Heterogeneity of L-alanine transport systems in brush-border membrane vesicles from rat placenta during late gestation.
The placental uptake of L-alanine was studied by using purified brush-border membrane vesicles from rat trophoblasts. Saturation curves were carried out at 37 degrees C in buffers containing 100 mM (zero-trans)-NaSCN, -NaCl, -KSCN, -KCl, or -N-methyl-D-glucamine gluconate. The uncorrected uptake results were fitted by non-linear regression analysis to an equation involving one diffusional component either one or two saturable Michaelian transport terms. In the presence of NaCl, two distinct L-alanine transport systems were distinguished, named respectively System 1 (S-1; Vm1 about 760 pmol/s per mg of protein; KT1 = 0.5 mM) and System 2 (S-2; Vm2 about 1700 pmol/s per mg; KT2 = 9 mM). In contrast, in the presence of K+ (KCl = KSCN) or in the absence of any alkali-metal ions (N-methyl-D-glucamine gluconate), only one saturable system was apparent, which we identify as S-2. When Na+ is present, S-1, but not S-2, appears to be rheogenic, since its maximal transport capacity significantly increases in the presence of an inside-negative membrane potential, created either by replacing Cl- with the permeant anion thiocyanate (NaSCN > NaCl) or by applying an appropriate K+ gradient and valinomycin. alpha-(Methylamino)isobutyrate (methyl-AIB) appears to be a substrate of S-1, but not of S-2. For reasons that remain to be explained, however, methyl-AIB inhibits S-2. We conclude that S-1 represents a truly Na(+)-dependent mechanism, where Na+ behaves as an obligatory activator, whereas S-2 cannot discriminate between Na+ and K+, although its activity is higher in the presence of alkali-metal ions than in their absence (Na+ = K+ > N-methyl-D-glucammonium ion). S-2 appears to be fully developed 2 days before birth, whereas S-1 undergoes a capacity-type activation between days 19.5 and 21.5 of gestation, i.e. its apparent Vmax. nearly doubles, whereas its KT remains constant.
Alonso-Torre
S R
SR
Departamento de Bioquímica y Biología Molecular, Facultad de Farmacia, Universidad de Salamanca, Spain.
Serrano
M A
MA
Medina
J M
JM
Alvarado
F
F
D000409:Alanine / Q000378:metabolism*; D000621:Aminoisobutyric Acids / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D000838:Anions; D001692:Biological Transport / Q000187:drug effects; D002412:Cations; D005260:Female; D007700:Kinetics; D008564:Membrane Potentials; D008871:Microvilli / Q000378:metabolism*; D010920:Placenta / Q000378:metabolism* / Q000648:ultrastructure; D011189:Potassium Chloride / Q000494:pharmacology; D011190:Potassium Cyanide / Q000494:pharmacology; D011247:Pregnancy; D051381:Rats; D017208:Rats, Wistar; D012965:Sodium Chloride / Q000494:pharmacology; D012966:Sodium Cyanide / Q000494:pharmacology; D014327:Trophoblasts / Q000378:metabolism; D014634:Valinomycin / Q000494:pharmacology
D000621:Aminoisobutyric Acids; D000838:Anions; D002412:Cations; D014634:Valinomycin; C017911:2-(methylamino)isobutyric acid; D012965:Sodium Chloride; D011189:Potassium Chloride; D011190:Potassium Cyanide; D012966:Sodium Cyanide; D000409:Alanine