Transport Of Alpha-aminoisobutyrate Research Articles

Na(+)‐ and H(+)‐gradient‐dependent transport of alpha‐aminoisobutyrate by luminal membrane vesicles from rabbit proximal tubule.

1. The characteristics of renal transport of alpha-aminoisobutyrate (AIB) by luminal membrane vesicles isolated from either the proximal convoluted part (pars convoluta) or the proximal straight part (pars recta) of rabbit proximal tubule were investigated. 2. Transport of AIB in vesicles from pars convoluta was mediated by both Na(+)-dependent and Na(+)-independent systems, which in the presence of an inwardly directed H+ gradient can drive the uphill transport of AIB into these vesicles. 3. By contrast, in luminal membrane vesicles from pars recta, transient accumulation of AIB was only dependent on Na+. Lowering pH without a H+ gradient (pHi = pH0 = 5.5) completely abolished the Na(+)-dependent transient accumulation of AIB in these vesicle preparations. 4. Attempts to determine the stoichiometry of both the Na(+)-AIB and H(+)-AIB transporters located in these two segments of proximal tubule suggested that one Na+ and one H+ ion may be involved in the transport of AIB. 5. Sodium-dependent uptake of AIB in vesicles from pars convoluta was competitively inhibited by L-serine and L-phenylalanine, whereas the presence of L-proline, L-alanine and glycine had no significant effect. By contrast, the H(+)-gradient-dependent uptake of AIB was drastically reduced (30% of the control value) by L-proline, L-alanine and glycine, while L-serine and L-phenylalanine had no significant effect. 6. On the other hand, pars recta vesicles exhibited a different transport specificity. L-Phenylalanine, L-serine, L-alanine and glycine, but not L-proline competitively inhibited the uptake of AIB, providing evidence for the existence of a common transport system for AIB, L-phenylalanine, L-serine, L-alanine and glycine in this segment of rabbit proximal tubule.

Na+‐dependent, active nucleoside transport in S49 mouse lymphoma cells and loss in AE‐1 mutant deficient in facilitated nucleoside transport

S49 murine lymphoma cells were examined for expression of various nucleoside transport systems using a non-metabolized nucleoside, formycin B, as substrate. Nitrobenzylthioinosine (NBTI)-sensitive, facilitated transport was the primary nucleoside transport system of the cells. The cells also expressed very low levels of NBTI-resistant, facilitated nucleoside transport as well as of Na(+)-dependent, concentrative formycin B transport. Concentrative transport was specific for uridine and purine nucleosides, just as the concentrative nucleoside transporters of other mouse and rat cells. A nucleoside transport mutant of S49 cells, AE-1, lacked both the NBTI-sensitive, facilitated and Na(+)-dependent, concentrative formycin B transport activity, but Na(+)-dependent, concentrative transport of alpha-aminoisobutyrate was not affected.

Amino acid transport in established adult rat liver epithelial cell lines.

The capacities of Na+-dependent transport of alpha-aminoisobutyrate, glutamine and glutamate in four established and three transformed rat liver epithelial cell lines were found to be considerably higher than those of isolated and cultured hepatocytes. At least for transport systems A and G- this seemed to be due to elevated values of Vmax, whereas the values for Km were quite comparable to those of hepatocytes. In contrast to hepatocytes, however, no significant hormonal stimulation of amino acid uptake could be detected in the cell lines. Each normal cell line expressed a distinct pattern of transport capacities with respect to the three systems measured and this was not altered by chemical transformation of the lines. The individual patterns of the lines showed no similarity to presumptive patterns of subpopulations of liver parenchymal cells. In particular, there was no evidence for a direct relationship of one of the cell lines with a small subpopulation of parenchymal cells located adjacent to hepatic venules as revealed by additional measurements of glutamine synthetase, a marker enzyme for this particular subpopulation. It is concluded that established rat liver epithelial cell lines express features characteristic of normal hepatocytes with respect to amino acid transport, but have developed a distinct phenotype adapted to a rapid, hormone-independent growth in vitro. Alteration of their phenotype by transformation is not coupled with a further increase in amino acid transport capacity.

Transport of alpha-aminoisobutyrate into Trypanosoma brucei brucei.

The uptake of alpha-aminoisobutyrate (AIB) by washed cell suspensions of bloodstream forms of Trypanosoma brucei brucei has been shown to be an energy-dependent process. No metabolism of AIB was detected under conditions leading to a 100-fold accumulation of AIB within the organism. Kinetic studies revealed that AIB uptake involved two components; that operating at low substrate concentrations had an apparent Km of 4.6 mM. Experiments with ionophores such as gramicidin and carbonylcyanide m-chlorophenylhydrazone were consistent with the AIB uptake system operating as a H+-symporter responding to the electrochemical gradient of H+, the major component of which was the membrane potential.

Sodium ion-substrate symport in a marine bacterium.

Anaerobic suspensions of Alteromonas haloplanktis accumulated alpha-aminoisobutyric acid, by a sodium-dependent process, in response to an artificially imposed membrane potential in the presence or absence of a transmembrane chemical gradient of sodium. These results suggest that the transport of alpha-aminoisobutyrate by this organism occurs via Na+-substrate symport.

Differentiation of ob 17 preadipocytes to adipocytes. Effect of insulin on the levels of insulin receptors and on the transport of alpha-aminoisobutyrate.

Cells of a clonal cell line (ob 17) isolated from the epididymal fat pad of ob/ob mouse possess insulin receptors. Their number was increased 1.5-fold after growth arrest, with no significant change in the Kd values of the "high affinity" sites determined by extrapolation of the high affinity portion of the curvilinear Scatchard plots. With chronic insulin exposure for 3 to 11 days after confluence, ob 17 cells showed a decrease in insulin receptor concentrations from 8,000 to 1,600 high affinity sites/cell (Kd from 0.45 to 1.10(-9) M) while similar levels of "low affinity" sites were found (80,000 to 100,000 sites/cell; Kd from 10(-8) to 3 x 10(-8) M). The loss of the high affinity binding sites is accompanied by the disappearance of the stimulatory effect by insulin of alpha-aminoisobutyrate uptake. Therefore, in contrast to 3T3-L1 fibroblasts, the ob 17 cells present, in culture, a self-modulation of insulin receptors and a loss of insulin sensitivity after chronic exposure to insulin.

The transport of alpha-aminoisobutyrate into Crithidia fasciculata.

The transport of alpha-aminoisobutyrate into Crithidia fasciculata was characterized under aerobic and anaerobic conditions. Kinetic data for alpha-aminoisobutyrate transport were consistent with the operation of a single system of broad specificity that showed no marked dependence on Na+. Under anaerobic conditions alpha-aminoisobutyrate transport was inhibited by uncouplers such as 2,4-dinitrophenol, lipophilic cations such as methyltriphenylphosphonium ion and adenosine triphosphatase inhibitors such as dicyclohexylcarbodi-imide and NaN3. A working model in which alpha-aminoisobutyrate enters this organism by an H+-symport mechanism, the electrochemical gradient of protons being maintained by an H+-translocating adenosine triphosphatase on the cytoplasmic membrane, is proposed.