Toxicokinetics

Absorption of lead from the gastrointestinal tract depends on host characteristics and on the physicochemical properties of the ingested material. Lead containing metallo-proteins and peptides are then transferred to soft tissues and bones, where lead accumulates with age. Lead is excreted primarily in urine and faeces, half-lives for lead in blood and bone are approximately 30 days and 10 to 30 years, respectively (Rabinowitz, 1991).

8.1.1. Absorption

Oral Exposure

Absorption of ingested soluble lead compounds appears to be higher in children than in adults (Alexander et al., 1974; Ziegler et al., 1978; Heard and Chamberlain, 1982; James et al., 1985; Rabinowitz et al., 1980), although kinetics of changes in stable isotope signatures of B-Pb in mothers and their children suggest that children aged 6 to 11 years and their mothers absorb a similar percentage of ingested lead (Gulson et al., 1997).

Studies in experimental animals provide additional evidence for an age-dependency of gastrointestinal absorption of lead. Absorption of lead, administered by oral gavage as lead acetate (6.37 mg lead/kg), was 38 % in juvenile Rhesus monkeys compared to 26 % in adult female monkeys (Pounds et al., 1978). Rat pups absorb approximately 40 to 50 times more lead via the diet than adult rats do (Aungst et al., 1981; Forbes and Reina, 1972; Kostial et al., 1978).

The presence of food decreases the absorption of water-soluble lead compounds (Blake and Mann, 1983; Blake et al., 1983; Heard and Chamberlain, 1982; James et al., 1985; Maddaloni et al., 1998; Rabinowitz et al., 1980), the reported absorption when taken with a meal varying from 3 to 21 % (average approximately 8 %). In adults, absorption of a tracer dose of lead acetate in water was approximately 63 % when ingested by fasting subjects, whereas it was only 3 % when ingested with a meal (James et al., 1985; Heard and Chamberlain, 1982). The arithmetic mean of reported estimates of absorption in fasting adults was 57 %, with reported fed/fasted ratios ranging from 0.04 to 0.2 (U.S. ATSDR, 2007, based on Blake et al., 1983; Heard and Chamberlain,1982; James et al., 1985; Rabinowitz et al., 1980).

Lead absorption in children is affected by nutritional iron status (Watson et al., 1986). A low iron intake (Cheng et al., 1998) and deficient iron status (Bárány et al., 2005) was associated with increased B-Pb. Evidence for the impact of iron deficiency on lead absorption has been provided by studies in rats, showing that iron deficiency increases lead absorption, possibly by enhancing its binding to iron binding carriers (Bannon et al., 2003; Barton et al., 1978; Morrison and Quaterman, 1987).

Dietary calcium intake appears to affect lead absorption. An inverse relationship has been observed between dietary calcium intake and B-Pb concentration in children, suggesting that children who are calcium-deficient may absorb more lead than calcium-replete children (Mahaffey et al., 1986; Ziegler et al., 1978). An effect of calcium on lead absorption is also evident in adults. In experimental studies e lead was ingested together with calcium carbonate (0.2 to 1 g calcium carbonate) than when the lead was ingested without additional calcium (Blake and Mann, 1983; Heard and Chamberlain, 1982). In experimental animal models, lead absorption was enhanced by dietary calcium depletion or administration of vitamin D (Mykkänen and Wasserman 1981, 1982). Although milk is a major source of calcium - and for more than a century milk has been recommended as a prophylactic for lead toxicity - it increases the uptake of lead (James et al., 1985). Lead salts and lead in milk appear to be absorbed by different mechanisms (Henning and Cooper, 1988). Lactose has a limited effect, whereas lactoferrin may cause an increase in absorption. Similar mechanisms may contribute to lead-iron and lead-calcium absorption interactions and to interactions between lead and other divalent cations such as cadmium, copper, magnesium and zinc.

Inhalation Exposure

Deposition and absorption of inhaled lead containing particles are influenced by their size and solubility. As compared to particles with lower density, lead fumes and lead-containing dusts tend to have a slightly different deposition, with a respirable fraction characterised by lower aerodynamic

diameter. Particles larger than five micron are deposited on the lining fluid of trachea and bronchi, and from there they are transferred by the mucociliary transport into the pharynx and then swallowed, with possible absorption of lead from the gastrointestinal tract. Smaller particles can be deposited in the distal parts of the respiratory tract, from where they can be absorbed after extracellular dissolution or ingestion by alveolar macrophages.

The deposition and clearance from the respiratory tract have been measured in adult humans exposed to lead-bearing particles with aerodynamic diameter below 1 m (Hursh and Mercer, 1970; Hursh et al., 1969; Morrow et al., 1980). Up to 95 % of deposited lead that is inhaled as submicron particles is absorbed (Hursh et al., 1969).

Dermal Exposure

Dermal absorption of lead compounds is generally considered to be much less than absorption by inhalation or oral routes of exposure. Dermal absorption has been estimated to be 0.06 % during normal use of lead-containing preparations (Moore et al., 1980), although few studies have provided quantitative estimates of dermal absorption of lead in humans.

8.1.2. Distribution

Several models have been proposed to characterise inter-compartmental lead exchange rates, retention of lead in various tissues, and relative rates of distribution amongst tissue groups, schematically presented in Figure 15.

Under steady-state conditions, lead in blood is found primarily in the red blood cells (96 to 99 %) (Bergdahl et al., 1997a, 1998, 1999; Hernandez-Avila et al., 1998; Manton et al., 2001; Schutz et al., 1996; Smith et al., 2002). At B-Pb concentrations <1.92 μM (400 μg/L), whole B-Pb levels increase linearly with serum levels. At higher B-Pb concentrations a non-linear relationship is apparent, and the serum to blood ratio increases dramatically as levels increase, due to saturation of binding in erythrocytes (WHO/IPCS, 1995). This kinetic relationship may be altered during pregnancy. From in

vitro data (Ong and Lee, 1980), fetal haemoglobin appears to have a greater affinity for lead than

adult haemoglobin.

Although the mechanisms by which lead crosses cell membranes have not been fully elucidated, results of studies in intact red blood cells and red blood cell ghosts indicate that the major pathway is likely to be an anion exchanger dependent upon HCO3

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(Simons, 1985, 1986a, 1986b, 1993). Lead and calcium may also share a permeability pathway, which might be a Ca2+-channel (Calderon-Salinas et al., 1999). Lead is extruded from the erythrocyte by an active transport pathway, most likely a (Ca2+, Mg2+)-ATPase (Simons, 1988).

Figure 14: Schematic presentation of lead distribution in humans

Most of the lead found in erythrocytes is bound to proteins, the primary binding ligand being delta- aminolevulinic acid dehydratase (ALAD) (Bergdahl et al., 1997a, 1998; Sakai et al., 1982; Xie et al., 1998). Lead binding capacity of ALAD is approximately 8500 µg/L red blood cells (or approximately 400 µg/L whole blood) and the apparent dissociation constant is approximately 1.5 µg/L (Bergdahl et al., 1998). Two other lead-binding proteins have been identified in the red cell, a 45 kDa protein (Kd 5.5 µg/L) and a smaller protein(s) having a molecular weight <10 kDa (Bergdahl et al., 1996, 1997a, 1998). Of the three principal lead-binding proteins identified in red blood cells, ALAD has the strongest affinity for lead (Bergdahl et al., 1998) and appears to dominate the ligand distribution of lead (35 to 84 % of total erythrocyte lead) at B-Pb concentrations below 400 µg/L (Bergdahl et al., 1996, 1998; Sakai et al., 1982).

Lead binding inhibits the activity of ALAD (Gercken and Barnes, 1991; Sakai et al., 1982, 1983), thereby inducing its synthesis, which however might also be due to secondary accumulation of both ALA and protoporphyrin, the latter as a consequence of lead-induced inhibition of ferrochelatase (Fujita et al., 1982).

ALAD is a polymorphic enzyme with two alleles (ALAD 1 and ALAD 2) and three genotypes: ALAD 1,1, ALAD 1,2 and ALAD 2,2 (Battistuzzi et al., 1981). Higher B-Pb levels have been

reported in individuals with the ALAD 1,2 and ALAD 2,2 genotypes compared to similarly exposed individuals with the ALAD 1,1 genotype (Astrin et al., 1987; Hsieh et al., 2000, Schwartz et al., 2000; Wetmur et al., 1991). This observation has prompted the suggestion that the ALAD-2 allele may have a higher binding affinity for lead than the ALAD 1 allele (Bergdahl et al., 1997b), a difference that might alter dose-response relationships between B-Pb and lead-mediated outcomes. However, the overall impact of this polymorphism on the pharmacokinetics of lead is presently unclear.

Approximately 40 to 75 % of lead in the plasma is bound to proteins, mainly albumin, though lead also complexes to sulphydryl groups of cysteine, and other ligands, in other proteins (Al-Modhefer et al., 1991).

In human adults, approximately 90 % of the total body burden of lead is found in the bones. In contrast, bone lead accounts for only 70 % of the body burden in children but its concentration increases with age. The large pool of lead in adult bone maintains elevated B-Pb concentrations long after exogenous exposure has ended (Fleming et al., 1997; Inskip et al., 1996; Kehoe, 1987; O'Flaherty et al., 1982; Smith et al., 1996).

Lead accumulation occurs predominantly in trabecular bone during childhood, and in both cortical and trabecular bone in adulthood (Aufderheide and Wittmers, 1992). In the former, bone lead is essentially inert, having a half-life of several decades. Although a high bone formation rate in early childhood results in the rapid uptake of circulating lead into mineralizing bone, bone lead is also recycled to other tissue compartments or excreted in association with a high bone resorption rate (O'Flaherty, 1995).

In some bones (e.g. mid femur and pelvic bone), lead content decreases with aging (Drasch et al., 1987). This decrease is most pronounced in females and may be due to osteoporosis and release of lead from bone to blood (Gulson et al., 2002). During pregnancy, the mobilisation of bone lead also increases, apparently as the bone is catabolised to produce the fetal skeleton. This mobilisation of bone lead may contribute to the increase in lead concentration that has been observed during the later stages of pregnancy (Gulson et al., 1997; Lagerkvist et al., 1996; Schuhmacher et al., 1996).

Bone resorption during pregnancy can be reduced by ingestion of calcium supplements (Janakiraman et al., 2003). Additional evidence for increased mobilisation of bone lead into blood during pregnancy comes from studies in nonhuman primates and rats (Franklin et al., 1997; Maldonado-Vega et al., 1996). Kinetic changes in the stable isotope signatures of B-Pb in postpartum women indicated that the release of maternal bone lead to blood appears to accelerate during lactation (Gulson et al., 2003, 2004). Using a similar approach, increased release of bone lead to blood in women, in association with menopause has been shown (Gulson et al., 2002). These observations are consistent with epidemiological studies that have shown increases in B-Pb after menopause and in association with decreasing bone density in postmenopausal women (Berkowitz et al., 2004; Hernandez-Avila et al., 2000; Nash et al., 2004; Popovic et al., 2005).

The relative distribution of lead in soft tissues, in both males and females, expressed in terms of liver tissue concentration ratios, was: liver, 1.0 (approximately 1 µg/g wet weight); kidney cortex, 0.8; kidney medulla, 0.5; pancreas, 0.4; ovary, 0.4; spleen, 0.3; prostate, 0.2; adrenal gland, 0.2; brain, 0.1; fat, 0.1; testis, 0.08; heart, 0.07; and skeletal muscle, 0.05 (Barry, 1975; Gross et al., 1975). In

1998; Goyer, 1990; Graziano et al., 1990). In a study of 159 mother-infant pairs, higher blood pressure and alcohol consumption late in pregnancy were associated with more lead in cord blood relative to maternal B-Pb. Higher haemoglobin levels and sickle cell trait were associated with reduced cord B-Pb relative to maternal B-Pb (Harville et al., 2005).

Maternal lead can be transferred to infants during breastfeeding. Although the breast milk/maternal blood concentration ratio is usually <0.1, values of up to 0.9 have been reported (Ettinger et al., 2006; Gulson et al., 1998). Lead in colostrum has also been found to be lower by at least one order of magnitude than in blood. Ettinger et al. (2006) found that B-Pb (mean, 80 to 90 μg/L; range 20 to 300) was a good predictor of breast milk lead (mean, 9 to 14 μg/L; range 2 to 80 μg/L), although Almeida et al. (2008) have reported that neither colostrum nor milk concentrations seem to correlate with B-Pb levels. Stable lead isotope dilution measurements in infant-mother pairs, measured as they came into equilibrium with a novel environmental lead isotope signature, suggested that lead in breast milk can contribute substantially to lead in infant blood (approximately 40 to 80 %; Gulson et al., 1998).

8.1.3. Excretion

The half-life of lead in blood is approximately 30 days in adult male humans, but varies with level of exposure, sex and age (Gulson, 2008).

Lead is excreted primarily in urine, most likely by passive diffusion, and faeces whilst sweat, saliva, hair and nails and breast milk are minor routes of excretion (Hursh and Suomela, 1968; Hursh et al., 1969; Kehoe, 1987; Rabinowitz et al., 1976; Stauber et al., 1994). Faecal excretion accounts for approximately one-third of total excretion of absorbed lead, and also of lead from inhalation of submicron lead particles (Hursh et al., 1969). Faecal lead includes both the unabsorbed fraction of ingested lead and the fraction of biliary excretion escaping any entero-hepatic re-circulation. The mechanisms for faecal excretion of absorbed lead have not been elucidated. However, there is evidence for lead excretion through bile (Ishihara and Matsushiro, 1986) and pancreatic juice (Ishihara et al., 1987). Possibly, the excretion in bile is in the form of a lead–glutathione complex (Alexander et al., 1986).