Biomedical Engineering Reference
In-Depth Information
2.2.
Bilirubin Phase-Partition
Binding of bilirubin to albumin is a fundamental phenomenon to be considered in de-
scribing bilirubin phase partition equilibria. In this section, bilirubin partition between two
albumin-containing aqueous phases and bilirubin solubility in a polymer membrane will be
considered, accounting for the effect of bilirubin-albumin binding. Bilirubin adsorption on
solid phases is discussed in a following section. It is worth noting that the following analy-
sis is referred to bilirubin; nevertheless, the same conclusions can be drawn for any tightly
albumin-bound compound.
Firstly, let us consider two aqueous phases with different albumin concentrations.
Bilirubin transfer between the two solutions, indicated as
Α
and
Β
, may be viewed as an
exchange reaction
AB
Α
+
A
Β
⇋
AB
Β
+
A
Α
(9)
As a consequence, equilibrium conditions correspond to
Μ
AB
+ Μ
A
= Μ
AB
+ Μ
A
(10)
or, assuming ideal solutions
C
AB
C
AB
C
A
C
A
=
(11)
If conditions (7) and (8) hold, the equilibrium condition (11) may be also rewritten in terms
of total bilirubin and albumin concentrations as
C
BIL
C
BIL
C
ALB
C
ALB
=
(12)
or as
C
ALB
−C
BIL
C
ALB
C
ALB
−C
BIL
C
ALB
=
(13)
Condition (12) corresponds to a bilirubin partition coefficient equal to the ratio of albumin
concentrations in the two phases, whereas condition (13) shows that, at equilibrium, equal
fractions of available albumin for bilirubin binding are present in both phases.
These simple equilibrium conditions apply to a dialysis process on an albumin rejecting
membrane and provide a rationale for some empirical findings reported in the literature. For
instance, Steiner et al. (2004) carried out albumin dialysis of human plasma spiked with
bromosulphthalein (BSP, a strongly albumin-bound compound, with a binding constant as
high as 2
10
7
M
−1
) and observed that, at equilibrium, the concentration of BSP in each
liquid phase was proportional to albumin concentration (see Fig. 2). The same authors
extrapolated a similar result also for bilirubin, by the analysis of the time course of bilirubin
concentration in a non-equilibrium experiment.
The applicability of equilibrium condition (12) to albumin-dialysis of strongly albumin-
bound toxins is also confirmed by data obtained by the authors of this chapter and reported
in Fig. 2. These data were collected at the end of albumin dialysis experiments of bilirubin-
containing solutions in a two compartment closed loop system, and refer to equilibrium
conditions.
