427
Martell & Smith 1974). This suggests a connection
between stability constant and degradability of a com-
plex, which is opposite to the results obtained for the
degradation of EDTA or NTA catalyzed by EDTA-
or NTA-oxidizing system, where no such relationship
between the stability constant and the degradability of
the different metal complexes was found (Witschel et
al. 1997; Xun et al., 1996). Thus, presently no general
rule concerning the enzymatic degradability of metal
complexes of aminopolycarboxylic acids as function
of their stability constant can be deduced. An expla-
nation for the close link between stability constant
of metal-[S,S]-EDDS-complexes and their enzymatic
degradability could be that during the process of sub-
strate binding the metal ligand of the [S,S]-EDDS
molecule has to be exchanged or removed. This step
in the reaction might become inhibited in the case
of metal-[S,S]-EDDS-complexes with high stability
constants.
It is interesting that, in the presence of metal
ions, the equilibrium between [S,S]-EDDS and the
two products of the enzymatic degradation, fumarate
and AEAA, was shifted towards [S,S]-EDDS. Un-
der these conditions, there are three other equilibria
which play a role: first of all the equilibrium be-
tween uncomplexed and complexed [S,S]-EDDS as
well as the equilibria between the uncomplexed prod-
ucts fumarate and AEAA and their complexed forms.
As far as we are aware, the stability constants for
metal-AEAA complexes are not known, but it seems
reasonable that they are lower than those of [S,S]-
EDDS and higher than those of fumarate. Since out
of the three compounds, [S,S]-EDDS is the ligand ex-
hibiting the highest stability constants, more of the
educt [S,S]-EDDS than of the two products is removed
from the equilibrium of the lyase catalyzed reaction in
the presence of metal ions. Hence, the equilibrium will
be shifted towards [S,S]-EDDS.
Schiff’s base which is then reduced to [S,S]-EDDS.
However, there is no evidence yet that this reaction of
AEAA with oxaloacetate takes place. It would also be
possible that a lyase was involved in the synthesis of
[S,S]-EDDS starting from the postulated intermediate
AEAA and fumarate. In fact, the reversible nature of
the EDDS-lyase catalyzed reaction fosters the specu-
lation that this enzyme might also play a role in the
synthesis of EDDS. Therefore, it would be interest-
ing to test whether EDDS-lyase activity is present in
cell-free extracts of A. orientalis. This could give an
indication whether or not the EDDS-lyase is responsi-
ble for both [S,S]-EDDS degradation and [S,S]-EDDS
synthesis.
Acknowledgements
We thank BUWAL for financial support. Furthermore,
we are indepted to D. Schowanek and C. Perkins
(The Procter and Gamble Company, Brussels, Bel-
gium and Cincinnati, USA) for supplying us with
the different EDDS isomers and with AEAA. We
further thank D. Hüglin (Ciba Chemikalien GmbH,
Grenzach-Wyhlen, Germany) and H. Zähner (Univer-
sity of Tübingen, Germany) for the generous gift of
[S,S]-EDDS. Our thanks also go to A.J.B. Zehnder for
helpful discussions.
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