The Cleavage of Pyrophosphate by Human Serum apo-transferrin

Objective: To determine the rate constants for the cleavage of pyrophosphate to phosphate by human serum apotransferrin. Methods: The cleavage was followed by 31P NMR spectroscopy. The data for concentration changes of pyrophosphate (as determined from its NMR peak intensity) with time, in the absence and the presence of MgCl2, at different pH values, were fitted to give second order rate constants. Results: The rate constants at 312 K were 8.83×10^(-4)L•mmol^(-1)•h^(-1) at pH 6.85, 9.59×10^(-4)L•mmol ^(-1)•h^(-1) at pH 7.4, and 1.38×10^(-3)L• mmol^(-1)•h^(-1) at pH 8.15, for reactions of apo-transferrin (0.5-1.0mmol/L) with 5 molar equivalence of pyrophosphate. The rate constant increased to 1.21×10^(-3)L•mmol^(-1)•h^(-1) at pH 7.4, 312 K when 2mmo1/L MgCl2 was added. Conclusion: The cleavage of pyrophosphate to phosphate by human serum apo-transferrin is very slow and follows the second-order kinetics. Mg(superscript (2+)) can slightly enhance the rate of the cleavage.