Discrimination of transport systems of L-tyrosine in mouse mammary gland: characterization of system T.

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 Km of 1.67 mM and maximal velocity 74.5 nmol.g-1 cell. 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 Km of 0.23 mM and maximum velocity of 31 nmol.g-1 cell.min-1. Second component was BCH-insensitive but tryptophan-sensitive, with an apparent Km of 15.75 mM and Vmax of 157.5 nmol.g-1cell.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.