Radioreceptor assays for sensitive detection and quantitation of saxitoxinand its analogues from strains of the freshwater cyanobacterium, Anabaena circinalis

Toxic freshwater cyanobacteria can contaminate water supplies and adversely effect humans, agricultural livestock, and wildlife. Toxicity is strain-sp ecific so morphological observations alone cannot predict the hazard level. Two microtiter plate based bioassays have emerged for measuring saxitoxin (STX) and its derivatives, commonly found in the freshwater cyanobacteria A nabaena and Aphanizomenon. They use radioactively labeled STX binding by so dium channels, STX's pharmacological target, or an unrelated protein, saxip hilin. These bioassays were challenged with extracts of toxic and nontoxic strains of Anabaena circinalis, and the results were compared with HPLC ana lysis. Both radioreceptor assays had detection limits of 2 mug STX equivale nts (STXeq)/L, which is below the concentration proposed for a health alert , namely 3 mug STXeq/L. In all cases, statistically significant correlation s existed between all toxicity measurements of the same extracts with the m ethods used herein. Sodium channel and saxiphilin assays however predicted less toxicity relative to HPLC analysis. The only exception to this was the equivalency observed between saxiphilin measurement and HPLC quantitation corrected for mammalian toxicity. Saxiphilin assay predicted toxicity in on e strain was 3 orders of magnitude more than by sodium channel assay, and n o STX was detected by HPLC. Lack of acetylcholinesterase inhibition showed this bioactivity was not anatoxin-a(S), a toxin also produced by this A. ci rcinalis with some resemblance to the region of STX bound by saxiphilin. Pr esence of anatoxin-a(S) was predicted for another strain by this same acety lcholinesterase assay that, if confirmed by chemical analysis, would be the first report of anatoxin-a(S) in an Australian cyanobacterium.