A novel arsenic containing riboside (arsenosugar) in three species of gastropod

Abstract

Arsenic compounds in three marine gastropods (Thais bitubercularis, Thais distinguenda, Morula musiva) from Phuket, Thailand were examined by HPLC using ICP-MS as an arsenic specific detector. Aqueous methanol treatment of the freeze-dried samples (initially 112–339 μg As g⁻¹ dry mass) extracted >96% of the total arsenic. HPLC-ICP-MS of the extracts demonstrated the presence of arsenobetaine (93–95% of total extractable arsenic), arsenocholine (3.1–4.6%), tetramethylarsonium ion (0.21–2.2%), two unknown arsenic compounds (each approx. 0.1%), and an unresolved mixture of arsenic compounds (∼1%). One of the unknowns was identified as a new natural product, the arsenosugar 2′,3′-dihydroxypropyl 5-deoxy-5-trimethylarsonioriboside, by co-chromatography with synthetic material. The presence of these arsenic compounds in the gastropods is consistent with the hypothesis that trimethylated arsenosugars are transformed into arsenobetaine via arsenocholine within animals.

Introduction

High arsenic concentrations in marine organisms were first reported in the early 1900s, and following considerable work over the last 20 years, the chemical structures of many of the marine arsenic compounds are now known (Francesconi and Edmonds, 1997). In marine animals arsenobetaine is by far the major arsenic compound. Tetramethylarsonium ion, arsenocholine, and trimethylarsine oxide are also found in marine animals but they generally occur only as minor compounds (see Fig. 1 for structures of arsenic compounds). The pattern of arsenic compounds in marine algae is quite different from that in animals. In algae, arsenic containing ribosides (arsenosugars) are the major arsenic compounds (Morita and Shibata, 1990), and arsenobetaine, tetramethylarsonium ion, arsenocholine, and trimethylarsine oxide are absent. Most of the arsenosugars identified in algae are dimethylated arsinoyl (Me₂AsO-) derivatives of ribosides, chiefly compounds 1–4 (Fig. 1). One trimethylated arsonio (Me₃As⁺-) riboside (arsenosugar 5) has also been reported from algal sources (Shibata and Morita, 1988; Francesconi et al., 1992a). In seawater and marine sediments, arsenic is present mainly as arsenate along with some arsenite and traces of methylarsonic acid and dimethylarsinic acid (Andreae, 1979; Reimer and Thompson, 1988; Santosa et al., 1994). Neither arsenobetaine nor arsenosugars have been detected thus far in seawater or sediments.

Two questions fundamental to our understanding of the cycling of arsenic in marine systems remain unanswered. First, what are the processes whereby inorganic arsenate in seawater is biotransformed by algae into arsenosugars; and, second, what is the origin of arsenobetaine in marine animals. A proposed biogenetic pathway for arsenosugars (Edmonds and Francesconi, 1987) has been supported by identification of key intermediates in marine biota (Francesconi et al., 1992a). The origin of arsenobetaine, however, is more speculative. Although some evidence has been presented for a biogenetic pathway to arsenobetaine beginning with dimethylated arsenosugars (Edmonds et al., 1982), the key intermediate in the proposed pathway, dimethylarsinoylethanol, has not been detected in biota, and the final biotransformation of dimethylarsinoylethanol to arsenobetaine was not observed in laboratory experiments designed to simulate natural marine environments (Francesconi and Edmonds, 1994). An alternative pathway beginning with trimethylated arsenosugars has some experimental support: under anaerobic microbial conditions the arsenosugar 5 was converted in high yield to arsenocholine (Francesconi et al., 1992b); and, in a second experiment, arsenocholine fed to fish was converted to arsenobetaine (Francesconi et al., 1989).

Arsenobetaine clearly plays an important role in the cycling of arsenic in marine systems, and recent results showing the presence of arsenobetaine in terrestrial organisms (Šlejkovec et al., 1997; Geiszinger et al., in press) suggest that its production is more widespread than previously thought. Elucidation of the biogenesis of arsenobetaine might be aided by the identification of naturally occurring arsenic compounds. Consequently, the search for key intermediates in the proposed biogenetic pathways leading to arsenobetaine forms the basis of the work reported here.