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.
Rekha
Animal Biochemistry Division, National Dairy Research Institute, Kamal, India.
Kansal
V K
VK
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D005260:Female; D008321:Mammary Glands, Animal / Q000378:metabolism*; D051379:Mice; D012964:Sodium / Q000502:physiology*; D013237:Stereoisomerism; D014443:Tyrosine / Q000493:pharmacokinetics*
D014443:Tyrosine; D012964:Sodium