Complex formation of inorganic pyrophosphate with magnesium. The influence of ionic strength, supporting medium and temperature

Mogens Hørder*

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Abstrakt

The influence of the composition of the medium, appropriate to PPi-phosphohydrolytic activity determinations, on the distribution of the equilibrium species in mixtures of magnesium and PPi has been investigated. A spectrophotometric method, based on the competition of PPi and 8-quinolinol for magnesium, was used. 8-quinolinol and its complex with magnesium were characterized at various temperatures and ionic strengths, and in different supporting media. Magnesium binding by PPi was measured in 50 mM Tris-HCl buffer at pH 8.6. The apparent stability constant K′, decreased with increasing ionic strength, viz. from 2·105 M-1 at 10 mM to 0.4·105 M-1 at 250 mM. The relationship was linearised by the function log10K′(-log10f{hook} Mg2+ ), where f{hook}Mg2+ is the activity coefficient of Mg2+. Stability constants at zero ionic strength, estimated from extrapolation of the linear function, increased about 2-fold if the temperature was raised from 15 to 37 °C. At 37 °C and pH 8.6 the stability constant at zero ionic strength, obtained from the extrapolation of values determined in tetraethylammoniumbromide, was 4.7·105 M-1 (S.D: 0.3·105). Concentrations of PP4- i, MgPP2- i and Mg2+, calculated on the basis of the determined stability constants, were significantly influenced by changes in ionic strength and temperature. Therefore, stability constants should be measured at the same conditions as those of the PPi-phosphohydrolytic activity determination.

OriginalsprogEngelsk
BogserieBBA - Enzymology
Vol/bind358
Udgave nummer2
Sider (fra-til)319-328
ISSN0005-2744
DOI
StatusUdgivet - 26. aug. 1974
Udgivet eksterntJa

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