Arsenic methylation by an arsenite S-adenosylmethionine methyltransferase from Spirulina platensis

• Published in: Journal of environmental sciences (China) Vol. 49; no. 11; pp. 162 - 168
• Main Authors: Guo, Yuqing, Xue, Ximei, Yan, Yu, Zhu, Yongguan, Yang, Guidi, Ye, Jun
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.11.2016
• Subjects: Arsenic; Arsenic - metabolism; Hazardous Substances - metabolism; Methylation; Methyltransferase; Methyltransferases - metabolism; S-Adenosylmethionine - metabolism; S-腺苷甲硫氨酸; Spirulina - physiology; Spirulina platensis; 亚砷酸盐; 半胱氨酸残基; 生物地球化学; 甲基丙烯酸甲酯; 甲基化; 甲基转移酶; 钝顶螺旋藻; Arsenic; Methylation; Spirulina platensis; Methyltransferase
• ISSN: 1001-0742; 1878-7320
• DOI: 10.1016/j.jes.2016.06.013

Arsenic-contaminated water is a serious hazard for human health. Plankton plays a critical role in the fate and toxicity of arsenic in water by accumulation and biotransformation.Spirulina platensis（S. platensis）, a typical plankton, is often used as a supplement or feed for pharmacy and aquiculture, and may introduce arsenic into the food chain, resulting in a risk to human health. However, there are few studies about how S. platensis biotransforms arsenic. In this study, we investigated arsenic biotransformation by S. platensis. When exposed to arsenite（As（Ⅲ））, S. platensis accumulated arsenic up to 4.1 mg/kg dry weight.After exposure to As（Ⅲ）, arsenate（As（Ⅴ）） was the predominant species making up 64% to86% of the total arsenic. Monomethylarsenate（MMA（Ⅴ）） and dimethylarsenate（DMA（Ⅴ））were also detected. An arsenite S-adenosylmethionine methyltransferase from S. platensis（Sp Ars M） was identified and characterized. Sp Ars M showed low identity with other reported Ars M enzymes. The Escherichia coli AW3110 bearing Spars M gene resulted in As（Ⅲ） methylation and conferring resistance to As（Ⅲ）. The in vitro assay showed that Sp Ars M exhibited As（Ⅲ） methylation activity. DMA（Ⅴ） and a small amount of MMA（Ⅴ） were detected in the reaction system within 0.5 hr. A truncated Sp Ars M derivative lacking the last 34 residues still had the ability to methylate As（Ⅲ）. The three single mutants of Sp Ars M（C59S, C186 S, and C238S） abolished the capability of As（Ⅲ） methylation, suggesting the three cysteine residues are involved in catalysis. We propose that Sp Ars M is responsible for As methylation and detoxification of As（Ⅲ） and may contribute to As biogeochemistry.