Zinc Interaction

Further study of the zinc-cadmium interaction indicated that a more drastic reduction in the frequency of brain defects could be accomplished if supplementary zinc was injected at 8-11 h after the cadmium injection (Table 2). Under these conditions, the frequency of eye defects was reduced to the level characteristic for BIO mice. Comparison of data obtained when all of the zinc supplement was provided at 8 h post-cadmium injection with data 

Data presented in Table 3 suggest that the importance of the timing of supplemental treatment with zinc relative to the time of cadmium injection is dependent on the embiyonic genotype. Differential responses to simultaneous injection of zinc and cad- 

Cadmium has been shown to be teratogenic in hamsters, mice, and rats, under experimental situations involving either embryonic exposure in utero or challenge in an in vitro test system. Interference with normal development of the brain and eye rudiments is an important manifestation of cadmium teratogenicity, but malformations may also be observed in other soft tissues and in the skeleton. Specific defects observed are related to developmental stage at the time of exposure. 

The following have participated in the research efforts reported from this laboratory Jean S. Haines, John Miska (deceased), Timothy Charlesbois, and Katherine Maurer. Clerical and typing assistance were provided by Julie Harley, Elizabeth Pastor and Joan Landon. 

Scientific Contribution No. 974 of the Storrs Agricultural Experiment Station. 

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Imamura (11,20,21) synthesized several similar perpendicular dimers exploiting axial coordination of the 4-pyridyl free-base porphyrin to Ru(II)CO (3) and Os(II)CO (4) porphyrins (Fig. 1). The pyridine-ruthenium and pyridine-osmium interactions are much stronger than the pyridine-zinc interaction, and the complexes are perfectly stable in solution and can be isolated by precipitation. One of the ruthenium dimers was characterized by FAB-MS (11). Complexation is accompanied by characteristic changes in JH NMR chemical shift, indicating... 

Smith (91) reported an X-ray crystal structure of a zinc porphyrin polymer (77, Fig. 32) where, unusually, the coordination bond is between a nitro group and the zinc center. The tetranitroporphyrin is highly substituted, and the resulting steric hindrance causes the macrocycle to be noticeably distorted. Adjacent porphyrin planes in the polymer are almost orthogonal. However, there is no evidence of polymerization in solution, and the nitro-zinc interaction is probably too weak to maintain this structure outside the solid state. 

Antimony, arsenic, selenium, tellurium, iridium, iron, molybdenum, osmium, potassium, rhodium, tungsten (and when primed with charcoal,) aluminium, copper, lead, magnesium, silver, tin, zinc. Interaction of lithium or calcium with chlorine tri- or penta-fluorides is hypergolic and particularly energetic. 

Zinc interacts with numerous chemicals, sometimes producing greatly different patterns of accumulation, metabolism, and toxicity when compared to zinc alone. Recognition of these interactions is essential to the understanding of zinc kinetics in the environment. 

Cadmium-zinc interactions are typical in that sometimes they act to the organism s advantage and sometimes not, depending on the organism, its nutritional status, and other variables. 

Current research needs include the development of protocols to (1) separate, quantitate, and verify the different chemical species of zinc (2) identify natural from anthropogenic sources of zinc (3) establish toxicity thresholds based on accumulation (4) evaluate the significance of tissue concentrations in target organs as indicators of zinc stress and (5) measure the long term consequences of zinc interactions with other nutrients using animals of various age and nutrient status. 

Wicklund, A., P. Runn, and L. Norrgren. 1988. Cadmium and zinc interactions in fish effects of zinc on the uptake, organ distribution, and elimination of 109Cd in the zebrafish, Brachydanio rerio. Arch. Environ. Contam. Toxicol. 17 345-354. 

Work from Sturtevant s laboratory detailed the kinetics and thermodynamics of zinc binding to apocarbonic anhydrase (carbonate dehydratase) selected data are recorded in Table II (Henkens and Sturtevant, 1968 Henkens etal., 1969). The thermodynamic entropy term A5 at pH 7.0 is 88 e.u. (1 e.u. = 1 cal/mol-K), and this is essentially matched by the binding of zinc to the hexadentate ligand cyclohexylenediamine tetraacetate where AS = 82 e.u. At pH 7.0 the enthalpy of zinc-protein association is 9.8 kcal/mol, but this unfavorable term is overwhelmed by the favorable entropic contribution to the free energy (AG = AH - T AS), where —TAS = -26.2 kcal/mol at 298 K (25°C). Hence, the kinetics and thermodynamics of protein-zinc interaction in this example are dominated by very favorable entropy effects. 

Currently, only a handful of examples of unique protein carboxylate-zinc interactions are available in the Brookhaven Protein Data Bank. Each of these entries, however, displays syn coordination stereochemistry, and two are bidentate (Christianson and Alexander, 1989) (Fig. 5). Other protein structures have been reported with iyw-oriented car-boxylate-zinc interactions, but full coordinate sets are not yet available [e.g., DNA polymerase (Ollis etal., 1985) and alkaline phosphatase (Kim and Wyckoff, 1989)]. A survey of all protein-metal ion interactions reveals that jyw-carboxylate—metal ion stereochemistry is preferred (Chakrabarti, 1990a). It is been suggested that potent zinc enzyme inhibition arises from syn-oriented interactions between inhibitor carboxylates and active-site zinc ions (Christianson and Lipscomb, 1988a see also Monzingo and Matthews, 1984), and the structures of such interactions may sample the reaction coordinate for enzymatic catalysis in certain systems (Christianson and Lipscomb, 1987). 

Fig. 5. (a and b) Two perpendicular orientations of direct carboxylate-zinc interactions retrieved from four metalloprotein structures contained in the Brookhaven Protein Data Bank. Orientation (a) represents the carboxylate group as found in Fig. 3. The coordination stereochemistry is syn for each example.

Fig. 18. The carboxylaie-histidine-zinc triad represents indirect carboxylate-zinc interaction across bridging histidine. Both tautomers of histidine are observed, and the hydrogen bond stereochemistry with carboxylate (either aspartate or glutamate) is generally syn. Experimental results and theoretical calculations suggest that the carboxylate-histidine- zinc form may be in equilibrium with the carboxylic acid-histidinate- zinc form, as shown.

Fig. 20. Schematic diagram of indirect carboxylate-zinc interactions through bridging hydroxyl groups, as observed in the (a) zinc proteases and (b) carbonic anhydrases.

There may be two proton transfers in the carbonic anhydrase II-catalyzed mechanism of CO2 hydration that are important in catalysis, and both of these transfers are affected by the active-site zinc ion. The first (intramolecular) proton transfer may actually be a tautomerization between the intermediate and product forms of the bicarbonate anion (Fig. 28). This is believed to be a necessary step in the carbonic anhydrase II mechanism, due to a consideration of the reverse reaction. The cou-lombic attraction between bicarbonate and zinc is optimal when both oxygens of the delocalized anion face zinc, that is, when the bicarbonate anion is oriented with syn stereochemistry toward zinc (this is analogous to a syn-oriented carboxylate-zinc interaction see Fig. 28a). This energetically favorable interaction probably dominates the initial recognition of bicarbonate, but the tautomerization of zinc-bound bicarbonate is subsequently required for turnover in the reverse reaction (Fig. 28b). 

The stereochemistry of anion-zinc interactions is important as inhibitors interact with the active site of carboxypeptidase A. For instance. 

Fig. 34. Glu-72- Zn interactions in native carboxypeptidase A and in carboxypep-tidase A-inhibitor complexes (inhibitors have been reviewed by Christianson and Lipscomb, 1989). When substrates or inhibitors bind to the enzyme active site and interact with the zinc ion, the interaction of the metal with Glu-72 tends from bidentate toward uniden-tate coordination. The flexibility of protein-zinc coordination may be an important aspect of catalysis in this system, and the Glu-72->Zn - coordination stereochemistry observed here is consistent with the stereochemical analysis of carboxylate-zinc interactions from the Cambridge Structural Database (Carrell et al., 1988 see Fig. 4).

Other multinuclear protein-zinc interactions may be implicated for the GAL4 protein in vivo. For example, since the GAL4 dimer binds to palindromic DNA sequences (Giniger et al, 1985), one possibility is that the dimer interface of the intact protein could comprise three zinc ions tetrahedrally coordinated by the 12 cysteines of two GAL4 monomers... 

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