Tetracyclines Metal-binding

Bone. Bone acts as a storage site for several toxic agents, especially heavy metals like lead. Also, drugs such as the tetracyclines, which bind to and form molecular complexes with the crystal components within the skeletal matrix, are stored within bone. 

Xanthate-modified cellulose binds metal-TC tetracycline complexes and is evaluated to be an effective controlled release system for TCs complexes, owing to the metal-binding properties of these antibiotics. ... 

The nitroimidazoles, sulfonamides, and tetracyclines all present analytical challenges because of metabolism and/or chemical degradation. In the case of the nitroimidazoles, this is further complicated by the relatively low requirements for detection. Method development therefore has to take into account both metabolites as additional target compounds and low detection limits. Sulfonamide analysis has to take into account the potential for conversion of N -acetyl metabolites back to the parent compound. In contrast, in the analysis of honey, deconjugation is regarded as necessary to accurately determine sulfonamide concentrations. The facile, reversible formation of epimers is of particular concern in the analysis of those tetracyclines that can epimerize in the 4 position. Protein and metal binding are other issues that have to be overcome for successful tetracycline residue determination. 

All of these methods fail to show the presence of the epimeric form of the tetracyclines and in most instances streaking of the spots is a problem. A basic improvement in the paper chromatography of these antibiotics was achieved by Selzer and Wright (47) and Kelly and Bryske (48) when they reported methods for the pretreatment of the paper with com-plexing agents to bind the metallic ions which may be present. 

The tetracyclines are well known for their ability to form complexes with polyvalent cations. This property changes their solubility characteristics in the mobile solvents and often results in troublesome streaking. To overcome this difficulty, Selzer and Wright used paper dipped in Mcllvaine s buffer (pH 3.5) which contains citrate ions capable of binding the metallic ions. The chromatograms were developed with a mixture of nitromethane, chloroform, and pyridine (20 10 3) on paper still damp from the treatment with the buffer solution. 

Antibiotics The AOAC has listed methods for sulfamethazine residues in swine tissues with determination either by GC-MS or GC-ECD of methylated derivatives and for sulfamethazine (and for the class of sulfonamides) in milk with determination by HPLC-UV. There is an AOAC method for the class of sulfonamide antimicrobials in animal tissues using solvent extraction and liquid partitioning with determination by TLC and fluorimetric scanning. For analysis of tetracyclines, AOAC describes methods based on buffer extraction from tissue samples and SPE (Cis) cleanup, or metal chelate affinity binding from milk samples, with determination in both cases by HPLC-UV. USDA/FSIS methods include (1) a method (similar to the AOAC GC-MS method for sulfamethazine) for confirmation of sulfonamide residues in edible tissues using solvent extraction and multiple liquid partitioning with determination of the methylated derivatives by GC-MS (2) methods for determination and confirmation of chloramphenicol in muscle by solvent extraction, liquid partitioning, and determination of the trimethylsilane (TMS) derivative by GC-ECD and GC-MS, respectively and (3) a method for determination of the beta-lactam antibiotic amoxicillin by aqueous extraction, cleanup by tricarboxylic acid precipitation, and ether extraction and formation of a fluorescent derivative for determination by LC. 

Evidence from fluorescence, absorption, and c.d. spectra suggests that the conformations of various tetracycline antibiotics in complexes with Ca + are different from those in the magnesium complexes. The optical properties of the catalytically active cobalt(n) form of liver alcohol dehydrogenase, which normally requires zinc for activity, indicate an unusual binding environment for the metal (c/. the implications of the absorption and c.d. spectra of the cobalt proteases, which have recently been discussed ). 

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