Amino acid transport in a polyaromatic amino acid auxotroph of Saccharomyces cerevisiae

The initiation of growth of a polyaromatic auxotrophic mutant of Saccharomyces cerevisiae was inhibited by several amino acids, whereas growth of the parent prototroph was unaffected. A comparative investigation of amino acid transport in the two strains employing (14)C-labeled amino acids revealed that the transport of amino acids in S. cerevisiae was mediated by a general transport system responsible for the uptake of all neutral as well as basic amino acids. Both auxotrophic and prototrophic strains exhibited stereospecificity for l-amino acids and a K(m) ranging from 1.5 x 10(-5) to 5.0 x 10(-5) M. Optimal transport activity occurred at pH 5.7. Cycloheximide had no effect on amino acid uptake, indicating that protein synthesis was not a direct requirement for amino acid transport. Regulation of amino acid transport was subject to the concentration of amino acids in the free amino acid pool. Amino acid inhibition of the uptake of the aromatic amino acids by the aromatic auxotroph did not correlate directly with the effect of amino acids on the initiation of growth of the auxotroph but provides a partial explanation of this effect.     

Amino acid transport in established adult rat liver epithelial cell lines

The capacities of Na⁺-dependent transport of -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 V _max , whereas the values for K _m 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.Abbreviations AIB -aminoisobutyrate - LPC liver parenchymal cells - N-Methyl-AIB N-methyl--aminoisobutyrate - TAT tyrosine aminotransferase     

Amino acid transport in Mycoplasma

The uptake of l-histidine by Mycoplasma fermentans and l-methionine by M. hominis was found to be dependent on temperature and pH and to follow saturation kinetics. Several metabolic inhibitors inhibited this uptake. The transport system for l-methionine was highly specific. The l-histidine transport system was less specific, and the uptake was competitively inhibited by l-arginine and l-lysine. l-Histidine accumulated in the intracellular pool of M. fermentans at a concentration about 200 times that found in the medium. Efflux of accumulated l-histidine was demonstrated at 37 C, but not at 0 C. The rate of efflux was greatly accelerated by addition of l-histidine to the medium. The findings indicate that the Mycoplasma cell membrane contains specific transport systems resembling the permease systems of other microorganisms.     

Amino acid transport in mammalian oocytes

Uptake of ¹⁴C-labelled amino acids into single oocytes was determined using ³H-labelled choline to correct for extracellular space. Cycloleucine, a non-metabolisable amino acid sharing an entry mechanism with methionine and phenylalanine, was transported in accord with Michaelis-Menten kinetics. At extracellular levels below 8 mM, cycloleucine was concentrated within the oocyte. The proportion of sheep oocytes having a functional amino acid transport system (i.e. cycloleucine flux > 1 nmole cm⁻² h⁻¹) was highest in pre-ovulatory follicles (97%), and lowest in atretic follicles (59%). Amino acid fluxes in functional germinal vesicle oocytes were similar at all stages of development studied. An increase in V_max but not K_m during meiotic maturation resulted in a doubling of amino acid uptake in metaphase II oocytes. These increased fluxes were under gonadotropic regulation and were independent of nuclear maturation. Amino acid uptake by mouse oocytes was approximately half that measured in sheep oocytes.     

Amino acid transport in Mycobacterium smegmatis

The transport of d-alanine, d-glutamic acid, and d-valine in Mycobacterium smegmatis was compared quantitatively with that of their l-isomers. It appeared that the uptake of d-alanine was mediated by an active process displaying saturation kinetics characteristic of enzyme function, whereas the uptake of d-glutamic acid was accomplished by a passive process showing diffusion kinetics. Both processes were involved in the uptake of l-alanine, l-glutamic acid, d-valine, and l-valine. d-Valine competed with l-valine for entry into the cell through a single active process. d-Alanine and l-alanine also utilized the same active process, but the d-isomer could not enter the cell through the passive process. The passive process exhibited characteristics of diffusion, but was sensitive to sulfhydryl-blocking reagents and showed competition among structurally related amino acids. These last findings suggested that the passive process is a facilitated diffusion.     

Amino acid transport in the retina

The uptake, exit, homoexchange, inhibitory pattern, and kinetic analysis of transport of three amino acids were studied in the isolated retina of adult rat under different metabolic conditions. Only in the case of glycine, uptake and exit were shown to duplicate the processes observed in brain slices. In the case of lysine, glucose and oxygen showed an inhibitory effect, but with glutamate spontaneous exit could not be measured. It was also found that the rate of homoexchange for glycine and glutamate, but not for lysine, increases in the presence of oxygen and glucose.