Arsenate metabolism in aquatic plants

Abstract

AbstractOf the several types of arsenic metabolic pathways in algae and aquatic higher plants, production of arsenoribosides is predominant and most interesting. Insertion of the ribosyl or adenosyl moiety by transfer from S‐adenosylmethioninine or possibly from adenosylcobalamin is the critical biochemical step which, as yet, has not been experimentally demonstrated. Oceanic or other environmental arsenate (AsO43−), absorbed in the plant's quest for phosphate, is fixed by reaction with ATP to yield the phosphoric arsenic anhydride APAs, a short‐lived but reducible intermediate which is converted to arsenic(III). The Hill Reaction, or thioredoxin reductase, reduces it to an arsine oxide (HAsO) or arsonous acid [HAs(OH)2] depending upon the water content of its environment. This readily diffusible reagent avidly attacks sulfhydryl groups of proteins to produce arsonous thioesters. The Sargassum group of algae appears to process arsenate no further than this. The reduced arsenic may be freed from its sulfur bondage by reaction with 2,3‐dimercaptopropan‐1‐ol (BAL) or dithiothreitol. In experiments with Sargassum fluitans and Sargassum natans no arsenoribosides were observed. Only protein‐bound arsenic was observed. It could be liberated by trituration with dithiothreitol to produce the cyclic arsonous dithioester.Most diatoms, dinoflagellates and macroalgae as well as freshwater higher plants release such protein‐bound arsenic as a result of sequential methylation and adenosylation. Ultimately the products are trialkylarsine oxides, innocuous substances which are slowly or not‐at‐all metabolized by herbivorous animals or bacteria. Fortunately mammals and most animals also excrete the arsenoribosides readily, unchanged. Arsenic metabolism by a cyanobacterium, Phormidium sp., was described by Matsuto et al. (Comp. Biochem. Physiol., 1984, 78c:377) as involving two modes of arsenate fixation, reduction, and excretion. We have extended those experiments with Phormidium persicinum. We have analyzed algae from 2‐ and 7‐day culture in radioarsenate media. The arsenic products included 80% of arsenolipid, similar if not identical to that formed in the brown algae. The water‐soluble products were in low concentration. Insoluble, protein‐bound arsonous thioesters accounted for 8% of the fixed arsenic. The mechanism of arsenic depuration in Phormidium appears to be primarily lipid‐mediated.