Hydrogen peroxide formation by reaction of peroxynitrite with HEPES and related tertiary amines. Implications for a general mechanism.
Organic amine-based buffer compounds such as HEPES (Good's buffers) are commonly applied in experimental systems, including those where the biological effects of peroxynitrite are studied. In such studies 3-morpholinosydnonimine N-ethylcarbamide (SIN-1), a compound that simultaneously releases nitric oxide (.NO) and superoxide (O-2), is often used as a source for peroxynitrite. Whereas in mere phosphate buffer H2O2 formation from 1.5 mM SIN-1 was low ( approximately 15 microM), incubation of SIN-1 with Good's buffer compounds resulted in continuous H2O2 formation. After 2 h of incubation of 1.5 mM SIN-1 with 20 mM HEPES about 190 microM H2O2 were formed. The same amount of H2O2 could be achieved from 1.5 mM SIN-1 by action of superoxide dismutase in the absence of HEPES. The increased H2O2 level, however, could not be related to a superoxide dismutase or to a NO scavenger activity of HEPES. On the other hand, SIN-1-mediated oxidation of both dihydrorhodamine 123 and deoxyribose as well as peroxynitrite-dependent nitration of p-hydroxyphenylacetic acid were strongly inhibited by 20 mM HEPES. Furthermore, the peroxynitrite scavenger tryptophan significantly reduced H2O2 formation from SIN-1-HEPES interactions. These observations suggest that peroxynitrite is the initiator for the enhanced formation of H2O2. Likewise, authentic peroxynitrite (1 mM) also induced the formation of both O-2 and H2O2 upon addition to HEPES (400 mM)-containing solutions in a pH (4.5-7.5)-dependent manner. In accordance with previous reports it was found that at pH >/=5 oxygen is released in the decay of peroxynitrite. As a consequence, peroxynitrite(1 mM)-induced H2O2 formation ( approximately 80 microM at pH 7.5) also occurred under hypoxic conditions. In the presence of bicarbonate/carbon dioxide (20 mM/5%) the production of H2O2 from the reaction of HEPES with peroxynitrite was even further stimulated. Addition of SIN-1 or authentic peroxynitrite to solutions of Good's buffers resulted in the formation of piperazine-derived radical cations as detected by ESR spectroscopy. These findings suggest a mechanism for H2O2 formation in which peroxynitrite (or any strong oxidant derived from it) initially oxidizes the tertiary amine buffer compounds in a one-electron step. Subsequent deprotonation and reaction of the intermediate alpha-amino alkyl radicals with molecular oxygen leads to the formation of O-2, from which H2O2 is produced by dismutation. Hence, HEPES and similar organic buffers should be avoided in studies of oxidative compounds. Furthermore, this mechanism of H2O2 formation must be regarded to be a rather general one for biological systems where sufficiently strong oxidants may interact with various biologically relevant amino-type molecules, such as ATP, creatine, or nucleic acids.