ALAD can catalyse the second step of heme synthesis. Accumulation of aminolevulinic acid results when ALAD is strongly inhibited by lead, which produces neurotoxic and genotoxic effects. Early studies used the phenotyping technique developed by Battistuzzi et al.
] to classify individuals as having ALAD 1-1, 1-2, or 2-2. In this procedure, whole blood samples are taken and the red blood cells are isolated and lysed. Isolation and electrophoresis of the ALAD protein permits distinction between phenotypes because of the charge differences among the isozymes. In 1991, Wetmur et al.
] developed the genotyping technique based on the polymerase chain used by most investigators. A 916-base-pair sequence containing the ALAD-1/2 polymorphic site is amplified and then cleaved with Msp 1. The cleavage products are then analysed by agarose gel electrophoresis. Since there was 100% genotype-phenotype correspondence, we used this technique during our investigation. Our studies also included positive and negative controls to avoid bias.
The prevalence of the ALAD-2 allele ranges from 0 to 20% depending on the population. Generally, Caucasians have the highest frequency of the ALAD-2 allele, with approximately 18% of the Caucasian population being ALAD 1-2 heterozygotes and 1% being 2-2 homozygotes [11
]. In comparison, African and Asian populations have low frequencies of the ALAD-2 allele, with few or no ALAD-2 homozygotes being found in such populations [12
Many of the studies presented that documented genotype or phenotype frequencies gave little detail about the study population (e.g., age or source of donors), making it hard to rule out any potential biases due to subject selection. Meanwhile, there is little evidence showing interethnic differences in the distribution of the ALAD gene polymorphism that result from extensive interethnic crosses between peoples from different regions, especially in heterogeneous populations such as the Chinese population.
The sole population-based study in Chinese was conducted by Hsieh et al.
] in a Taiwanese population (n = 660). They measured blood and found the ALAD 1-1 genotype in 95.5%, the ALAD 1-2 genotype in 4.4%, and the ALAD 2-2 genotype was detected in 0.2%. ALAD1 and ALAD2 allele frequency were 0.976 and 0.024, respectively. In our study, we ensured that all participants were Han subjects, and the distribution of ALAD variants were consistent with Hsieh’s reported from Taiwan.
Early studies conducted on the ALAD polymorphism and lead poisoning focused on differences in blood lead levels by genotype in populations with different occupations. Ziemsen et al.
] were the first to describe differences in blood lead levels by genotype. They found that lead-exposed workers with the ALAD 1-2 genotype had higher blood lead levels than ALAD 1-1 homozygotes and that ALAD 2-2 homozygotes had highest blood lead levels. Wetmur et al.
] found similar results.
Schwartz et al.
] found that the ALAD-2 allele was not clearly associated with higher blood levels. Smith et al.
] found no association between the ALAD genotype and blood lead levels, which implied that the ALAD genotype may be a modifier of blood lead level only at high blood lead concentrations. However, Shaik et al.
] reported blood lead levels did not differ significantly among ALAD 1-1, 1-2, and 2-2 genotypes.
In our investigation, results showed that workers’ with the ALAD 1-1 genotype had blood lead level that was higher than those with the ALAD 1-2 genotype. The findings should be discussed with caution due to the very low number of ALAD 1-2 genotypes present in this population. And most ALAD 1-2 individuals (nine out of ten) are females. Thus the lower blood lead levels of ALAD 1-2 carriers might be related to gender-specific exposure and further study was needed.
The reason that elevated blood and urine lead were more common in battery storage workers was attributed to the more intensive work environment and much poorer protection against lead exposure. These issues were quite common among battery storage factories in China, especially in the southwestern part.
Lead inhibits ALAD stoichiometrically, and ALAD inhibition results in the buildup of aminolevulinic acid. Aminolevulinic acid resembles γ-aminobutyric acid and can stimulate γ-amino-butyric acid receptors in the nervous system; this is thought to be one of the primary mechanisms of lead-induced neurotoxicity [18
]. Other studies revealed that lead could interfere with catecholaminergic and particularly dopaminergic neurotransmission, inducing antioxidant defences and oxidative damage in brain [19
]. Further research showed that lead may also decrease beta-adrenergic receptor density and adenylate cyclase activity directly [20
There have limited studies mentioning about the relationship between self-awareness and ALAD genotype [22
]. Self-reported symptoms may serve as effect markers of lead toxicity. Our study of the self-conscious symptom survey showed that the self-conscious symptom constituted the same sequence between two ALAD genotype, but the incidences of dreaminess, insomnia, dizziness and the abdominal pain were much higher in those with the ALAD 1-2 genotype than those with the ALAD 1-1 genotype. Self-conscious symptoms were also analysed by ALAD genotype stratification. The result showed that the incidences of debilitation, amnesia and dreaminess were much higher in the ALAD 1-1 genotype subgroup especially with tenure or lead exposure greater than 5 years. Schwartz et al.
] reported similar findings. We proposed that the ALAD-2 subunit of the protein could maintain lead in a nonbioavailable form, such that these individuals were protected from lead’s effects and could tolerate longer exposures to lead than those with ALAD-1 subunit.
Interactions between calcium and lead were previously documented [25
]. Competition for Ca-binding proteins may underlie a mechanism for lead absorption. Pires et al.
] reported that calcium supplementation during lactation appeared to blunt the lactation-induced increase in maternal blood lead. Ettinger et al
] reported similar findings. Varnai et al.
] also concluded that higher calcium intake might be a way of efficient reduction of lead absorption during the suckling period. However, Markowitz et al.
] found that calcium supplementation had no significant effect on the change in blood lead levels.
There is evidence that iron could be a neuroprotective factor in lead-induced attention deficit disorder with hyperactivity [31
]. Muwakkit et al.
] found that iron deficiency was associated with elevated blood lead levels. Wang et al.
] reported that iron supplement among lead-exposed rats maintained the normal ultra-structure of the blood-brain barrier and restored the expression of occluding to normal levels. Kim et al.
] confirmed the effectiveness of dietary iron intake as a secondary preventive intervention against lead toxicity. However, Kordas et al.
] found that iron and zinc supplementation did not improve behaviour in children exposed to lead. Serwint et al.
] reported there was no difference in iron status between children with low or moderate lead exposure.
Zinc is an important element that can prevent lead poisoning [37
]. The protective effect may be due to competition between lead and zinc or displacement of lead by zinc. Batra et al.
] found that zinc supplementation ameliorated lead-induced testicular damage both at the cellular and subcellular level. However, Rico et al.
] revealed that daily supplementation with iron and/or zinc might be less effective in improving cognition in school children than in younger children.
We evaluated the interaction of serum calcium, iron, and zinc levels with blood lead levels in our study. In the ALAD genotype stratification, correlation analysis indicated that the lead-exposed workers’ blood lead levels negatively correlated with serum calcium, iron and zinc levels, and was consistent with earlier studies [40
]. Furthermore, the interactions between zinc and lead would be significant when blood lead levels are relatively high.