Kinetics and mechanism of the epimerization and decomposition of threonine in fossil foraminifera

Determination of alloThr/Thr ratios in foraminiferal tests ranging in age from Recent to ～ 700,000 yr B.P. has shown that threonine epimerization ratios approach a steady-state value of ～0.2. This is well below the equilibrium ratio of ～0.9 as measured in buffered aqueous solutions. It has also been found that the initial rate of decomposition of threonine in fossil foraminfera is about an order of magnitude faster than would be predicted on the basis of extrapolation from high temperature kinetic studies on free threonine in buffered aqueous solution. These results are apparently a consequence of metal cation catalysis of both the decomposition and the epimerization of free threonine, which is being continually released by the slow hydrolysis of peptide and proteinaceous material present in the fossil shells.