Cyclic Octa- and Decapeptides as Ionophores for Magnesium

A rationally designed oligopeptide shows significant conformational changes upon binding to sulphate ions

Oligopeptides that interact with oxoanions were developed by rational design methods. The substrate-binding site of the enzyme purine nucleoside phosphorylase served as a model for the design of the ionophores. The amino acids involved in the complexation of oxoanions were linked through flexible spacer residues. These spacers were chosen such that the relative orientation of the interacting amino acids was conserved. Several peptide sequences were preselected based on intermolecular H-bond frequencies. These frequencies were calculated from molecular dynamics trajectories of the corresponding peptide-anion complexes and used to score the binding properties of the peptides. The most promising peptides were prepared using solid phase peptide synthesis. Anion binding of the peptide ionophores was screened using circular dichroism (CD) and confirmed by NMR spectroscopy. CD measurements performed in methanol revealed a significant conformational change of a linear undecapeptide upon binding to sulphate ions. Two-dimensional-NMR experiments confirmed that a conformation with high helical content is formed in the presence of sulphate ions. These conformational changes induced by the anion stimulate the development of new transduction mechanisms in chemical sensors.

Measurement of ionized magnesium with AVL 988/4 electrolyte analyzer: preliminary analytical and clinical results

Only free magnesium has biological activity: technology for measuring the ionized fraction of magnesium is now available via ion-selective electrodes. We have evaluated an instrument (AVL 988/4) which determines ionized magnesium (cMg2+) with an ion-selective electrode based on the ionophore ETH 7025. The selectivity of the electrode is adequate for the ions normally present in plasma, except for calcium: the interference is automatically corrected by simultaneous measurements of calcium with compensation for the calcium interference to the magnesium signal. We have first verified possible interference caused by sampling procedures: known silicon interference has been avoided by use of glass tubes (BD Vacutainer with no additive, code 7626); heparin interference has been measured and found significant above 20 UI.mL-1 of plasma. Instrument evaluation according to NCCLS protocol gives the following imprecision results on 20 replicated analyses: cMg2+ (mmol.L-1) 1.29, 0.76, 0.23, CVs% (within-run) 0.67, 0.67, 3.00 and CVs% (between-run) 4.06, 3.91, 5.89 respectively. Linearity (in the range 0.23-1.60 mmol.L-1) was: measured cMg2+ = 0.981.(calculated cMg2+) + 0.009 mmol/L; r = 0.999. In healthy adults (n = 103) cMg2+ was in the range 0.46-0.74 mmol.L-1 (with a mean of 0.60 mmol/L and normal distribution). These values represent 57% to 84% of serum total magnesium concentration (TMg) (mean 71%). pH dependence of cMg2+ is present, usually to a lower extent with respect to cCa2+, but it seems different in patients with real or in vitro provoked acidosis and in hemodialyzed patients. Citrate interference on ionized magnesium measurements was found both in vitro and in vivo, whilst that due to lactate was demonstrated only in vitro. On a wide range of cMg2+ (n = 100), a good correlation is obtained both with TMg and ultrafiltrable Mg (UFMg): cMg2+ = 0.723.TMg + 0.008 mmol.L-1, r = 0.978; cMg2+ = 0.912.UFMg + 0.10 mmol.L-1, r = 0.968, respectively. The ionized magnesium in ultrafiltrate was found 25% lower than that in serum. The lifespan of the electrode, evaluated on the basis of both time from installation and on number of measured samples, was estimated longer than 4 months and able to analyze more than 1500 samples, whichever comes first. The four electrodes we used during 18 months behaved all the same way. The correlation between measurements performed in whole blood (WB-cMg2+) and in the corresponding serum (S-cMg2+) was excellent: WB-cMg2+ = 0.954.S-cMg2+ +0.02 mmol.L-1; r = 0.998; n = 60.

The roles of magnesium in biotechnology

This review highlights the important roles played by magnesium in the growth and metabolic functions of microbial and animal cells, and therefore assigns a key role for magnesium ions in biotechnology. The fundamental biochemical and physiological actions of magnesium as a regulatory cation are outlined. Such actions are deemed to be relevant in an applied sense, because Mg2+ availability in cell culture and fermentation media can dramatically influence growth and metabolism of cells. Manipulation of extracellular and intracellular magnesium ions can thus be envisaged as a relatively simplistic, but nevertheless versatile, means of physiological cell engineering. In addition, biological antagonism between calcium and magnesium at the molecular level may have profound consequences for the optimization of biotechnological processes that exploit cells. In fermentation, for example, it is argued that the efficiency of microbial conversion of substrate to product may be improved by altering Mg:Ca concentration ratios in industrial feedstocks in a way that makes more magnesium available to the cells. With particular respect to yeast-based biotechnologies, magnesium availability is seen as being crucially important in governing central pathways of carbohydrate catabolism, especially ethanolic fermentation. It is proposed that such influences of magnesium ions are expressed at the combined levels of key enzyme activation and cell membrane stabilization. The former ensures optimum flow of substrate to ethanol and the latter acts to protect yeasts from physical and chemical stress.