Journal of Lipid Research, Vol 33, 1327-1334, Copyright © 1992 by Lipid Research, Inc.
Water-phase palmitate concentrations in equilibrium with albumin-bound palmitate in a biological system
IN Bojesen and E Bojesen
Department of Biochemistry B, University of Copenhagen, Panum Institute, Denmark.
The palmitate (PA) binding and transport capacity of human and bovine red
cell membranes enables us to establish, in a biological system, the
existence of a well-defined monomer concentration in equilibrium with PA
bound to bovine serum albumin (BSA, 30 microM) inside the resealed red cell
ghosts. Supernatants of suspensions of the [3H]PA-labeled ghosts contain a
tiny quantity of dissolved binding capacities besides the monomer PA. This
is demonstrated by linear regression of supernatant tracer concentrations
versus ghost concentrations in a dilution series. The extrapolated value,
corresponding to zero ghost concentration, is the monomer PA concentration
in equilibrium with PA bound to BSA within the ghosts in molar ratio (nu).
Measurements have been carried out for nu between 0.1 and 1.5 and at 0
degrees C, 10 degrees C, 23 degrees C and 38 degrees C. The important
nu-dependent binding of PA to the ghost membrane itself enables us to use
preparations of BSA-free ghosts in the same way, whereas this is impossible
in the case of arachidonic acid. Within the physiological range of nu the
PA monomer concentrations are accounted for by an apparent dissociation
equilibrium constant (Kd) 3.4 10(-8) M at 38 degrees C calculated on basis
of three equivalent binding sites per mol BSA. Kd depends on temperature
with a well-defined enthalpy of 38.4 kJ/mol.