The effect of alpha-tocopherol on the nitration of gamma-tocopherol by peroxynitrite

It has been proposed (S. Christen ct al. Proc. Natl. Acad. Sci. USA 94, 321 7-3222, 1997) that although alpha-tocopherol (alpha-TH) is an efficient ant ioxidant, the presence of gamma-tocopherol (gamma-TH) may be required to sc avenge peroxynitrite-derived reactive nitrogen species. To investigate the reactions between alpha-TH, gamma-TH, and peroxynitrite, endogenous levels of both alpha-TH and gamma-TH were monitored when low-density lipoprotein w as oxidized in the presence of the peroxynitrite generator 5-amino-3-(4-mor pholinyl)-1,2,3-oxadiazolium (SIN-1). SIN-1 oxidized alpha-TH while gamma-T H levels remained constant, The sparing of gamma-TH was also demonstrated w hen 1,2-dilauroyl-sn-glycero-3-phosphocholine liposomes containing alpha-TH and gamma-TH were incubated with either SIN-1 or peroxynitrite. Our data s how that alpha-TH inhibits peroxynitrite-mediated gamma-TH nitration, i.e., 5-NO2-gamma-tocopherol formation. The rate constants for the reactions bet ween both alpha-TH and gamma-TH with peroxynitrite suggest that the sparing of gamma-TH by alpha-TH does not occur by competitive scavenging, but may be due to the formation of a transient gamma-TH intermediate. Nitration of gamma-TH becomes significant. cant only after alpha-TH levels have been dep leted. We conclude alpha-TH alone is sufficient to remove any peroxynitrite -derived reactive nitrogen species, as the presence of alpha-TH attenuates nitration of both gamma-TH and tyrosine. The present results also indicate that a bolus addition of peroxynitrite or SIN-1 to liposomes containing gam ma-TH forms 5-NO2-gamma-tocopherol in similar yields. This is in contrast t o their reaction profile with tyrosine in aqueous solution. Under these con ditions, SIN-1 does not form nitrotyrosine at detectable yields. (C) 1999 A cademic Press.