MEMBRANE-POTENTIAL DEPENDENCE OF THE KINETICS OF CATIONIC AMINO-ACID-TRANSPORT SYSTEMS IN HUMAN PLACENTA

Citation
N. Eleno et al., MEMBRANE-POTENTIAL DEPENDENCE OF THE KINETICS OF CATIONIC AMINO-ACID-TRANSPORT SYSTEMS IN HUMAN PLACENTA, Journal of physiology, 479(2), 1994, pp. 291-300
Citations number
15
Categorie Soggetti
Physiology
Journal title
ISSN journal
00223751
Volume
479
Issue
2
Year of publication
1994
Pages
291 - 300
Database
ISI
SICI code
0022-3751(1994)479:2<291:MDOTKO>2.0.ZU;2-O
Abstract
1. Mediated influx of L-lysine into human placental brush-border membr ane vesicles occurs through two systems, one of lower affinity but hig h capacity, the other of very high affinity but low capacity. These tr ansporters have features characteristic of systems y(+) (the classical system) and y(+)L (recently described in the erythrocyte), respective ly. 2. In solutions containing sodium the entry of lysine through the high-affinity system (y(+)L) is inhibited by the neutral amino acids L -leucine, L-methionine and L-glutamine with comparable high affinity. The removal of sodium reduces the affinity but not the maximal extent of this inhibition. Leucine and methionine, but apparently not glutami ne, inhibit lysine entry through system y(+) with a much lower affinit y. 3. The influx of lysine through system y(+) changes markedly in res ponse to alterations of membrane potential. In the presence of an inwa rdly directed negative diffusion potential created by an inwardly dire cted thiocyanate (SCN-) gradient, the influx of lysine through this ro ute is accelerated; with an inwardly directed positive potassium diffu sion potential, lysine influx through this route is reduced. The influ x of lysine through system y(+)L is much less sensitive to such altera tions of potential. 4. Analysis of the kinetic constants characterizin g system y(+) shows that with a change of potential front zero to nega tive (approximately -60 mV) the maximum velocity (V-max) is roughly do ubled and the half-saturation constant (K-m) halved leading to a 4-fol d increase in permeability. For system y(+)L smaller changes are seen and K-m does not change; the resulting increase in y+L permeability is 1.5-fold. 5. These findings are discussed with respect both to the me chanism of membrane transport and placental epithelial function.