Lead poisoning and the inactivation of 5-aminolevulinate dehydratase as modeled by the tris(2-mercapto-1-phenylimidazolyl)hydroborato lead complex, {[Tm(Ph)]Pb}[ClO4] [14]

Full text of DOI:10.1021/ja001530y

7140
J. Am. Chem. Soc. 2000, 122, 7140-7141
Scheme 1
Lead Poisoning and the Inactivation of
5-Aminolevulinate Dehydratase as Modeled by the
Tris(2-mercapto-1-phenylimidazolyl)hydroborato
Lead Complex, {[TmPh]Pb}[ClO4]
Brian M. Bridgewater and Gerard Parkin*
Department of Chemistry, Columbia UniVersity
New York, New York 10027
ReceiVed May 3, 2000
tioned active site of ALAD are not common. Nevertheless, we
have recently reported a series of complexes with the requisite
motif, which incorporates sterically demanding tris(mercap-
toarylimidazolyl)borate ligands, [TmAr] (Ar ) Ph, Mes), to model
the binding of the zinc center to the three cysteine residues, for
example, [TmPh]ZnX (X ) I, NO3) and [TmMes]ZnX (X ) Cl,
I).15 The availability of such complexes has allowed us to explore
the potential for lead to replace zinc in a synthetic system for
which the coordination environment mimics that in ALAD.
Significantly, we have observed that lead readily displaces the
zinc in [TmPh]ZnX derivatives. For example, both [TmPh]ZnI15
and {[TmPh]Zn(NCMe)}(ClO4)16 react rapidly with Pb(ClO4)2‚
xH2O to give yellow {[TmPh]Pb}(ClO4) (Scheme 1). The latter
compound may also be obtained independently by reaction of
[TmPh]Li with Pb(ClO4)2.
Lead is the most commonly encountered toxic metal pollutant
in the environment as a result of its current and previous use in,
for example, batteries, gasoline, plumbing, and paints.1 Consider-
able effort is, therefore, being directed toward solving this
environmental problem.2 In this regard, the toxicological properties
of lead3 are associated with its interactions with proteins and, in
particular, 5-aminolevulinate dehydratase (ALAD).4-6 The influ-
ence of lead on the latter enzyme is particularly harmful because
ALAD is responsible for the asymmetric dimerization of 5-ami-
nolevulinic acid (ALA) to porphobilinogen, a monopyrrole which
is essential for heme synthesis.7-9 Thus, not only does inactivation
of ALAD result in anemia because it inhibits the formation of
heme, and hence hemoglobin, but it also results in a build-up of
ALA, a neuropathogenic agent.7,8 ALAD is a zinc-dependent
enzyme,10 and, in this paper, we address aspects of lead inactiva-
tion of ALAD by investigating the reactivity of a synthetic
analogue towards PbII.
Recent crystallographic studies have demonstrated that the
catalytic site of yeast ALAD possesses the composition [(Cys)3ZnII-
(OH2)].7,11,12 Such a composition must be regarded as truly unusual
since the active sites (as opposed to structural sites) of most zinc
enzymes include at least one histidine ligand.13,14 Furthermore,
due to the proclivity of sulfur containing ligands to bridge more
than one zinc center, mononuclear tetrahedral zinc complexes with
sulfur-rich coordination environments that mimic the aforemen-
The molecular structure of {[TmPh]Pb}(ClO4) has been deter-
mined by X-ray diffraction (Figure 1), illustrating the monomeric
trigonal pyramidal nature of the {[TmPh]Pb}+ cation with lead at
the apex; thus, other than coordination to the three sulfur donors
[d(Pb-S) ) 2.693(2) Å] the next closest interaction is with a
disordered perchlorate counteranion at a distance [d(Pb‚‚‚OClO3)
≈ 2.94 Å] which is substantially greater than the sum of the
covalent radii of lead and oxygen.17 Of particular relevance, the
structure of {[TmPh]Pb}+ is very similar to the active site of PbII-
ALAD.7 Specifically, neither water nor acetonitrile coordinates
to the lead centers in {[TmPh]Pb}+ or Pb-ALAD, both of which
possess trigonal pyramidal geometry, with very similar average
Pb-S bond lengths of 2.7 and 2.8 Å,7 respectively. The signi-
ficance of this similarity is underscored by the observation that
mononuclear trigonal pyramidal lead complexes with a sulfur
coordination sphere are rare.18,19 Indeed, only one such example,
namely [Pb(SPh)3]-,20,21 is listed in the Cambridge Structural
Database (CSD).22,23
(1) (a) Hynes, M. J.; Jonson, B. Chem. Soc. ReV. 1997, 26, 133-146. (b)
Castellino, N.; Castellino, P.; Sannolo, N. Inorganic Lead Exposure: Me-
tabolism and Intoxication; Lewis Publishers: Boca Raton 1995.
(2) See, for example: Rampley, C. G.; Ogden, K. L. EnViron. Sci. Technol.
1998, 32, 987-993.
(3) (a) Todd, A. C.; Wetmur, J. G.; Moline, J. M.; Godbold, J. H.; Levin,
S. M.; Landrigan, P. J. EnViron. Health Perspect. 1996, 104, 141-146. (b)
Haley, B. A.; Ashley, P. J. J. Urban Technol. 1999, 6, 37-58. (c) Bergdahl,
I. A. Analusis 1998, 26, M81-M84. (d) Bressler, J.; Kim, K.-A.; Chakraborti,
T.; Goldstein, G. Neurochem. Res. 1999, 24, 595-600. (e) Srianujata, S. J.
Toxicol. Sci. 1998, 23, 237-240.
(4) ALAD is also known as porphobilinogen synthase (PBGS).
(5) Jaffe, E. K. Acta Crystallogr., Sect. D 2000, 56, 115-128.
(6) (a) Spencer, P.; Jordan, P. M. Biochem. J. 1995, 305, 151-158. (b)
Spencer, P.; Jordan, P. M. Biochem. J. 1994, 300, 373-381.
(7) Warren, M. J.; Cooper, J. B.; Wood, S. P.; Shoolingin-Jordan, P. M.
Trends Biochem. Sci. 1998, 23, 217-221.
(8) (a) Campagna, D.; Huel, G.; Girard, F.; Sahuquillo, J.; Blot, P.
Toxicology 1999, 134, 143-152. (b) Sithisarankul, P.; Weaver, V. M.; Davoli,
C. T.; Strickland, P. T. Biomarkers 1999, 4, 281-289. (c) Wetmur, J. G.;
Lehnert, G.; Desnick, R. J. EnViron. Res. 1991, 56, 109-119.
(9) Battersby, A. R.; Leeper, F. J. Top. Curr. Chem. 1998, 195, 143-193.
(10) A class of magnesium dependent ALAD enzymes is also known. See
ref 5 and Frankenberg, N.; Erskine, P. T.; Cooper, J. B.; Shoolingin-Jordan,
P. M.; Jahn, D.; Heinz, D. W. J. Mol. Biol. 1999, 289, 591-602.
(11) (a) Erskine, P. T.; Senior, N.; Awan, S.; Lambert, R.; Lewis, G.; Tickle,
L. J.; Sarwar, M.; Spencer, P.; Thomas, P.; Warren, M. J.; Shoolingin-Jordan,
P. M.; Wood, S. P.; Cooper, J. B. Nat. Struct. Biol. 1997, 4, 1025-1031. (b)
Erskine, P. T.; Norton, E.; Cooper, J. B.; Lambert, R.; Coker, A.; Lewis, G.;
Spencer, P.; Sarwar, M.; Wood, S. P.; Warren, M. J.; Schoolingin-Jordan, P.
M. Biochemistry 1999, 38, 4266-4276.
(12) It should be noted that ALAD enzymes contain more than one type
of metal binding site. For further discussion of structures of the various sites,
see ref 5.
(13) Lipscomb, W. N.; Stra¨ter, N. Chem. ReV. 1996, 96, 2375-2433.
(14) Parkin, G. Probing of Proteins by Metal Ions and Their Low-Molecular-
Weight Complexes. In Metal Ions in Biological Systems; Sigel, A., Sigel, H.,
Eds.; M. Dekker: New York, 2000; Vol. 38, in press.
The trigonal pyramidal structure of the lead center in {[TmPh]-
Pb}+ is in marked contrast to the tetrahedral zinc counterpart
{[TmPh]Zn(NCMe)}+.16 Such an observation indicates that, by
(15) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Chem.
Commun. 1999, 2301-2302.
(16) {[TmPh]Zn(NCMe)}(ClO4) is obtained by reaction of [TmPh]Li with
Zn(ClO4)2 in MeCN and has been structurally characterized by X-ray
diffraction. See Supporting Information.
(17) Furthermore, the Pb‚‚‚Pb separation of 3.99 Å is significantly greater
than twice the covalent radius of lead (3.08 Å).
(18) For a review of the structures of PbII complexes, see: Shimoni-Livny,
L.; Glusker, J. P.; Bock, C. W. Inorg. Chem. 1998, 37, 1853-1867.
(19) (a) Parr, J. Polyhedron 1997, 16, 551-566. (b) Holloway, C. E.;
Melnik, M. Main Group Met. Chem. 1997, 20, 399-495.
(20) (a) Dean, P. A. W.; Vittal, J. J.; Payne, N. C. Inorg. Chem. 1984, 23,
4232-6. (b) Christou, G.; Folting, K.; Huffman, J. C. Polyhedron 1984, 3,
1247-53.
(21) Simple [Pb(SAr)2] derivatives are polymeric. See, for example (a)
Krebs, B.; Brommelhaus, A.; Kersting, B.; Nienhaus, M. Eur. J. Solid State
Inorg. Chem. 1992, 29, 167-180. (b) Rae, A. D.; Craig, D. C.; Dance, I. G.;
Scudder, M. L.; Dean, P. A. W.; Kmetic, M. A.; Payne, N. C.; Vittal, J. J.
Acta Crystallogr., Sect. B 1997, 53, 457-465.
(22) Cambridge Structural Database (version 5.18). 3D Search and Research
Using the Cambridge Structural Database; Allen, F. H.; Kennard, O. Chemical
Design Automation News 1993, 8 (1), pp 1, 31-37.
(23) The average Pb-S bond length in [Pb(SPh)3]- (2.65 Å) (ref 20) is
comparable to that in {[TmPh]Pb} [2.693(2) Å].
10.1021/ja001530y CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/11/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7141
An understanding of the debilitating effects of lead also requires
a comprehension of the factors that distinguish the coordination
chemistry of zinc and lead. In particular, it is essential to quantify
the ability of lead to replace zinc in an environment which
resembles that of ALAD. For this reason, we have determined
the preference of [TmPh] to bind lead over zinc. Specifically, the
reaction between {[TmPh]Pb}(ClO4) and Zn(ClO4)2 in MeCN
(equation 1) may be conveniently monitored by 1H NMR
spectroscopy, thereby allowing determination of the equilibrium
constant (K).
The study indicates that the preference of [TmPh] to coordinate
PbII over ZnII in this system is ∼500:1,28 a value that is
substantially greater than the ∼25:1 relative affinity of these
metals to reside at the active site of human erythrocyte ALAD.29
Most interestingly, even though the preference for [TmPh] to
coordinate PbII is substantially greater than that for ZnII, zinc may
be readily induced to replace lead in {[TmPh]Pb}+ by addition of
Figure 1. Molecular structure of the cation {[TmPh]Pb}+.
1
NaI. Thus, H NMR spectroscopy demonstrates that addition of
comparison to zinc, trigonal pyramidal lead centers have a reduced
tendency to bind an additional ligand. In this regard, it should be
noted that MeCN is present as a solvent of crystallization in
{[TmPh]Pb}(ClO4), but does not bind to the three-coordinate lead
center. Consideration of the structures of other zinc and lead
complexes reinforces this notion that lead complexes have
significantly different structural preferences with respect to
tetrahedral and trigonal pyramidal coordination. Thus, whereas
tetrahedral coordination is most common for zinc in the solid
state,24 there are no examples of tetrahedral PbII complexes listed
in the CSD; in preference four-coordinate PbII complexes typically
have “hemidirected”25 geometries, with a “saw-horse” structure.18
Furthermore, calculations on a variety of four-coordinate PbII
complexes indicates that this geometry is favored over tetrahe-
dral.18,26
NaI to a mixture of {[TmPh]Pb}(ClO4) and Zn(ClO4)2 in aceto-
nitrile results in the formation of {[TmPh]Zn(NCMe)}+, ac-
companied by the deposition of yellow PbI2. In essence, the
equilibrium represented by eq 1 is shifted to the right by
precipitation of PbII as PbI2. The ability to sequester lead by this
simple method is of some importance in view of the fact that a
completely effective means to ameliorating the toxic effects of
lead in the human body is not yet known, despite efforts to
develop lead complexing agents.30
In summary, the use of the [TmPh] ligand has enabled the
replacement of zinc by lead to be studied in a synthetic analogue
system. Of particular relevance, a quantitative study demonstrates
that coordination of [TmPh] to PbII is a factor of ∼500 greater
than that for ZnII, and that the trigonal pyramidal geometry of
the lead center in the cation {[TmPh]Pb}+ is very similar to the
active site of PbII-ALAD. Finally, despite the fact that PbII shows
a greater preference to bind to [TmPh], ZnII may be induced to
displace PbII from {[TmPh]Pb}+ in the presence of iodide.
The reduced Lewis acidity of a trigonal pyramidal PbII versus
ZnII center is presumably associated with the stereochemically
active PbII lone pair which tempers its electrophilicity, and we
postulate that this is of significance to the inactivity of PbII-
ALAD. Specifically, of the two mechanisms of action that have
been proposed for ALAD, both involve activation of ALA by
interaction of the ketone group with the ZnII center.5,6b The degree
of such activation is clearly a function of the Lewis acidity of
the metal center,27 which is considerably greater for a trigonal
pyramidal ZnII center than for a corresponding PbII center. As
such, the formation of a tetrahedral species of the type [(Cys)3PbII-
ALA], a required intermediate in the proposed mechanisms of
action of ALAD, would be inhibited.
Acknowledgment. We thank the National Institutes of Health (Grant
GM46502) for support of this research, and also the National Science
Foundation and the U.S. Department of Energy for support via the
Environmental Molecular Sciences Institute at Columbia University (NSF
CHE-98-10367).
Supporting Information Available: Experimental and crystal-
lographic information (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA001530Y
(24) Bock, C. W.; Katz, A. K.; Glusker, J. P. J. Am. Chem. Soc. 1995,
117, 3754-3763.
(28) The equilibrium constant (K) as defined for eq 1, including MeCN as
(25) “Hemidirected” geometries are those in which the ligands are
distributed over only a portion of the coordination sphere, while “holodirected”
geometries are those in which the ligands are distributed more evenly about
the whole surface of the coordination sphere. See ref 18.
a reagent, is 1.1(4) × 10-4 M-1. If KZn and KPb are the respective binding
constants for coordination of [TmPh
]
to ZnII and PbII (neglecting MeCN as
-
a reagent), the equilibrium constant for eq 1 may be expressed as K ) KZn
{KPb[MeCN]}, thereby allowing the ratio KPb/KZn to be determined.
(29) Simons, T. J. B. Eur. J. Biochem. 1995, 234, 178-183.
/
(26) For example, the “hemidirected” geometry of [Pb(SH2)4]2+ is 9.5 kcal
mol-1 higher in energy than the tetrahedral structure. See ref 18.
(27) Vallee, B. L.; Auld, D. S. Acc. Chem. Res. 1993, 26, 543-551.
(30) See, for example: Abudari, K.; Karpishin, T. B.; Raymond, K. N.
Inorg. Chem. 1993, 32, 3052-3055.