Discrimination of transport systems of L-tyrosine in mouse mammary gland: characterization of system T
Rekha; Kansal, V.K.
Indian Journal of Experimental Biology 34(8): 750-757
1996
ISSN/ISBN: 0019-5189 PMID: 8979480 Document Number: 468884
On the basis of inhibition analysis, tyrosine uptake in mouse mammary gland was found to be mediated by Na+-dependent and Na+-independent systems. Na+-dependent system was insensitive to 2-(methylamino) isobutyric acid (MeAIB), with an apparent K-m of 1.67 mM and maximal velocity 74.5 nmol cntdot g-1 cell cntdot min-1. Competition experiments showed the presence of two distinct Na+-independent components of tyrosine uptake. One component was sensitive to 2-aminobicyclo (2,2,1) heptane-2-carboxylic acid (BCH) and was similar to the system L, with an apparent K-m of 0.23 mM and maximum velocity of 31 nmol cntdot g-1 cell cntdot min-1. Second component was BCH-insensitive but tryptophan-sensitive, with an apparent K-m of 15.75 mM and V-max of 157.5 nmol cntdot g-1 cell cntdot min-1. BCH-insensitive, tryptophan-sensitive system was a low affinity system. It approached steady state slowly and was more sensitive, relative to the system L, to n-ethylmaleimide inhibition. Tyrosine uptake through this system did not respond to trans-stimulation, whereas system L mediated uptake responded considerably. BCH-insensitive, tryptophan-sensitive component of Na+-independent tyrosine uptake is attributed to the system T, previously described only in human red blood cells and rat liver cells.