LEAD TOXICITY AND THE ALA-DEHYDRATASE POLYMORPHISM

δ -氨基乙酰丙酸脱水酶（ALAD, EC 4.2.1.4）是一种锌

Delta-aminolevulinic acid dehydratase (ALAD, EC 4.2.1.4) is a zinc metalloenzyme whose inhibition by lead is the first and most sensitive indicator of lead exposure and whose decreased activity has been clearly implicated in the pathogenesis of lead poisoning. The enzyme is encoded by two alleles, ALAD1 (p = 0.9) and ALAD2 (q = 0.1). A molecular test for ALAD genotyping has been developed based on the DNA sequences of the ALAD1 and ALAD 2 alleles. Evidence from our laboratory indicates that lead- exposed individuals heterozygous or homozygous for ALAD 2 have mean blood lead levels 9 micrograms/dl greater than similarly exposed individuals homozygous for ALAD1. These results suggest that there are genetically susceptible individuals who would be at an increased health risk if exposed to lead in the workplace or environment. Moreover, since lead is a known teratogen, the fetuses of women with such a genetic susceptibility may be at higher risk for lead-induced congenital abnormalities. In this application we propose three studies: First, investigation of the relationship of the ALAD alleles to lead poisoning will be extended to include integrated measures of lead exposure. Blood lead is an acute measure of lead exposure and does not measure the amount of lead ultimately retained. ALAD genotypes in lead-exposed populations will also be related to bone or deciduous teeth dentine lead levels, both integrated measures of lead exposure. Second, mouse models will be investigated to relate (a) mouse ALAD activity and/or concentration to lead retention and distribution in body tissues or (b) human ALAD activity and/or concentration to lead retention and distribution in tissues. These models will involve lead exposure of (a) existing mouse strains with differing ALAD gene doses as well as (b) transgenic mice expressing the human ALAD1 or ALAD2 allele. Lead uptake, the tissue-specific distribution of lead and the inhibition of tissue ALAD by lead will be determined in order to clarify further the role of ALAD in the pathophysiology of lead poisoning. Finally, purified recombinant human ALAD isozymes will be used to determine their differential affinity for lead binding, inhibition by lead, and protease inhibition. The recombinant isozymes will also be made available for X-ray determination of their crystal structures. These studies are aimed at determining the biochemical basis for the differential genetic susceptibility of individuals to lead exposure.