Heterogeneity of histidine transport in the Ehrlich cell

We have reexamined the heterogeneity shown by histidine in its uptake by the Ehrlich ascites tumor cell, in the face of a contradiction of our earlier interpretation. We again find the fraction of histidine uptake at neutral pH inhibitable by the model substrate for System A, 2-(methylamino)-isobutyric acid, to be fully dependent on the presence of Na+ or Li+. The small Na+-independent component not attributable to System L can be identified with System Ly+ through its inhibitability by homo-arginine. This component increases as the pH is lowered with an apparent pK'a of 6.1. The simultaneous decrease in the uptake by the neutral systems could be identified, for System L, with the same titration of histidine to its cationic form, but for System A the sharp decrease is identified with the protonation of a structure on the membrane rather than one on the substrate. The action of H+ in the latter case proved approximately non-competitive with Na+ when tested with ordinary substrates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21638/1/0000019.pd

Amino acid stimulation of ATP cleavage by two Ehrlich cell membrane preparations in the presence of ouabain

Two membrane fractions prepared from the Ehrlich ascites-tumor cell show non-identical stimulatory responses to certain amino acids in their Mg2+-dependent activity to cleave ATP, despite the presence of ouabain and the absence of Na+ or K+. The first of these, previously described, shows little (Na+ + K+)-ATPase activity, and is characteristically stimulated by the presence of certain diamino acids with low pK2, and at pH values suggesting that the cationic forms of these amino acids are effective. The evidence indicates that these effects are not obtained through occupation of the kinetically discernible receptor site serving characteristically for the uphill transport of these amino acids into the Ehrlich cell. The second membrane preparation was purified with the goal of concentrating the (Na+ + K+)-ATPase activity. It also is stimulated by the model diamino acid, 4-amino-1-methylpiperidine-4-carboxylic acid, and several ordinary amino acids. The diamino acids were most effective at pH values where the neutral zwitterionic forms might be responsible. Among the optically active amino acids tested, the effects of ornithine and leucine were substantially stronger for the than for isomers. The list of stimulatory amino acids again corresponds poorly to any single transport system, although the possibility was not excluded that stimulation might occur for both preparations by occupation of a membrane site which ordinarily is kinetically silent in the transport sequence. The high sensitivity to deoxycholate and to dicyclohexylcarbodiimide of the hydrolytic activity produced by the presence of -ornithine and 4-amino-1-methyl-piperidine-4-carboxylic acid suggests that the stimulatory effect is not merely a general intensification of the background Mg+-dependent hydrolytic activity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/21748/1/0000142.pd

ABBREVIATIONS: GABA, -y-aminobutyric acid; HEPES, 4-(2-hydroxyethyl)-1-piperazineethane sulfonic acid; TBPS, t-butylbicyclophosphorothionate; PFU, plaque-forming units. 801 Comparison of Interactions of [3H]Muscimol, t- Butylbicyclophosphoro[35S]thionate

SUMMARY The al f32 and al f32y2 subtypes, common subtypes of -y-aminobutync acid (GABA)A receptors in the brain, are known to share many ligands, but only the latter interacts with benzodi