Biomedical Engineering Reference
In-Depth Information
Tabl e 4. 2
Stability constants
logK
stab
of EDTA and
Cysteine for a selection of
metal ions
Cysteine
a
Ion
Sulfide
EDTA
pK
sol
b
logK
stab
b
logK
stab
Fe
2C
17
14:3
Fe
3C
25:1
Co
2C
21
16:3
9
Ni
2C
22
18:6
10
Cd
2C
26
16:5
10
Ag
C
50
7:3
15
Hg
2C
53
21:8
14
a
Values taken from [127
]
b
pK's and logK's are rounded to the nearest integer
accumulation of metal ions in tissues. It is a tool for sequestration, making metal
ions unavailable to exert toxic effects and contributes to metal homeostasis of the
body.
Studies on MTs remain an active field till today (to name a few papers from
Prof. De Ley's laboratory:[
128
-
130
]). MTs play an important role in the mainte-
nance of body's immune system. Although the research emphasis is on the role of
zinc and copper, other trace elements entering the body are, as already stated, com-
peting partners for the reaction with thionein (homeostasis!). A recent review of
metalloproteins has been published in
Nature
(
460
, 813-862, 2009).
4.2.3
Multiple Interactions
Electrochemical potential.
The high stability of some complexes encompasses a
few consequences. The redox potential of
nC
/
is depressed to more negative
values and are more eagerly oxidized. The more metal oxidized, the more metal
ions dissolved in the body fluids. Part of these ions are evacuated through urine and
feces, part travels around, either as ion or as complex, occasionally accumulating in
target tissues for some ions. The sensitivity of stainless steel to corrosion by sulfide
as discussed in Chap.
2
originates in the affinity of nickel to sulfide.
Rate of excretion and accumulation.
The decrease of body concentration
M
M
c
as
function of time
can be estimated from pharmacokinetic models but it is not
a straightforward business because of the complex interplay between absorption,
distribution, biotransformation and excretion. The overall excretion process can be
expressed by a general differential equation:
t
d
D bc;
(4.6)
t
where
b
is the elimination constant. Rearranging and integrating between zero and
t
and
c
0
and
c
t
gives:
