Marie Vahter

Mechanisms of arsenic biotransformation

Inorganic arsenic, a documented human carcinogen, is methylated in the body by alternating reduction of pentavalent arsenic to trivalent and addition of a methyl group from S-adenosylmethionine. Glutathione, and possibly other thiols, serve as reducing agents. The liver is the most important site of arsenic methylation, but most organs show arsenic methylating activity. The end metabolites are methylarsonic acid (MMA) and dimethylarsinic acid (DMA). These are less reactive with tissue constituents than inorganic arsenic and readily excreted in the urine. However, reactive intermediates may be formed. Absorbed arsenate (As(V)) is fairly rapidly reduced in blood to As(III), which implies increased toxicity. Also, intermediate reduced forms of the methylated metabolites, MMA(III) and DMA(III), have been detected in human urine. In particular MMA(III) is highly toxic. To what extent MMA(III) and DMA(III) contribute to the observed toxicity following exposure to inorganic arsenic remains to be elucidated. There are marked differences in the metabolism of arsenic between mammalian species, population groups and individuals. There are indications that subjects with low MMA in urine have faster elimination of ingested arsenic, compared to those with more MMA in urine.

Tubular and glomerular kidney effects in Swedish women with low environmental cadmium exposure

Cadmium is a well-known nephrotoxic agent in food and tobacco, but the exposure level that is critical for kidney effects in the general population is not defined. Within a population-based women’s health survey in southern Sweden (Women’s Health in the Lund Area, WHILA), we investigated cadmium exposure in relation to tubular and glomerular function, from 1999 through early 2000 in 820 women (71% participation rate) 53–64 years of age. Multiple linear regression showed cadmium in blood (median, 0.38 μg/L) and urine (0.52 μg/L; density adjusted = 0.67 μg/g creatinine) to be significantly associated with effects on renal tubules (as indicated by increased levels of human complex-forming protein and N-acetyl-β-d-glucosaminidase in urine), after adjusting for age, body mass index, blood lead, diabetes, hypertension, and regular use of nephrotoxic drugs. The associations remained significant even at the low exposure in women who had never smoked. We also found associations with markers of glomerular effects: glomerular filtration rate and creatinine clearance. Significant effects were seen already at a mean urinary cadmium level of 0.6 μg/L (0.8 μg/g creatinine). Cadmium potentiated diabetes-induced effects on kidney. In conclusion, tubular renal effects occurred at lower cadmium levels than previously demonstrated, and more important, glomerular effects were also observed. Although the effects were small, they may represent early signs of adverse effects, affecting large segments of the population. Subjects with diabetes seem to be at increased risk.

Cadmium-induced effects on bone in a population-based study of women

High cadmium exposure is known to cause bone damage, but the association between low-level cadmium exposure and osteoporosis remains to be clarified. Using a population-based women’s health survey in southern Sweden [Women’s Health in the Lund Area (WHILA)] with no known historical cadmium contamination, we investigated cadmium-related effects on bone in 820 women (53–64 years of age). We measured cadmium in blood and urine and lead in blood, an array of markers of bone metabolism, and forearm bone mineral density (BMD). Associations were evaluated in multiple linear regression analysis including information on the possible confounders or effect modifiers: weight, menopausal status, use of hormone replacement therapy, age at menarche, alcohol consumption, smoking history, and physical activity. Median urinary cadmium was 0.52 μg/L adjusted to density (0.67 μg/g creatinine). After multivariate adjustment, BMD, parathyroid hormone, and urinary deoxypyridinoline (U-DPD) were adversely associated with concentrations of urinary cadmium (p < 0.05) in all subjects. These associations persisted in the group of never-smokers, which had the lowest cadmium exposure (mainly dietary). For U-DPD, there was a significant interaction between cadmium and menopause (p = 0.022). Our results suggest negative effects of low-level cadmium exposure on bone, possibly exerted via increased bone resorption, which seemed to be intensified after menopause. Based on the prevalence of osteoporosis and the low level of exposure, the observed effects, although slight, should be considered as early signals of potentially more adverse health effects.

Toxic and essential elements in placentas of Swedish women.

OBJECTIVES To evaluate interactions between toxic and essential elements in the mother-fetus relationship and possible predictors of trace element concentrations in placenta and cord blood. DESIGN AND METHODS A group of 106 Swedish women was investigated for concentrations of cadmium, lead, and several essential elements in placenta as well as cadmium, lead, zinc, and selenium in venous blood collected at gestational week (gw) 36 and umbilical cord blood. Relations between these elements and maternal and childs characteristics were examined. RESULTS The concentrations of cadmium in placenta ranged from 10 to 170 nmol/kg, with the median value (Md) being 46 nmol/kg. Cord blood cadmium (Md of 0.19 nmol/L) was only about 10% of that in maternal blood. Smokers had significantly higher cadmium concentrations in blood (p < 0.001) and placenta (p = 0.001) than non-smokers. The median placental concentration of lead was 26 nmol/kg (range 0-630 nmol/kg). The lead levels in cord blood (Md of 54 nmol/L) were almost the same as in maternal blood. Statistically significant negative associations were found between cord blood lead, on one hand, and childs weight, length, and head circumference, on the other. The placental levels (medians and ranges) of the essential elements (micromol/kg) were 160 (120-280) for zinc, 2.4 (2.0-3.3) for selenium, 15 (10-20) for copper, 0.084 (0.02-0.32) for cobalt, 0.055 (0.03-0.12) for molybdenum, and 1.2 (0. 65-5.1) for manganese, respectively. Several of the essential elements in placenta correlated significantly with each other. Multiparous mothers had significantly lower concentrations of zinc (p = 0.002) and selenium (p = 0.049) in serum as well as zinc (p = 0. 001) and calcium (p = 0.004) in placenta than nulliparous ones. Also, cord blood zinc decreased with parity. CONCLUSIONS The results showed that lead, but not cadmium crossed easily the placental barrier. There were no negative effects of cadmium on the zinc status. Cord blood lead, on the other hand, was a negative predictor of childs birth weight, length and head circumference, indicating that lead might have negative influence on growth in children even at very low exposure levels. There was a depletion of maternal stores of essential elements with increasing parity.

Genetic polymorphism in the biotransformation of inorganic arsenic and its role in toxicity

Arsenic is a recognized human carcinogen, but experimental cancer studies are negative. There is a variation in susceptibility among individuals, which probably is related to variation in metabolism. Inorganic arsenic is methylated to methylarsonic acid (MMA) and dimethylarsinic acid (DMA), which are less toxic and more readily excreted in urine than the inorganic arsenic. The rate of methylation of arsenic varies considerably between species. Most population groups studied so far have on average 10-30% inorganic, 10-20% MMA, and 60-70% DMA in urine, but there is a considerable inter-individual variation. Also, recent studies have identified groups with unusually low or high urinary excretion of MMA. Thus, there seems to be a genetic polymorphism in the biomethylation of arsenic. However, the methyltransferases involved in arsenic methylation have not been characterized.

Biotransformation of trivalent and pentavalent inorganic arsenic in mice and rats

Abstract In mice exposed to 74 As-labeled trivalent or pentavalent inorganic arsenic, dimethylarsinic acid was found to be the major urinary metabolite. As(III) was methylated to a greater extent than As(V) but caused higher whole-body retention. Elimination, thus seems to be less dependent on methylation following exposure to As(V) than following exposure to As(III). Low doses of As(III) were methylated to such an extent (about 80% of the dose) that the retention was comparable to that of As(V). For both valence forms, retention increased with increasing dose, approximately parallel to a decrease in the methylation (in percentage of the dose). Whole-body retention of arsenic was about 20 times higher in rats than in mice. In vitro binding of arsenic to erythrocytes was similar in blood from rats and mice though considerably higher for As(III) than for As(V). Methylation was, however, much lower in the rats and the possibility that this might be one reason for the extremely high retention of arsenic in the rat is discussed.

Role of metabolism in arsenic toxicity.

In humans, as in most mammalian species, inorganic arsenic is methylated to methylarsonic acid (MMA) and dimethylarsinic acid (DMA) by alternating reduction of pentavalent arsenic to trivalent and addition of a methyl group from S-adenosylmethionine. The methylation of inorganic arsenic may be considered a detoxification mechanism, as the end metabolites, MMA and DMA, are less reactive with tissue constituents, less toxic, and more readily excreted in the urine than is inorganic arsenic, especially the trivalent form (AsIII, arsenite). The latter is highly reactive with tissue components, due to its strong affinity for sulfhydryl groups. Thus, following exposure to AsV the first step in the biotransformation, i.e. the reduction to AsIII, may be considered a bioactivation. Also, reactive intermediate metabolites of high toxicity, mainly MMAIII, may be formed and distributed to tissues. Low levels of MMAIII and DMAIII have been detected in urine of individuals chronically exposed to inorganic arsenic via drinking water. However, the contribution of MMAIIIand DMAIII to the toxicity observed after intake of inorganic arsenic by humans remains to be elucidated. The major route of excretion of arsenic is via the kidneys. Evaluation of the methylation of arsenic is mainly based on the relative amounts of the different metabolites in urine. On average human urine contains 10-30% inorganic arsenic, 10-20% MMA and 60-80% DMA.

A unique metabolism of inorganic arsenic in native Andean women

The metabolism of inorganic arsenic (As) in native women in four Andean villages in north-western Argentina with elevated levels of As in the drinking water (2.5, 14, 31, and 200 micrograms/1, respectively) has been investigated. Collected foods contained 9-427 micrograms As/kg wet weight, with the highest concentrations in soup. Total As concentrations in blood were markedly elevated (median 7.6 micrograms/1) only in the village with the highest concentration in the drinking water. Group median concentrations of metabolites of inorganic As (inorganic As, methylarsonic acid (MMA) and dimethylarsinic acid (DMA)) in the urine varied between 14 and 256 micrograms/1. Urinary concentrations of total As were only slightly higher (18-258 micrograms/1), indicating that inorganic As was the main form of As ingested. In contrast to all other populations studied so far, arsenic was excreted in the urine mainly as inorganic As and DMA. There was very little MMA in the urine (overall median 2.2%, range 0.0-11%), which should be compared to 10-20% of the urinary arsenic in all other populations studied. This may indicate the existence of genetic polymorphism in the control of the methyltransferase activity involved in the methylation of As. Furthermore, the percentage of DMA in the urine was significantly higher in the village with 200 micrograms As/1 in the water, indicating an induction of the formation of DMA. Such an effect has not been observed in other studies on human subjects with elevated exposure to arsenic.

Effects of Arsenic on Maternal and Fetal Health

Arsenic, which is commonly found in drinking water, is a potent toxicant, but little is known about its effects on maternal health. Arsenics modes of action include enzyme inhibition and oxidative stress as well as immune, endocrine, and epigenetic effects. A couple of studies reported increased blood pressure and anemia during pregnancy. Susceptibility to arsenic is dependent on the biomethylation, which occurs via one-carbon metabolism. Methylarsonic acid and dimethylarsinic acid are main metabolites in urine, and elevated methylarsonic acid is considered a general risk factor. Arsenic easily passes the placenta, and a few human studies indicate a moderately increased risk of impaired fetal growth and increased fetal and infant mortality. The fetus and infant are probably partly protected by the increased methylation of arsenic during pregnancy and lactation; the infant is also protected by low arsenic excretion in breast milk. Early-life exposure may induce changes that will become apparent much later in life.

Intracellular interaction and metabolic fate of arsenite and arsenate in mice and rabbits

In vitro incubation of [74As]arsenite, -arsenate or -dimethylarsinic acid (DMA, the main metabolite of inorganic arsenic) with liver, lung and kidney homogenate of mice and rabbits showed that arsenite is the main form of arsenic bound to tissues. Injection of arsenite in mice and rabbits (0.04 mg As/kg body wt.) caused higher concentration of arsenic in the liver and the lungs than did the same dose of arsenate. This was less marked in the mice than in the rabbits, mainly due to the faster methylation to DMA. The relatively high degree of binding of arsenic to tissue constituents which also followed injection of arsenate may be explained by in vivo reduction to arsenite. The similar binding pattern after exposure to arsenite and arsenate indicates further that one and the same form of arsenic, arsenite, is retained independent of the form of exposure to inorganic arsenic. In contrast to the liver and lungs the kidneys showed a higher retention of arsenic after injection of arsenate than after injection of arsenite. Following injection of [74As]DMA in the animals excretion was essentially completed within 24 h, indicating low affinity for the tissues in vivo.