Portion Of Uptake Research Articles

Effect of Nitrogen Supply on the Kinetics and Regulation of Nitrate Assimilation in Chlamydomonas reinhardtii Dangeard

Concentration-dependent NO3 uptake by C. reinhardtii was studied in cells cultured under three NO3-growth regimes: 10 mM NO3 (NOJ -grown), 0-25 mM NO3 (N-limited) and an 18 h period of N-deprivation following growth on 10 mM NO J (N-starved). Two NO3 uptake isotherms, with an apparent phase transition in uptake at IT mM NO3, were resolved in all three cell types over the concentration range tested (0025 to 1-8 mM N03). The apparent Kmax for uptake by N-limited cells (0-72 x 10-8 /.?mol cell-1 h-1) exceeded that for both NO3-grown (0-42 x 10-8 /??mol cell-1 h-1) and N-starved (0-49 x 10-8 /??mol cell-1 h-1) cells at NO3 concentrations less than IT mM (isotherm 0). When supplied with NOj concentrations in excess of IT mM (isotherm 1) the apparent Kmax values for uptake by N-limited (0-64 x 10-8 /?mol cell-1 h-1) and N-starved (0-41 x 10-8 /?mol cell-1 h-1) cells were depressed relative to those for isotherm 0. In contrast, NO3-grown cells exhibited a greater apparent Vmax for uptake (0-63 x 10-8/?mol cell-1 h-1) at the higher NO3 concentrations. When N-limited and N-starved cells were supplied with 1-6 mM NO J (isotherm 1), the pattern of uptake showed a rapid linear increase after 60 min. This enhanced pattern of uptake did not occur in NO J-grown cells supplied with either 0-6 or 1-6 mM NO3 or in N-limited and N-starved cells supplied with 0-6 mM NO J. Cycloheximide (10 /?g cm-3), but not actinomycin D (50 /?g cm-3), blocked the apparent 'induction' of NO3 uptake in N-starved cells supplied with l-6mM NO J. These results suggest that mechanisms controlling the observed 'inducible' aspect of NO3 uptake in N-starved cells are dependent on a translational or post translational event. It is possible that the NO3 -concentration dependent 'inducible' portion of uptake may provide the means by which C. reinhardtii cells may regulate uptake in accordance with their capacity for assimilation.

Sodium-dependent neutral amino acid transport by human liver plasma membrane vesicles.

The activities of several selected Na(+)-dependent amino acid transporters were identified in human liver plasma membrane vesicles by testing for Na(+)-dependent uptake of several naturally occurring neutral amino acids or their analogs. Alanine, 2-(methylamino)isobutyric acid, and 2-aminoisobutyric acid were shown to be almost exclusively transported by the same carrier, system A. Kinetic analysis of 2-(methylamino)isobutyric acid uptake by the human hepatic system A transporter revealed an apparent Km of 0.15 mM and a Vmax of 540 pmol.mg-1 protein.min-1. Human hepatic system A accepts a broad range of neutral amino acids including cysteine, glutamine, and histidine, which have been shown in other species to be transported mainly by disparate carriers. Inhibition analysis of Na(+)-dependent cysteine transport revealed that the portion of uptake not mediated by system A included at least two saturable carriers, system ASC and one other that has yet to be characterized. Most of the glutamine and histidine uptake was Na(+)-dependent, and the component not mediated by system A constituted system N. The largest portion of glycine transport was mediated through system A and the remainder by system ASC with no evidence for system Gly activity. Our examination of Na(+)-dependent amino acid transport documents the presence of several transport systems analogous to those described previously but with some notable differences in their functional activity. Most importantly, the results demonstrate that liver plasma membrane vesicles are a valuable resource for transport analysis of human tissue.