Biology Reference
In-Depth Information
Chorus and Bartram, 1999). Paralysis of skeletal muscles and respiratory muscles leads to death of
the individual within minutes (Ressom
et al
., 1994; Singh
et al
., 1999).
A) Anatoxin-a:
It was fi rst isolated from
A
.
fl os-aquae
. Some species of
Anabaena
,
Planktothrix
,
Aphanizomenon
and
Cylindrospermum
(Sivonen and Jones, 1999; Ballot
et al
., 2010),
M
.
aeruginosa
(Park
et al
., 1993)
Phormidium favosum
(Gugger
et al
., 2005) and
Oscillatoria
PCC 6506 (Méjean
et al
.,
2009) also are reported to produce this toxin.
Structurally, anatoxin-a is an alkaloid considered to be a secondary amine, 2-acetyl-9 azabicyclo(4-
2-1) non-ene (Fig. 12; Huber, 1972). It has a molecular weight of 165 D (Carmichael, 1992). It is a
structural analogue of cocaine and neurotransmitter acetylcholine (Mazur-Marzec, 2006). The
biosynthesis of anatoxin-a has been shown to be analogous to tropane group of alkaloids found in
higher plants (Gallon
et al
., 1990, 1994). This was countered by Hemscheidt
et al
. (1995) based on
14
C-labelling and enzymatic experiments. They suggested that pyrroline, a precursor of anatoxin-a,
is synthesized in
A
.
fl os
-
aquae
starting from ornithine or arginine via putrescine. The derivation of
carbon skeletons of anatoxin-a from either acetate or glutamate has been confi rmed by
14
C-labelling
experiments and the retention of glutamic acid residue during transformation of anatoxin-a further
does not lend support to the biosynthesis of anatoxin-a similar to tropane group of alkaloids
(Hemscheidt
et al
., 1995). However, the biosynthesis of anatoxin-a and homoanatoxin-a has been
reported to be mediated by the gene products of eight contiguous genes (
anaA
to
anaH
) in
Oscillatoria
PCC 6506. It is initiated by the loading of 1-proline on acyl-carrier protein where its ring structure gets
oxidized into pyrroline oxidation state. This is soon followed by three successive PKS modules for
elongation, reduction and cyclization and methylation. The loading of 1-proline in the fi rst reaction
has been confi rmed by the mediation of purifi ed protein of AnaC. The existence of gene sequences
anaC
,
anaE
,
anaF
and
anaG
in all anatoxin-a producing toxic strains and their absence from non-toxic
strains of cyanobacteria has also been demonstrated (Méjean
et al
., 2009).
The stability of anatoxin-a has been studied. Photolysis of anatoxin-a was dependent on the
intensity of sunlight and pH of the medium with a half-life of 1-2 h whereas in absence of sunlight
or metal ions such as copper and iron, the half- life was several days. The breakdown products of
anatoxin-a exhibited no toxicity in mouse bioassay (Stevens and Krieger, 1991).
The mode of action of anatoxin-a is similar to acetylcholine and acts as a stereo-selective
agonist of the acetylcholine receptors in both neurons and endplates with 8-10 fold higher potency
(Carmichael
et al
., 1975; Spivak
et al
., 1980; Ressom
et al
., 1994; Dow and Sowboda, 2000). Under
normal conditions, acetylcholine released by neurons binds to acetylcholine receptor results in
opening up of sodium channels. Continuous fl ow of sodium ions to muscle cells activates them. The
degradation of acetylcholine by acetylcholinesterase leads to closure of sodium channel that makes
O
H
2
+
CH
3
N
Figure 12:
Structure of anatoxin-(a).
