Interactions in Metal Toxicology - Toxicology of Metals - page 122

interactions for ALAD genotype, lead metabolism, and
toxicity (reviewed in Kelada et al. , 2001). ALAD is the
major binding site for lead in red blood cells (Bergdahl
et al. , 1997; 1998). This protein is polymorphic, and the
main variant analyzed is the Lys/Asn substitution at
amino acid position 59. The two alleles are tradition-
ally labeled ALAD -1 for the Lys variant and ALAD -2
for the Asn variant, and the resulting genotypes are
named ALAD 1-1 for Lys/Lys carriers, ALAD 1-2 for
Lys/Asn, and ALAD 2-2 for Asn/Asn. The frequency
of the ALAD-2 allele varies considerably worldwide:
the highest frequency is found in Northern Europe (up
to 20%), whereas this allele is rarer among Asian pop-
ulations and almost absent among Africans (Packer
et al. , 2006). The Lys59Asn amino acid position is not
positioned near the Zn-binding sites, where lead prob-
ably binds (Jaffe et al. , 2000; 2001). However, because
asparagine is a neutral amino acid whereas lysine is
positively charged, this amino acid substitution results
in a more electronegative enzyme. This fact has gen-
erated the hypothesis that the ALAD-2 protein binds
positively charged lead more tightly than the ALAD-1
protein and that a protective effect of ALAD-2 exists
because of this tight binding, maintaining lead in the
intravascular space in a less bioavailable form (Wet-
mur, 1994; Wetmur et al. , 1991). It is important to stress
that, so far, limited proof exists of molecular differ-
ences between the two isozymes. Jaffe et al. have con-
structed recombinant ALAD variants in E. coli . They
found no functional dissimilarities, apart from a small
difference in the half-time for recovery from lead inhi-
bition between ALAD-1 and ALAD-2 in vitro (Jaffe
et al. , 2000; 2001).
In early studies of effect modifi cation of ALAD on
lead, the individuals analyzed were from populations
with relatively high levels of exposure from occupa-
tional or home environment. These studies displayed
association of the ALAD -2 genotype with higher blood
lead levels. Wetmur and colleagues demonstrated in a
study based on more than 1000 children from New York
City that ALAD -2 individuals had higher lead levels in
blood (Wetmur 1994; Wetmur et al. , 1991). However,
conclusions of this large study are hampered by the
selection of study subjects: they all had high protopor-
phyrin levels in an initial screening. This means that the
conclusion may either be that ALAD -2 carriers demon-
strate higher lead levels, which in turn result in higher
protoporphyrin levels, or alternately, that ALAD -2
carries more lead relative to ALAD -1 but is protected
from the negative effects of lead. Several other studies
have shown an effect of ALAD genotype on blood lead
levels among individuals with a high lead exposure
(Alexander et al. , 1998; Fleming et al. , 1998), whereas
others have not (Bergdahl et al. , 1997; Sakai et al. , 2000;

Schwartz et al. , 1995; 1997; Sithisarankul et al. , 1997;
Süzen et al. , 2003). Epidemiological studies of blood
lead levels at background exposure levels show no
clear association of effect of ALAD genotype (Bergdahl
et al. , 1997; Hu et al. , 2001; Smith et al. , 1995), although
Hsieh et al. found nonsignifi cant higher levels among
ALAD -2 individuals in a Taiwanese population (Hsieh
et al. , 2000).
Schwartz and colleagues found that the 1-2 geno-
type (Lys/Asn carriers) was associated with occupa-
tional exposures of more than 6 years (Schwartz et al. ,
1995). This genotype distribution could be the result
of genotype selection, and the authors suggested that
ALAD -2 subjects were protected from effects of lead
and could tolerate longer exposures to lead than ALAD
1-1. This is in line with the alternative conclusion of the
New York study mentioned previously.
Kim et al. analyzed whether Korean lead work-
ers with the ALAD 1-1 genotype were more suscep-
tible to the hematological effects of lead exposure
(Kim et al. , 2004). They found that ALAD 1-2/2-2 was
associated with lower log zinc protoporphyrin (ZPP)
and higher hemoglobin levels. Moreover, among
individuals with normal iron status, those with the
ALAD 1-1 genotype were more likely to be anemic.
Alexander et al. (1998) found signifi cantly lower lev-
els of ZPP at high blood lead levels among ALAD -2
genotypes as well, and lower but nonsignifi cant ZPP
levels among carriers of the ALAD -2 allele have been
reported elsewhere (Sakai et al. , 2000; Schwartz et al. ,
1997; Sithisarankul et al. , 1997). Reduced levels of
plasma aminolevulinic acid among ALAD -2 carriers
have also been seen in the studies on the Korean bat-
tery workers (Sakai et al. , 2000; Schwartz et al. , 1997;
Sithisarankul et al. , 1997), as well as among Japanese
workers (Sakai et al. , 2000). Accumulation of plasma
aminolevulinic acid has been suggested to confer
a greater risk for neurotoxic effects of lead. Bioavail-
able lead has also been analyzed by measuring the
amount of dimercaptosuccinic acid (DMSA)-chelata-
ble lead in urine. There is some evidence that ALAD -2
individuals display lower levels of DMSA-chelatable
lead levels (Schwartz et al. , 1997; 2000).
ALAD genotype has been analyzed in relation to
kinetics of bone lead as well. However, the results are
diffi cult to interpret; some studies suggest an impact of
ALAD on bone lead (Bellinger et al. , 1994; Hu et al. , 2001;
Kamel et al. , 2003), whereas others do not fi nd any effect
modifi cation of the polymorphism (Bergdahl et al. , 1997;
Fleming et al. , 1998; Schwartz et al. , 2000). Moreover, a
few studies have analyzed effect modifi cation of ALAD
on lead's effects on kidney parameters (Bergdahl et al. ,
1997; Smith et al. , 1995; Weaver et al. , 2003). Also the
results of these studies are contradictive.