Transport of amino acid amide sarcosinamide and sarcosinamide chloroethylnitrosourea in human glioma SK-MG-1 cells

Abstract

The transport of the amino acid amide N-[3H]sarcosinamide (methyl glycinamide) was investigated in human glioma SK-MG-1 cells. Sarcosinamide uptake was found to be temperature dependent, sodium independent, and linear up to 1 min at 22 degrees C. Equilibrium was reached after 10 min at 22 degrees C with accumulation slightly above unity. Sarcosinamide was not metabolized in the cells as shown by thin layer chromatography. The uptake of sarcosinamide was significantly decreased when the extracellular pH was lowered from 7.5 to 6.0 and significantly enhanced at pH values above 7.5. The latter effect may be due mainly to increased cell permeability at high pH. The uptake of the labeled sarcosinamide was trans-stimulated by excess cold sarcosinamide. Sarcosinamide uptake over a 200-fold range of concentrations followed Michaelis-Menten kinetics with a Km of 0.284 +/- 0.041 mM and a Vmax of 0.154 +/- 0.024 nmol/10(6) cells/min. The uptake of sarcosinamide was significantly reduced by iodoacetate but not by the metabolic poisons NaF, ouabain, or dinitrophenyl, suggesting that the uptake is not dependent on energy, rather it proceeds by facilitated diffusion. Several naturally occurring substrates were unable to inhibit the uptake of sarcosinamide. Leucine significantly reduced the uptake of sarcosinamide, while sarcosinamide was a weak inhibitor of leucine transport. 2-Aminobicyclo[2,2,1]heptane-2-carboxylic acid a specific substrate for the sodium-independent, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid-sensitive amino acid system L failed to inhibit the uptake of sarcosinamide. Epinephrine reduced the uptake of sarcosinamide and sarcosinamide was equally potent as an inhibitor of epinephrine transport. Dixon plot analysis demonstrated that epinephrine (Km = 0.270 mM) inhibits the uptake of sarcosinamide competitively (Ki = 0.260 mM). These results indicate that sarcosinamide is a substrate for the catecholamine transporter. The alkylating agent, sarcosinamide chloroethylnitrosourea, was tested for its ability to inhibit the uptake of sarcosinamide. The results of Dixon plot analysis were consistent with competitive inhibition of sarcosinamide uptake and the inhibition constant Ki for SarCNU was found to be 3.26 +/- 0.57 mM. The steady-state intracellular concentration of SarCNU was found to be significantly higher (cell:medium ratio of 1.03 +/- 0.01) than that of BCNU cell:medium ratio of 0.52 +/- 0.12). These findings indicate that SarCNU and sarcosinamide share the same carrier for uptake in SK-MG-1 cells. This transport mechanism may be responsible for the increased accumulation of SarCNU as compared to BCNU, a nitrosourea which enters cells by passive diffusion.