Metabolite transport at cell membranes

Distribution of some non-physiological amino acids (AIB, DAB, MeAIB) in mice

The distribution of labelled 2-aminoisobutyric acid (AIB), 2,4-diaminobutyric acid (DAB) and 2-(methylamino)-isobutyric acid (MeAIB) was studied by whole body autoradiography in mice. All the amino acids were rapidly cleared from the blood and were taken up in most of the organs in the body. The results suggest the presence of operating transport systems for these amino acids into the majority of different cell types in mice. In the central nervous system, small amounts could be registered and in the pancreas there was a marked difference between DAB, which was not taken up, and the other two amino acids which were heavily accumulated. In an early embryo only MeAIB was observed in a high concentration.

Kinetics of thiamine transport across the blood-brain barrier in the rat

1. By measurement of the rate of disappearance of injected tracer thiamine from the bloodstream, a programme for the continuous injection of thiamine at a variable rate has been devized by which a steady raised level can be achieved rapidly and maintained in the circulation. By this means the flux of radioactive thiamine across the blood-brain barrier has been measured. 2. In separate experiments progressively higher levels of thiamine were maintained in the bloodstream. Evidence was obtained that the transport of thiamine across the blood-brain barrier is a carrier-mediated process which can be saturated by raised levels of thiamine. 3. The saturation of the transport process was incomplete: kinetic analysis showed that there was a non-saturable component of the transport which was probably due to passive diffusion. 4. The contribution of the non-saturable component was normally small and is probably insufficient to meet the needs of the brain for the vitamin unless the concentration of the vitamin in the blood is raised considerably above normal. 5. This two-component transport process had substantially similar kinetic parameters in different regions of the brain.

Compartmentation in amino acid transport across the blood brain barrier

Under steady-state conditions, the transport rates for amino acids from blood to brain have been found to be about half that seen using the intraarterial injection technique. Using a method that mathematically mimics the constant infusion procedure, we were able to reconcile this apparent discrepancy. At less than 1 min after subcutaneous injection of [14C]tyrosine in mice, we have observed a rate of entry into brain of 19.7 nmol/g/min, while from 1-15 min we have measured the rate at 6.4 nmol/g/min. Using methionine sulfoximine as an inhibitor of the gamma-glutamyl cycle, the early rate was reduced to 10.0 nmol/g/min and the later rate to 3.7 nmol/g/min. These data are consistent with a two-compartment system regulating amino acid transport into the neurons. A mathematical model fit to these data indicates that the first compartment contains 8.3 nanomoles of tyrosine per gram brain or about 6.7% of the brain total. It is speculated that the first compartment consists primarily of the astrocytes.

Interaction between parallel transport systems examined with tryptophan and related amino acids

Most of the neutral amino acids are transported across the plasma membrane by two or more parallel transport systems. In this study, we have limited transport interactions to System A and L by studying amino acids not transported by System ASC. Results are presented to emphasize that such amino acids as leucine, phenylalanine and tryptophan do enter the Ehrlich cell to substantial degrees by System A. We show how the phenomenon of competitive stimulation presents a strong argument that these systems operate between the same two compartments, extracellular and cellular. To discover what is needed to bring two amino acids into the transport relation shown by tryptophan and methionine, we arbitrarily set up two classes of amino acids: (1) those whose steady-state gradients will be increased when System L is deleted; (2) those whose gradients will instead be decreased. On blockading System L with 2-aminorbornane-2-carboxylic acid applied in symmetry to the two sides of the plasma membrane, we show as predicted that the gradient maintained for methionine is strongly increased, that for trypotphan decreased. The initial rate of uptake of all the amino acids mentioned is highly sensitive to the nature of the cellular amino acid pool. A high influx by exchange, relative to net influx, appears to characterize the amino acids that respond to competitive stimulation. Albumin-bound tryptophan appears not directly accessible to the tested carriers.