Essentiality Toxicity Carcinogenicity Lead(Pb) Exposure Toxicokinetics Toxicity Neurologic, Neurobehavioral, and Developmental Effects in Children Mechanisms of Effects on the Developing Nervous System Peripheral Neuropathy Hematologic Effects Renal Toxicity Lead and Gout Effects on Cardiovascular System Immunotoxicity Bone Effects Reproductive Effects Birth Outcomes Carcinogenicity Other Effects Dose Response Treatment Organic Lead Compounds Mercury (Hg) Exposure Disposition and Toxicokinetics Metabolic Transformation Cellular Metabolism Toxicology Biological Indicators Treatment Nickel (Ni) Exposure Toxicokinetics Essentiality Toxicity Nickel Carbonyl Poisoning Dermatitis Indicators of Nickel Toxicity

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Toxic effects of metals

Humans are exposed occupationally and environmentally to metal aerosols including lead (Pb2+) and cadmium (Cd2+). These toxicants accumulate in male reproductive organs. Epidemiological studies have been equivocal about effects of Pb2+ and Cd2+ on hormone concentrations, male fertility and sperm parameters. Comparison of Pb2+ and Cd2+ concentrations in fertile and infertile men are problematic. Problem areas include failure to control confounding variables, but genetic polymorphisms as in somatic diseases may modulate Pb2+ and Cd2+ damage. Multiple calcium (Ca2+) and potassium (K+) channel isoforms have been identified in human testes and spermatozoa. These Ca2+ and K+ channels are involved in early events of acrosome reactions. Ca2+ channel are susceptible to Cd2+ poisoning and K+ channels to Pb2+. These channels offer entry paths for metallic toxicants into mature spermatozoa. Ion channel polymorphisms may cause differential sensitivities to Cd2+ and Pb2+, explaining in part prospective blinded studies showing high Cd2+ in varicocele-related human infertility and high Pb2+ in unexplained infertility. In both forms of male infertility the ability to undergo an acrosome reaction decreases. Reverse transcriptase-polymerase chain reaction assays for Ca2+ and K+ channel isoforms may identify susceptibility subgroups with lower resistance to environmental exposures.

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Male infertility and environmental exposure to lead and cadmium

The second enzyme in the heme biosynthetic pathway, delta-aminolevulinate dehydratase (ALAD), is a homooctameric protein encoded by a gene localized to human chromosome 9q34. Expression of the two common alleles, ALAD1 (p = .9) and ALAD2 (q = .1), results in a polymorphic enzyme system with three distinct charge isozymes, designated 1-1, 1-2, and 2-2. Individuals heterozygous (2pq = .18) or homozygous (q2 = .01) for the ALAD2 allele have significantly higher blood lead levels than do ALAD1 homozygotes, when exposed to low or high levels of lead in the environment. To investigate the molecular nature of this common polymorphism, total RNA from an ALAD2 homozygote was oligo-dT primed and reverse transcribed, and then the ALAD2 cDNA was amplified, subcloned, and sequenced. Compared with the ALAD1 sequence, the only difference in the ALAD2 cDNA was a G-to-C transversion of nucleotide 177 in the coding region, which created an MspI restriction site. This base substitution predicted the replacement of a positively charged lysine by a neutral asparagine (K59N), an amino acid change consistent with the more electronegative charge of the ALAD-2 subunit. The ALAD1 and ALAD2 alleles were easily detected by amplification of a 916-bp region of genomic DNA and MspI digestion which results in 582- and 511-bp products, respectively. Molecular analysis of 85 ALAD1/ALAD2 heterozygotes and of eight ALAD2 homozygotes revealed no discrepancy between the predicted genotype and the erythrocyte isozyme phenotype, indicating that all the ALAD2 alleles analyzed had the G-to-C transversion.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular characterization of the human delta-aminolevulinate dehydratase 2 (ALAD2) allele: implications for molecular screening of individuals for genetic susceptibility to lead poisoning.

Although population exposure to lead has declined, chronic lead toxicity remains a major public health problem in the United States affecting millions of children and adults. Important gaps exist in knowledge of the pathophysiology of chronic lead intoxication. These gaps have impeded development of control strategies. To close current gaps in knowledge of chronic lead toxicity, we propose an integrated, multidisciplinary, marker-based research program. This program combines a) direct measurement of individual lead burden by 109Cd X-ray fluorescence analysis of lead in bone, b) determination of ALA-D phenotype, an index of individual susceptibility to lead, and c) assessments of subclinical injury produced by lead in the kidneys, nervous system and, reproductive organs. Data from this research will provide answers to questions of great public health importance: a) Are current environmental and occupational standards adequate to prevent chronic lead intoxication? b) is lead mobilized from the skeleton during pregnancy or lactation to cause fetal toxicity? c) Is lead mobilized from bone during menopause to cause neurotoxicity? d) What is the significance of genetic variation in determining susceptibility to lead? e) What is the contribution of lead to hypertension, renal disease, chronic neurodegenerative disease or declining sperm counts? f) Is chelation therapy effective in reducing body lead burden in persons with chronic overexposure to lead?

Unraveling the chronic toxicity of lead: an essential priority for environmental health.

ALAD is a zinc metalloenzyme whose inhibition by lead is the first and most sensitive indicator of lead exposure and whose decreased activity has been implicated in the pathogenesis of lead poisoning. This heme biosynthetic enzyme is encoded by a gene located at chromosome 9q34, which has two codominant alleles, ALAD1 and ALAD2. The occurrence of two frequent alleles for ALAD stimulated an investigation into the possible pharmacogenetic role of the enzyme polymorphism in lead poisoning. In a New York City population at high risk for lead exposure, individuals heterozygous or homozygous for the less common allele, ALAD2, had blood lead levels greater than or equal to 30 micrograms/dl more frequently than expected. These findings suggest a potential genetic susceptibility to lead poisoning in individuals with the ALAD 1-2 and 2-2 phenotypes.

δ-Aminolevulinic Acid Dehydratase Isozymes and Lead Toxicitya

For several years, a 4-12-fold increase of the upper normal limit in erythrocyte protoporphyrin concentrations persisted in two men 34 and 39 years of age who were chronically exposed to lead We are dealing with a zinc protoporphyrinemia in both cases, without lead intoxication or anemia The 34-year-old had been a regular blood donor for 10 years and had already been treated for iron deficiency several times Hemoglobin, red cell counts, hematocrit, and iron were at the lower normal limit The activity of porphobilinogen synthase (PBG-S), uroporphyrinogen-synthase and -decarboxylase as well as urinary porphyrin precursors and porphyrin excretion were normal Protoporphyrinemia was said to be due to a prelatent/latent iron deficiency In the 39-year-old, the activity of PBG-S was lowered to 388 mumol/1  h, as compared to the mean of controls (1,190 +/- 210, x +/- SD, n = 50), in connection with a slightly elevated excretion of delta-aminolevulinic acid and coproporphyrin in the urine and a high-normal blood lead level In his family there was no history of either a protoporphyrinemia or a hematological disturbance Six of eight family members in three generations showed a diminished activity of PBG-S: 600 +/- 160, P less than 0001 compared to controls These family members are heterozygous with regard to the PBG-S deficiency; they are clinically unobtrusive in comparison to homozygotes with an acute prophyria syndrome Activation by zinc and reactivation by dithiothreitol were normal in contrast to PBG-S from patients with lead intoxication The cause of biochemical symptoms of subclinical lead intoxication developed by the propositus is probably due to the hereditary PBG-S deficiency which sensitizes him to low-level lead exposure The determination of red cell PBG-S activity can be recommended as a test detecting heterozygotes The hereditary PBG-S deficiency is recognized as a new molecular basis for the pathogenesis of lead intoxication

Persistent protoporphyrinemia in hereditary porphobilinogen synthase (delta-aminolevulinic acid dehydrase) deficiency under low lead exposure. A new molecular basis for the pathogenesis of lead intoxication.

G Kufner

Papers

Persistent protoporphyrinemia in hereditary porphobilinogen synthase (delta-aminolevulinic acid dehydrase) deficiency under low lead exposure. A new molecular basis for the pathogenesis of lead intoxication.