Plasma proteins containing damaged L-isoaspartyl residues are increased inuremia: Implications for mechanism

Background. Several alterations of protein structure and function have been reported in uremia. Impairment of a transmethylation-dependent protein rep air mechanism possibly related to a derangement in homocysteine metabolism is also present in this condition, causing erythrocyte membrane protein dam age. Homocysteine may affect proteins via the accumulation of its parent co mpound S-adenosylhomocysteine (AdoHcy), a powerful in vivo methyltransferas e inhibitor. However, since plasma homocysteine is mostly protein bound, a direct influence on protein structures cannot be ruled out. We measured the levels of L-isoaspartyl residues in plasma proteins of uremic patients on hemodialysis. These damaged residues are markers of molecular age, which ac cumulate when transmethylation-dependent protein repair is inhibited and/or protein instability is increased. Methods. L-isoaspartyl residues in plasma proteins were quantitated using h uman recombinant protein carboxyl methyl transferase (PCMT). Plasma concent rations of homocysteine metabolites were also measured under different expe rimental conditions in hemodialysis patients. Results. The concentration of damaged plasma proteins was increased almost twofold compared to control (controls 147.83 +/- 17.75, uremics 282.80 +/- 26.40 pmol of incorporated methyl groups/mg protein, P < 0.003). The major protein involved comigrated with serum albumin. Although hyperhomocysteinem ia caused a redistribution of thiols bound to plasma proteins, this mechani sm did not significantly contribute to the increase in isoaspartyl residues . The S-adenosylmethionine (AdoMet)/AdoHcy concentration ratio, an indicato r of the Aux of methyl group transfer, was altered. This ratio was partiall y corrected by folate treatment (0.385 +/- 0.046 vs. 0.682 +/- 0.115, P < 0 .01), but protein L-isoaspartate content was not. Conclusions. Plasma protein damage, as determined by protein L-isoaspartyl content, is increased in uremia. This alteration is to be ascribed to an in creased protein structural instability, rather than the effect of hyperhomo cysteinemia.