Kinetics of 3-[123I]iodo-l-α-methyltyrosine transport in rat C6 glioma cells

3-[123I]Iodo-l-α-methyltyrosine (123I-IMT) is used for the diagnosis and monitoring of brain tumours by means of single-photon emission tomography (SPET). To date, little has been known about the system for the transport of 123I-IMT into brain tumour cells. It is assumed that 123I-IMT is transported by a specific carrier for large, neutral amino acids (L-system). In this study, rat C6 glioma cells were used to characterize the uptake system of 123I-IMT and to investigate its precise kinetics. The time course of 123I-IMT uptake into the cells was examined for a range of 1-60 min. 123I-IMT uptake rates with varying concentrations of 123I-IMT (2.5-50 μM) in the medium were quantified to assess the kinetic parameters of 123I-IMT transport. Furthermore, competition of 123I-IMT with other amino acids was investigated to identify the distinct transport systems involved in 123I-IMT uptake. 123I-IMT uptake into C6 glioma cells was linear for approximately 10 min and reached a steady-state level within 30 min. The analysis of the rate of uptake of 123I-IMT at different concentrations was concordant with the predominance of a single uptake system. The apparent Michaelis constant (Km) of 123I-IMT was 26.2±1.9 μM, and the maximum transport velocity (Vmax) was 35.4±1.7 nmol/mg protein per 10 min. 77%±10% of 123I-IMT transport was sodium independent and 23%±3% was sodium dependent. Competitive inhibition of 123I-IMT uptake by 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid, α-(methylamino)isobutyric acid and naturally occurring amino acids revealed a major 123I-IMT transport via the sodium-independent system L (72%) and a minor uptake via the sodium-dependent system B0,+ (17%). Our results show that 123I-IMT transport into C6 glioma cells is principally mediated by the L-system and to a minor extent by the B0,+-system. The kinetic parameters of 123I-IMT uptake are in the range of those of naturally occurring amino acids. (orig.)