Brain tissue accumulates 67copper by two ligand-dependent saturable processes. A high affinity, low capacity and a low affinity, high capacity process.
We characterized the mechanism of copper accumulation by the brain, using rat hypothalamic tissue slices incubated with 67Cu as a model system. Two ligand-dependent saturable processes were discerned: a high affinity, low capacity process and a low affinity, high capacity process. Vo versus [S] for the high affinity process was a hyperbolic function having an apparent Km and Vmax of 6 microM copper and 23 pmol/min/mg protein, respectively. Vo versus [S] for the low affinity process was a sigmoidal function having an "apparent Km" (So5) and maximal velocity at saturating [S] of 40 microM copper and 425 pmol/min/mg protein, respectively. The two processes were similar in that each exhibited: (a) a requirement for complexing of copper for optimal 67Cu accumulation; (b) a broad ligand specificity with respect to amino acids (histidine, cysteine, threonine, glycine) and peptides (Gly-His-Lys, glutathione) and ineffectiveness of albumin in serving as a facilitatory ligand; (c) a requirement for thermic but not metabolic energy. In spite of these similarities, a 50- or 1000-fold molar excess of ligand (histidine) inhibited 67Cu accumulation by the low affinity process by 60 and 85%, respectively, whereas excess histidine facilitated 67Cu accumulation by the high affinity process by 1.6-4-fold. These results are consistent with 1) a carrier-mediated facilitated diffusion, analogous to that of neutral amino acids, as a means of transporting complexed copper into brain tissue, and 2) the existence of two distinct carrier sites interacting in a positive cooperative manner: a high and a low affinity site.
Hartter
D E
DE
Department of Obstetrics and Gynecology, University of Texas Health Science Center, Dallas 75235.
Barnea
A
A
D000818:Animals; D001921:Brain / Q000378:metabolism*; D003300:Copper / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D006639:Histidine / Q000494:pharmacology; D007031:Hypothalamus / Q000378:metabolism; D007700:Kinetics; D008297:Male; D051381:Rats; D013816:Thermodynamics
D006639:Histidine; D003300:Copper