Damage of amino acids and proteins induced by nitrogen dioxide, a free radical toxin, in air

Abstract

Damage of amino acids and proteins induced by nitrogen dioxide, a free radical toxin in polluted air, was investigated. When nitrogen dioxide (30–90 ppm) in air was exposed to a solution of an amino acid at pH 7.5 for several hours, tryptophan and tyrosine were damaged. Degradation of tryptophan was accompanied by formation of a nitroindole derivative. Decrease of tyrosine was accompanied by formation of 3-nitrotyrosine and fluorescent dityrosine. When nitrogen dioxide was exposed to a solution of bovine serum albumin, human γ-globulin and bovine eye lens α-crystallin, the proteins were cross-linked by nondisulfide bonds. Tryptophan and tyrosine residues in the proteins were extensively decreased, and significant amounts of 3-nitrotyrosine and fluorescent dityrosine were formed. The modification of the proteins with nitrogen dioxide in air may have toxicological significance. Because fluorescent dityrosine is detected in a wide variety of natural proteins, nitrogen dioxide may play a role in its occurrence in natural proteins.