NONENZYMATIC CONVERSION OF 3,4-DIHYDROXYPHENYLALANINE TO 2,4,5-TRIHYDROXYPHENYLALANINE AND 2,4,5-TRIHYDROXYPHENYLALANINE QUINONE IN PHYSIOLOGICAL SOLUTIONS

2,4,5-Trihydroxyphenylalanine (TOPA) oxidizes in solution to form a qu inone derivative that is a non-N-methyl-D-aspartate agonist and neurot oxin. Although pathways have been postulated for the formation of both TOPA and TOPA quinone from closely related catecholamines, the genera tion of these compounds has not been conclusively demonstrated by anal ytical techniques. Reverse-phase HPLC with a dual electrode coulometri c detector was used to analyze TOPA containing solutions in an effort to rigorously characterize the behavior of this substance under physio logical conditions. Electrode potential, buffer system, and methanol c oncentration were varied to obtain optimal conditions to selectively d etect and quantify TOPA and TOPA quinone from closely related catechol amines. TOPA was shown to rapidly autoxidize to TOPA quinone by a proc ess dependent on pH. TOPA was the dominant species at acidic pHs (belo w 5-6), whereas TOPA quinone was dominant at physiological pHs. This c onversion was reversible upon acidification. In addition, we found tha t 3,4-dihydroxyphenylalanine can autoxidize to form both TOPA and TOPA quinone under physiological conditions. This partial conversion (0.5% ) is time dependent and can be substantially decreased (0.2%) in acidi c conditions (pH less-than-or-equal-to 3). These results suggest that some of the excitatory and excitotoxic properties that some investigat ors have attributed to DOPA may actually be due to its conversion to T OPA and TOPA quinone.