PLUTONIUM
120
3. HEALTH EFFECTS
Although Pu(IV) forms complexes with
a variety of plasma proteins, including albumin, -globulins, and
low molecular weight
proteins, the dominant complex is with transferrin. The dissociation constant of
Pu(IV)-transferrin complex has not been measured; however, the complex appears to be less stable than
Fe(III)-transferrin complex (K
d
≈
10
-22
M) (Aisen and Listowsky 1980). As a result, binding of Fe(III) to
transferrin can influence the degree of binding of Pu(IV). Plutonium also forms complexes with
nonprotein ligands, polycarboxylates (e.g., citrate, lactate). The stability constants for the mono- and
di-citrate complexes are approximately 10
15
and 10
30
M, respectively (Taylor 1973).
Distribution within Soft Tissues.
Plutonium, because it is strongly bound
to proteins in blood, does not
escape easily from the vasculature. However, at sites within the body where blood sinusoids are present
(e.g., in the liver, red bone marrow), protein-plutonium complexes in plasma can leave the vasculature
and distribute to sites within tissues. Plutonium can exist within tissues as an ion bound to binding sites
on proteins, including those associated with iron metabolism (e.g., transferrin, haemosiderin, and ferritin)
or as insoluble particulates. Particulates may derive from inhalation of particles of plutonium (e.g., PuO
2
)
or may form by aggregation of polymeric hydrolysis products of more soluble Pu(IV) compounds (e.g.,
plutonium citrate and plutonium nitrate) (Taylor 1973). In the lung, plutonium accumulates within
alveolar macrophages and Type I alveolar epithelial cells (both of which phagocytize plutonium
particles), and in lung-associated lymph nodes (Bair et al. 1973). Aggregation
of macrophages can result
in localized regions of high activity that can become encapsulated in fibrotic material, inhibiting the
dissolution of the plutonium and the further migration of the macrophages from the lung. Plutonium is
also found associated with hemosiderin (in bone marrow macrophages) and with ferritin in liver and other
tissues (e.g., spleen, bone marrow) where ferritin is expressed (Gorden et al. 2003; Taylor 1973). The
sequestration of plutonium into ferritin may contribute to the relatively long retention time of plutonium
in liver. Following intravenous administration of
239
Pu-citrate or
239
Pu(NO
3
)
4
to rats, plutonium was
found in hepatocytes and sinusoidal cells. Distribution of plutonium within liver was relatively
homogeneous following injection of
239
Pu-citrate compared to a heterogeneous pattern of aggregation in
liver following injection of
239
Pu(NO
3
)
4
(Fouillit et al. 2004). A similar pattern has been observed in dogs
(Gearhart et al. 1980). These observations suggest distinct mechanisms of transfer of the two compounds
into and/or within liver. The temporal changes in the distribution pattern of plutonium in
liver and lung
also occur. In liver, this may derive from regeneration of injured tissue. In the lung, plutonium deposits
become focalized, over time, within macrophages. Within the lungs, after intakes of insoluble plutonium
particles the number of contaminated macrophages decreases and the remaining macrophages become
loaded with even larger amounts of plutonium. These often come together to
form local hotspots that
PLUTONIUM
121
3. HEALTH EFFECTS
sometimes form a fibroid capsule around them, which inhibits both the dissolution of the plutonium and
the further migration of the macrophages.
Distribution within Bone.
Plutonium in bone initially distributes to bone surfaces adjacent to blood
sinusoids in bone marrow and can subsequently be redistributed within bone volume during bone growth
and remodeling. Redistribution of plutonium from bone to bone marrow can also occur, at least in part,
from macrophage phagocytosis of plutonium released from bone during bone resorption. Various studies
of bone uptake of injected plutonium suggest the following general pattern of deposition of plutonium on
bone surfaces (DOE 1989; Leggett 1985; Priest 1990; Rosenthal et al. 1972a, 1972b; Vaughan et al.
1973): (1) monomeric complexes of plutonium (e.g., monomeric plutonium citrate) deposit preferentially
at bone surfaces, with relatively little initial distribution to marrow; whereas, highly polymeric plutonium
deposits preferentially in marrow; (2) initial deposition is greater on trabecular compared
to cortical bone
surfaces; (3) initial deposition occurs preferentially on endosteal surfaces compared to periosteal surfaces;
(4) deposition is greater on surfaces where active resorption is occurring compared to surfaces undergoing
mineralization; and (5) deposition is greater on surfaces of the axial skeleton (i.e., skull, hyoid bone,
sternum, ribs, and vertebrae) then on the appendicular skeleton (i.e., limbs). Mechanisms of plutonium
deposition at bone surfaces are not completely understood. Plutonium (IV) can form complexes with
bone glycoproteins, collagen, and bone mineral (Vaughan et al. 1973).
The deposition pattern in bone is age-dependent. A comparison of bone distribution
of plutonium in
juvenile (3 months) beagles, compared to young adult (17–20 months) and mature (60 months) beagles
that received a single injection of plutonium citrate showed the following patterns (Bruenger et al. 1991a):
(1) deposition (per cent of dose) was higher in juveniles; (2) a larger fraction of the skeletal deposition
occurred in limb bones of juveniles; and (3) plutonium in bone volume (as opposed to bone surface) was
more pronounced in juveniles. These observations are consistent with the concept that plutonium
preferentially deposits in regions adjacent to red marrow, which has a wider distribution in juveniles than
in adults, and is more prominent in trabecular bone than
in cortical bone, and in bones of the axial
skeleton. High bone turn-over in juveniles contributes to more rapid distribution of plutonium from bone
surface to bone volume as a result of burial of surface deposits, uncovering buried deposits, and recycling
of the plutonium between marrow, bone, and blood (Bruenger et al. 1991a; Leggett 1985; Priest 1990;
Vaughan et al. 1973).
Metabolism.
Plutonium metabolism in physiological systems consists, primarily, of hydrolytic
reactions and formation of complexes with protein and nonprotein ligands.