Metal-ion-directed site-specificity of hydroxyl radical detection

Abstract

A wide variety of ·OH detectors are in use for determinatio of biological ·OH production. The chemical generation of ·OH is site-specific with respect to the metal-binding site, and thus ·OH detectors with metal-binding properties may affect the biological damage and bias ·OH detection. The present study shows that both salicylate and phenylalanine, added as low molecular weight ·OH indicators, decreased Cu(II) binding to erythrocyte ghosts. In a cell-free system, Cu(II) complexed to both salicylate and phenylalanine. Phenylalanine is a stronger Cu(II) chelator that salicylate, both when competing for Cu(II) bound to ghosts and when competing directly with each other. When OH radicals were generated by ascorbate and Cu(II), the amount of ·OH detected as dihydroxybenzoates was proportional to the amount of ·OH produced. However, when phenylalanine was added to this system, the efficiency of ·OH detection by salicylate strongly decreased, concomitant with the transfer of Cu(II) binding from salicylate to the amino acid. This decrease was larger than that predicted by calculations for random competition of the two detectors for ·OH. Deoxyribose and mannitol, which do not bind copper apperciably, competed poorly with salicylate for the ·OH. Hydroxylation fo phenylalanine, on the other hand, was only slightly affected by the presence of salicylate and unaffected by deoxyribose and mannitol. These results suggest that the detection of ·OH by low molecular weight ·OH indicators was related to the relative affinity of the detectors for the catalyzing, and thus partially site-specific. Furthermore, glutamate, which does not contain an aromatic ring but binds Cu(II) with considerable affinity, competed strongly with salicylate for the ·OH, indicating that metal-binding properties rather that the presence of an aromatic ring were the cause of the deviation from random competition. The results indicate that ·OH indicators with metal-binding properties affect the distribution of catalytic metal ions in a biological system, causing a shift of free radical damage and localizing a site-specific reaction of ·OH on these detectors, with a resulting positive bias in the apparent ·OH production.