Chapter XI.

 

Biochemical Studies - DNA and Protein Binding

 

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Summary:

 

Uranium (uranyl salts) has been used for more than 40 years to stain DNA for electron microscopy.  Other stains were favored by electron microscopists, which may be due to the more recent findings (1990s) that uranyl ion can catalyze hydrolysis of DNA (strand breaks) in the presence of light.  The long-recognized fact that uranium binds to nucleic acids and nucleotides and catalyzes the chemical modification of these molecules would clearly implicate this heavy metal as a cytotoxin.

 

Details:

 

Zobel (1) presents the effects of pH, salt concentration and structural integrity of DNA on the chemical interaction of DNA with uranyl salts.

 

Huxley (2) showed in 1961 that DNA can take up nearly its own dry weight of uranyl acetate from fixing solution, indicating high affinity for uranium. Constantinescu (3) demonstrated that the uranyl ions bind through the phosphate groups forming the backbone of DNA.

 

Nielson (4) showed that visible light can cleave single-strand DNA that has been pretreated to uranyl ions. Nielson (5), Mollegard (6) and Sonnichsen (7) all reported studies showing further details relating to this reaction.

 

Tracz (8) in 1997 showed  that uranyl ion binds to paired helical filaments in nerve tissue, suggesting that uranyl ions may have neurological effects.

 

Ananyev (9) demonstrated that uranyl ion strongly inhibits photooxidation of Mn2+ in photosystem II indicating a potentially debilitating effect of uranium contamination in the environment and photosynthesis in exposed plants.

 

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1.            Electron Stains: I. Chemical Studies on the Interaction of DNA with Uranyl Salts, by R Zobel, et al., J. Biophys. Biochem. Cytol. Vol. 10, 1961 (pp. 336-346).

Discusses the effects of pH, salt concentration and structural integrity of DNA on the chemical interaction of uranyl salts, using spectrophotometric characterization of binding.

 [Zobel1961xxJBBCv10nxp336]

 

2.            Preferential staining of nucleic acid-containing structures for electron microscopy, by HE Huxley, et al., J. Biophys. Biochem. Cytol. Vol. 11, 1961 (pp. 273-296). 

Used uranyl acetate staining to reveal excellent fine structure in nucleohistone fibers.  Purified DNA can take up nearly its own dry weight of uranyl acetate from 2 % fixing solution, indicating high affinity.

[Huxley1961xxJBBCv11nxp273]

 

3.      Metachromasia through uranyl ions: a procedure for identifying the nucleic acids and the nucleotides, by DG. Constantinescu, et al., Anal. Biochem. Vol. 62, 1974 (pp. 584-587). 

Describe binding of uranyl ions through phosphate groups of nucleic acids and nucleotides.

[Constantinescu1974xxABv62nxp583]

 

4.      Uranyl salts as photochemical agents for cleavage of DNA and probing of protein-DNA contacts, by PE Nielsen, et al., FEBS Letts. Vol. 235, 1988 (pp. 122-124). 

Used uranyl acetate and nitrate to bind uranyl ion to single stranded DNA and induce nicks (cleavage) with visible light at 420 nm.  Nicks are random, but proteins bound to DNA protect that region.

[Nielsen1988xxFEBSLv235nxp122]

 

5.      DNA binding and photocleavage by uranyl salts, by PE Nielsen, et al., J. Amer. Chem. Soc. Vol. 114, 1992 (pp. 4967-4975). 

Describe binding of uranyl ion in the minor groove of DNA to induce photocleavage.  The binding constant is estimated to be 1010 M-1 at pH 4. Photocleavage not influenced by O2 and occurs either 3' or 5' to the deoxyribose.

[Nielsen1992xxJACSv114nxp4967]

 

6.      Uranyl photoprobing of a four-way DNA junction: evidence for specific metal ion binding, by NE Mĝllegaard, et al., EMBO J. Vol. 13, 1994 (pp. 1508-1513). 

Indicates uranyl, hexaminecobalt(III) and spermidine compete for the same high affinity binding site at a four-way DNA junction.

[Mollegaard1994xxEMBOJv13nxp1508].

 

7.            Enhanced uranyl photocleavage across the minor groove of all (A/T)4 sequences indicates a similar narrow minor groove conformation, by SH Sönnichsen, et al., J. Molec. Recogn. Vol. 9, 1996 (pp. 219-227).

Shows preferential cleavage of dsDNA in the minor groove at AT rich regions, i.e., least polar region of DNA molecules.

[Sonnichsen1996xxJMRv9nxp219]

 

8.            Paired helical filaments in corticobasal degeneration (CBD): the fine fibrillary structure with NanoVan, by E Tracz, et al., Brain Res. Vol. 773, 1997 (pp. 33-44).

Discuss using uranyl acetate, aurothioglucose, and NanoVan (vanadium compd) to identify CBD filaments by electron microscopy.  This shows that uranyl ion binds to paired helical filaments in nerve tissue.  It isn't certain what effect uranyl ion may have on neurodegenerative diseases.

[Tracz1997xxBRv773nxp33]

 

9.      Remarkable affinity and selectivity for Cs+ and uranyl (UO22+) binding to the manganese site of the apo-water oxidation complex of photosystem II, by GM Ananyev, et al., Biochemistry  Vol. 38, 1999 (pp. 7200-7209). 

Studied binding of several different alkali and alkaline earth (divalent) metal ions to apo-water oxidizing complex of PSII in spinach (that had Mn(II), Ca(II) and Cl- removed). Uranyl ion strongly inhibits photooxidation of Mn2+ in PSII. Uranyl Kd = 15.3 :M.  They argue that uranyl may block first or second photo-activation step, or alternatively accelerate the decay of IM1 (first oxidized intermediate). One must keep in mind that uranyl is a strong oxidant, just like the photooxidized Mn complex in PSII.

[Ananyev1999xxBv38nxp7200]

  

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