Anatoxin homoanatoxin

Hemscheidt, T., Rapala, J., Sivonen, K., and Skulberg, O. M., Biosynthesis of anatoxin-A inAnabaena flos-aquae and homoanatoxin-A in Oscillatoriaformosa, J. Chem. Soc. Chem. Commun., 1361, 1995. 

Fig. 6.1 (a) The neurotransmitter acetylcholine (ACh) (small circles) bind to the ACh receptor and stimulate muscle contraction. Acetylcholinesterase (black partial circle) degrades ACh allowing the muscle cells to return to a resting state, (b) Anatoxin-a, homoanatoxin-a and analogs (gray ovals)... 

Homoanatoxin-a, obtained from various freshwater cyanobacteria, is a relatively rare natural analog of anatoxin-a for which the C-11 side chain is extended by one methylene unit (Figure 6.4). It was originally isolated from Planktothrix sp. (formerly Oscillatoria) in 1992 [51]. It has recently been isolated from Raphidiopsis mediterranea Skuja from Japan [52] and Planktothrix (formerly Oscillatoria) formosa blooms in Ireland [53]. 

The total synthesis of homoanatoxin-a, as an analog of anatoxin-a, was also accomplished in 1992 [33]. It is a potent nicotinic agonist active at the postsynaptic nicotinic ACh receptor channel complex [54]. Although not as potent as anatoxin-a, homoanatoxin-a has provided valuable insight for SAR studies of anatoxin-a and its homologs. 

Biosynthesis of homoanatoxin-a was examined using Oscillatoria formosa and a mechanism similar to that for anatoxin-a was proposed [13,14]. The origin of the C-12 methyl group that distinguishes homoanatoxin-a from anatoxin-a, was shown through feeding experiments performed with L-[methyl- C]-methionine in the culture of Raphidiopsis mediterranea Skuja. It was proposed that the S-methyl of methionine is transferred to the toxin via S-adenosyl-L-methionine (SAM)-mediated methylation [55]. 

Araoz, R., Nghiem, H.O., Rippka, R., Ealihroda, N., de Marsac, N.T., and Herdman, M. 2005. Neurotoxins in axenic oscilla-torian cyanobacteria coexistence of anatoxin-a and homoanatoxin-a determined by ligand-binding assay and GC/MS. Microbiol 151, 1263-1273. 

Namikoshi, M., Murakami, T, Watanabe, M.E, Oda, T, Yamada, I, Tsujimura, S., Nagai, H., and Oishi, S. 2003. Simultaneous production of homoanatoxin-a, anatoxin-a, and a new non-toxic 4-hydroxyhomoanatoxin-a by the cyanobacterium Raphidiopsis mediterranea Skuja. Toxicon 42, 533-538. 

Zotou, A., Jefferies, T.M., Brough, P.A., and Gallagher, T. 1993. Determination of anatoxin-a and homoanatoxin in blue-green algal extracts by high-performance liquid chromatography and gas chromatography-mass spectrometry. Analyst 118, 753-758. 

The neurotoxins known to be produced by freshwater cyanobacteria include anatoxin-a and homoanatoxin-a, ana-toxin-a(s), and saxitoxins. Their target is the neuromuscular system, and they can paralyze peripheral, skeletal muscle, including respiratory muscles. Death ensues as a result of respiratory arrest within a few minutes to a few hours (see reviews by Duy et al, 2000 Kuiper-Goodman et al, 1999). 

In 1992, a methylene analogue of AN, homoanatoxin-a (HMAN) (Figure 38.1 R=C2H5), was isolated from Planktothrix (Oscillatoria) formosa in Norway (Skulberg et al., 1992). AN and HMAN act by enhancing the release of acetylcholine (ACh) from peripheral cholinergic nerves (Lilleheil et al, 1997). HMAN is much less common than AN but has been found in Ireland and Japan (Furey et al., 2003a), (Namikoshi et al., 2003). Anatoxin-a(s) is structurally umelated to AN but was also... 

James, K. J., Eurey, A., Sherlock, I. R., Stack, M. A., Twohig, M., Caudwell, F. B., and Skulberg, O. M. (1998). Sensitive determination of anatoxin-a, homoanatoxin-a and their degradation products by liquid chromatography with fluorimetric detection. J. Chromatogr. 798, 147-157. 

Anatoxin-a and homoanatoxin-a are potent agonists of the nicotinic acetylcholine receptor, and they provoke the rapid death of animals by acute asphyxia when ingested [73]. Cases of animal death due to cyanobacterial toxin exposure are regularly reported in different places around the world [74-76]. Hence, it is recognized that the release of cyanobacterial toxins in waters and water supplies has major implications for public health and for the environment. Therefore, several countries... 

The first studies on the biosynthesis of these neurotoxin involved feeding experiments using labeled precursors, which, although allowed the researchers to establish that anatoxin-a results from the condensation of three acetate units on (5)-l-pyrroline-5-carboxylate (P5C) that is derived from glutamate followed by a decarboxylation and that homoanatoxin-a would result from methylation of anatoxin-a, could not lead to the elucidation of the individual chemical steps of the biosynthetic pathway [77]. 

Figure 5.19 Proposed biosynthetic pathway to anatoxin-a and homoanatoxin-a starting from proline. KS, ketosynthase AT, acyltransferase KR, ketoreductase DH, dehydrogenase ER, enoylreductase ACP, acyl carrier protein Cy, cyclase and CM, methyllransferase.

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