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Claudia Hopenhayn-Rich,¹ Mary Lou Biggs,¹ Allan H. Smith,¹ David A. Kalman,² and Lee E. Moore¹

¹School of Public Health, University of California-Berkeley, Berkeley, CA 94720 USA; ²School of Public Health and Community Medicine, University of Washington, Seattle, WA 98195 USA

Abstract

Methylation is considered the detoxification pathway for inorganic arsenic (InAs) , an established human carcinogen. Urinary speciation analysis is used to assess the distribution of metabolites [monomethylarsonate (MMA) , dimethylarsinate (DMA) , and unmethylated arsenic (InAs) ], as indicators of methylation capacity. We conducted a large biomarker study in northern Chile of a population chronically exposed to high levels of arsenic in drinking water. We report the results of the methylation study, which focused on the effects of exposure and other variables on the percent InAs, MMA, DMA, and the ratio of MMA to DMA in urine. The study consisted of 122 people in a town with arsenic water levels around 600 g/l and 98 participants in a neighboring town with arsenic levels in water of about 15 g/l. The corresponding mean urinary arsenic levels were 580 g/l and 60 g/l, of which 18.4% and 14.9% were InAs, respectively. The main differences were found for MMA:DMA ; exposure, smoking, and being male were associated with higher MMA:DMA, while longer residence, Atacameño ethnicity, and being female were associated with lower MMA:DMA. Together, these variables explained about 30% of the variability in MMA:DMA. Overall, there was no evidence of a threshold for methylation capacity, even at very high exposures, and the interindividual differences were within a much wider range than those attributed to the variables investigated. The differences in percent InAs were small and within the ranges of other studies of background exposure levels. The biological significance of MMA:DMA, which was more than 1.5 times greater in the exposed group, and its relationship to sex, length of exposure, and ethnicity need further investigation because its relevance to health risk is not clear. Key words: arsenic, arsenic methylation, arsenic speciation, Chile, water pollution. Environ Health Perspect 104:620-628 (1996)

Address correspondence to A. H. Smith, School of Public Health, University of California-Berkeley, 140 Warren Hall, Berkeley, CA 94720 USA.

Support for this work was provided by grants P30-ES01896 and P42-ES04705 from the National Institute of Environmental Health Sciences. Additional support was received from the Health Effects Component of the University of California Toxic Substances Program. We thank Nella Marchetti and the Instituto de Salud Pública de Chile for support and assistance in Chile ; Martin Beeris, Verónica Moreno, Carmen Oyanguren, Fernando Toroco, Leda Mondaca, Mario Banchón, Brian Rich, Olivier Robert, Jill Dale, and many others who participated in the field work in Chile ; and all study participants from San Pedro and Toconao. We also thank Irva Hertz-Picciotto, Michael Bates, and Mark van der Laan for helpful comments and Veronica Barroga for assisting with manuscript preparation.

Received 13 December 1995 ; accepted 15 March 1996.