Applications of Synchrotron Radiation to the Structure, Localization, and Quantitation of Zinc in Biological Systems.

Abstract

This thesis describes the application of synchrotron X-ray sources for localization, quantification and characterization of biological zinc sites. Erythrocytes infected with the malaria parasite Plasmodium falciparum are known to accumulate zinc ~3-fold over basal levels. Attempts to reduce parasite viability through the use of zinc chelators had shown previously that the membrane permeable chelator TPEN was several hundred-fold more effective than the impermeable chelator EDTA, despite these having similar zinc binding affinities. XRF imaging data presented here indicates that both chelators were able to prevent zinc uptake in infected erythrocytes and suggests that parasites cope with the lack of extracellular zinc by making greater use of host zinc stores. EXAFS studies indicated that zinc sites in zinc-depleted, but viable, EDTA-treated samples were significantly more sulfur-rich compared to TPEN-treated samples and untreated controls, suggesting that changes in zinc speciation are another method of coping with zinc loss.

Apoptotic cells exhibit early characteristic volume decrease, thought to be linked to small ion fluxes. These fluxes are commonly studied through the use of fluorophores, which lack element specificity and which probe only the labile ion pools. XRF imaging revealed increases in chloride and decreases in calcium not previously recorded in the literature. Within 140 minutes after exposure to staurosporine, potassium and chloride levels were identical to those of the surrounding media. In addition, calcium was observed to co-localize with zinc in the cytosol, although the target of this localization could not be determined.

Although EXAFS can yield accurate zinc active site structures, the technique lacks sensitivity to ligand identity. Kβ2,5 (valence-to-core) X-ray fluorescence was used to study structurally-characterized zinc active site mimics. Zinc carboxylates, imidazoles, and thiolates were all found to have characteristic Kβ2,5 lineshapes, which open up the possibility of site-selective EXAFS.