and 60% (P=0.0006) respectively, following 1 min of incubation. This effect was specific and dose-dependent, being maximal at 5mM concentration (-82%). In contrast, the pre-exposure of cells to tryptophan for up to 2h had no effect on the subsequent uptake of [(125)I]T3, in the absence of tryptophan. The effect of D-tryptophan on saturable T3 uptake was not different from that obtained using the L-stereoisomer. Tryptophan reduced the V(max) of the initial rate of saturable [(125)I]T3 uptake by two-thirds without affecting the apparent K(m) (2.2 nM) of the process, thus indicating the non-competitive nature of the inhibition. In sodium-free medium the saturable [(125)I]T3 uptake was reduced by 43%. The inhibitory effect of tryptophan on [(125)I]T3 uptake was exerted in both the presence and the absence of sodium. In fact, the inhibitory effect of tryptophan on T3 transport was greater and significantly different (P=0.0046) from that obtained by sodium depletion alone.


CONCLUSIONS
Tryptophan interferes with both the sodium-dependent and -independent components of [(125)I]T3 uptake by a dose-dependent, non-competitive mechanism which operates in cis-modality at the plasma membrane level of mouse thymocytes.
Centanni
M
M
Department of Experimental Medicine and Pathology, University 'La Sapienza' of Rome, Italy.
Canettieri
G
G
Viceconti
N
N
Sibilla
R
R
Bei
A
A
Andreoli
M
M
D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D007457:Iodine Radioisotopes; D007700:Kinetics; D051379:Mice; D008807:Mice, Inbred BALB C; D012964:Sodium / Q000494:pharmacology; D013950:Thymus Gland / Q000187:drug effects* / Q000378:metabolism*; D014284:Triiodothyronine / Q000378:metabolism*; D014364:Tryptophan / Q000494:pharmacology*
D007457:Iodine Radioisotopes; D014284:Triiodothyronine; D014364:Tryptophan; D012964:Sodium