Uptake of purine bases by HGPRT deficient erythrocytes

Purine and pyrimidine transport and permeation in human erythrocytes

Time courses of the uptake of radiolabeled hypoxanthine, adenine and uracil were measured by rapid kinetic techniques over substrate ranges from 0.02 to 5000 microM in suspensions of human erythrocytes at 25 or 30 degrees C. At concentrations above 25 microM, the rate of intracellular phosphoribosylation of hypoxanthine and adenine was insignificant relative to their rates of entry into the cell and time courses of transmembrane equilibration of the substrates could be measured and analyzed by integrated rate analysis. Hypoxanthine and uracil are transported by simple facilitated carriers with directional symmetry, high capacity and Michaelis-Menten constants of about 0.2 and 5 mM, respectively. Adenine is probably transported by a carrier with similar properties but no saturability was detectable up to a concentration of 5 mM. Cytosine entered the cells much more slowly than the other three nucleobases, and its entry seems not to be mediated by a carrier. The hypoxanthine transporter resembles that of one group of mammalian cell lines, which does not exhibit any overlap with the nucleoside transporter and is resistant to inhibitors of nucleoside transport. Results from studies on the effects of the nucleobases on the influx and countertransport of each other were complex and did not allow unequivocal conclusions as to the number of independent carriers involved. At concentrations below 5 microM, radiolabel from adenine and hypoxanthine accumulated intracellularly to higher than equilibrium levels. Part of this accumulation reflected metabolic trapping, especially when the medium contained 50 mM phosphate. But part was due to an apparent concentrative accumulation of free adenine and hypoxanthine up to 3-fold at medium concentrations much less than 1 microM and when cells were incubated in phosphate-free medium. This concentrative accumulation could be due to the functioning of additional high-affinity, low-capacity, active transport systems for adenine and hypoxanthine, but other factors could be responsible, such as saturable binding to intracellular components.

Adenine transport and binding in cultured mammalian cells deficient in adenine phosphoribosyltransferase

Rapid kinetic techniques were employed to measure the transport of adenine in adenine phosphoribosyltransferase-deficient L929 and Chinese hamster ovary (CHO) cells in zero-trans entry and exit and equilibrium exchange procedures. The kinetic parameters of transport were computed by fitting appropriate integrated rate equations to time courses of transmembrane equilibration of radiolabeled adenine. Adenine transport conformed to the simple carrier model with directional symmetry and equal mobility of loaded and empty carrier. The Michaelis-Menten constants and maximum velocities for various strains of L929 cells fell between 2.3 and 3.5 mM and 90 and 150 pmol/microliters of cell water per s, respectively, values similar to those previously reported for CHO and Novikoff hepatoma cells. The corresponding values for hypoxanthine transport in L929 cells were 413 microM and 16 pmol/microliters of cell water per s. Adenine transport velocities were directly proportional to adenine concentrations between 0.03 and 50 microM in both CHO and Novikoff cells. The results indicate that adenine is transported in these cells by a single, low-affinity, high-capacity transporter. Adenine transport was inhibited by hypoxanthine in some cell strains, but not in others. Adenine also rapidly bound to L929 cells in a saturable manner (KD = 18 microM), presumably to the cell surface (about 3 X 10(7) sites per cell).

Adenine transport and binding in cultured mammalian cells deficient in adenine phosphoribosyltransferase

Rapid kinetic techniques were employed to measure the transport of adenine in adenine phosphoribosyltransferase-deficient L929 and Chinese hamster ovary (CHO) cells in zero-trans entry and exit and equilibrium exchange procedures. The kinetic parameters of transport were computed by fitting appropriate integrated rate equations to time courses of transmembrane equilibration of radiolabeled adenine. Adenine transport conformed to the simple carrier model with directional symmetry and equal mobility of loaded and empty carrier. The Michaelis-Menten constants and maximum velocities for various strains of L929 cells fell between 2.3 and 3.5 mM and 90 and 150 pmol/microliters of cell water per s, respectively, values similar to those previously reported for CHO and Novikoff hepatoma cells. The corresponding values for hypoxanthine transport in L929 cells were 413 microM and 16 pmol/microliters of cell water per s. Adenine transport velocities were directly proportional to adenine concentrations between 0.03 and 50 microM in both CHO and Novikoff cells. The results indicate that adenine is transported in these cells by a single, low-affinity, high-capacity transporter. Adenine transport was inhibited by hypoxanthine in some cell strains, but not in others. Adenine also rapidly bound to L929 cells in a saturable manner (KD = 18 microM), presumably to the cell surface (about 3 X 10(7) sites per cell).

Purine metabolism of erythrocytes in myotonic dystrophy

The uptake and subsequent metabolism of adenine and adenosine was studied with the intact erythrocytes of patients with myotonic dystrophy (MD). Washed erythrocytes of both controls and patients were incubated with 14C-labeled adenine or adenosine at 20 degrees C for 5 to 120s to characterize the uptake process. No differences in the uptake process of adenine or adenosine were observed between normal and MD erythrocytes. Formation of adenine nucleotides was determined at 37 degrees C after incubation for 30 min. Compared to controls, the incorporation of adenine into adenine nucleotides was 2.6 times higher in MD erythrocytes. This depends mainly on an increase of ATP formation. The mean ratios of ATP:ADP + AMP for normal red cells and MD erythrocytes were 0.92 and 1.39 respectively. No difference was found in the conversion of adenosine to adenine nucleotides. In spite of a normal amount of intracellular ATP a greater demand for ATP exists. This might be due to leakage of adenine nucleotides out of MD erythrocytes.

Purine metabolism in human lymphocytes

In peripheral human blood lymphocytes the uptake and metabolism of adenine, guanine, and hypoxanthine was investigated. This was achieved by incubation of purified lymphocytes with 14C-purine bases, separation of cells from the incubation medium by a rapid filtration technique, and subsequent separation of the acid soluble material by thin-layer chromatography. No perferential uptake for one of the purine bases was observed. In all cases only traces of 14C-purine bases not added originally and labeled nucleosides could be demonstrated. Approximately 2/3 of adenine and 1/2 of guanine or hypoxanthine were converted to nucleotides. Separation of formed nucleotides showed that adenine and guanine were metabolized mainly to their corresponding nucleotides; hypoxanthine was converted to a considerable amount to adenine nucleotides and only to a small proportion into its own nucleotides. These results demonstrate the predomonance of adenine nucleotide formation in normal human lymphocytes.