N-Acetyl-D-tyrosinol

During reaction of N-acetyl-l-tyrosine with the nitrating system (15)NO(-)(2)/H(2)O(2)/HRP at pH 7.5, emission is observed in the (15)N NMR signals of 3-nitro-N-acetyl-l-tyrosine and the 1-nitrocyclohexa-2,5-dien-4-one derivative of N-acetyl-l-tyrosine. The occurrence of (15)N CIDNP proves the formation of radical pairs. The nitration products are formed by recombination of (15)NO(*)(2) and N-acetyl-l-tyrosinyl radicals Tyrac(*). The (15)N NMR signals of (15)NO(-)(2) and of (15)NO(-)(3) show enhanced absorption. The (15)N CIDNP effects are built up in radical pairs [(15)NO(*)(2), Tyrac(*)](F) formed by diffusive encounters of the radicals. Nuclear polarization is transferred into (15)NO(-)(2) and (15)NO(-)(3) by disproportionation reactions of (15)NO(*)(2) and by H abstraction of (15)NO(*)(2) from N-acetyl-l-tyrosine. If N-acetyl-l-tyrosine is not present, (15)NO(-)(3) does not show (15)N CIDNP, indicating that peroxynitrite is not formed as an intermediate during the reaction. Peroxynitrate is not formed either, which is indicated by the absence of (15)N CIDNP during reaction or a (15)N NMR signal after reaction. Emission also appears in nitrated tyrosine residues of bovine serum albumin during reaction of albumin with the (15)NO(-)(2)/H(2)O(2)/HRP system. Analogous but smaller effects are observed during reaction of the nitrating system (15)NO(-)(2)/HOCl with N-acetyl-l-tyrosine at pH 7.5, proving the radical character of this reaction, too. The (15)N CIDNP effects are weak because of the low yield of nitration products.

Airway epithelium is a principal target for inhaled oxidants like cigarette smoke, which induce epithelial injury and thereby provoke pathogenesis of chronic airway diseases. Alterations in airway epithelial glutathione (GSH) metabolism are central in causing a loss of reducing environment, however, data are scarce on epithelial cells from larger bronchi. We showed a transient depletion of intracellular GSH in human bronchial epithelial cells after exposure to cigarette smoke condensate (CSC), which later followed by a prolonged elevation. Of the GSH-regulating enzymes, CSC increased mRNA expression of both catalytic (GCLC) and modifier (GCLM) subunits of glutamate-cysteine ligase. UPF1, a tetrapeptide GSH analogue, 4-methoxy-L-tyrosinyl-γ-L-glutamyl-L-cysteinyl-glycine, known to possess a 50-fold higher hydroxyl radical scavenging efficiency than does GSH, normalized the intracellular GSH level in the human bronchial epithelial cells under oxidative stress caused by CSC. UPF1 restored the GCLM and GSH reductase mRNA levels, which were significantly augmented by CSC treatment, back to the level of untreated control cells, referring to a successful establishment of control by UPF1 upon the over-accumulation of GSH. Moreover, UPF1 showed a significantly more potent antioxidant capacity than did N-acetyl-L-cysteine (NAC) and, compared to NAC, demonstrated a better potential to assure the whole GSH homeostasis in human bronchial epithelial cells. The current study suggests that UPF1 is capable of maintaining intracellular GSH level under CSC-induced oxidative stress in bronchial epithelial cells via balanced control over GSH-regulating enzymes, reflecting an improved perception of cellular redox conditions and thereby warranting improved adjustment of GSH accumulation.