A quantitative investigation of proton— and lead(II)—dithioerythritol complex equilibria under physiological conditions

Abstract

Dithioerythritol (DTE) is frequently employed as a reducing agent or a protective reagent for thiol groups in biological assays. Owing to its known inhibiting properties in enzyme catalysis reactions, lead is also commonly used in such experiments, and often simultaneously with DTE. Given the potential affinity of these two reactants, their measured individual effects may well depend on their interactions in the medium used. Any quantitative assessment of these interactions necessitates, however, that the complex equilibria between lead and DTE be investigated beforehand. To test this hypothesis, the formation constants of lead(II) complexes with DTE under physiological conditions (37°, NaCl 0.15M) have been calculated from the results of glass electrode potentiometry, with the help of the MINIQUAD and ESTA computer programs. The p K values for dissociation of DTE have been found equal to 8.926 ± 0.003 and 9.840 ± 0.003. The following lead—DTE species have been characterized: ML (12.774 ± 0.037), MLH −1 (2.858 ± 0.037), M 2LH −1 (13.349 ± 0.025) and M 6L 5 (86.586 ± 0.099); the log *β-values are given in the parentheses. Appropriate computer simulations effectively show that the interactions of the two reactants are indeed quite significant within the concentration ranges commonly used in in vitro biological assays. They should thus be taken into account in interpretation of the effects observed.