URINARY PORPHYRINS AS BIOLOGICAL INDICATORS OF OXIDATIVE STRESS IN THE KIDNEY - INTERACTION OF MERCURY AND CEPHALORIDINE

Reduced porphyrins (hexahydroporphyrins, porphyrinogens) are readily o xidized in vitro by free radicals which are known to mediate oxidative stress in tissue cells. To determine if increased urinary porphyrin c oncentrations may reflect oxidative stress to the kidney in vivo, we m easured the urinary porphyrin content of rats treated with mercury as methyl mercury hydroxide (MMH) or cephaloridine, both nephrotoxic, oxi dative stress-inducing agents. Rats exposed to MMH at 5 ppm in the dri nking water for 4 weeks showed a 4-fold increase in 24-hr total urinar y porphyrin content and a 1.3-fold increase in urinary malondialdehyde (MDA), an established measure of oxidative stress in vivo. Treatment with cephaloridine alone (10-500 mg/kg, i.p.) produced a dose-related increase in urinary MDA and total porphyrin levels up to 1.6 and 7 tim es control values, respectively. Injection of MMH-treated rats with ce phaloridine (500 mg/kg) caused a synergistic (20-fold) increase in uri nary porphyrin levels, but an additive (1.9-fold) increase in the MDA concentration. Studies in vitro demonstrated that cephaloridine stimul ated the iron-catalyzed H2O2-dependent oxidation of porphyrinogens to porphyrins in the absence of either microsomes or mitochondria. Additi onally, porphyrinogens were oxidized to porphyrins in an iron-dependen t microsomal lipid peroxidation system. Moreover, porphyrinogens serve d as an effective antioxidant (EC(50) similar to 1-2 mu M) to lipid pe roxidation. These results demonstrate that MMH and cephaloridine syner gistically, as well as individually, promote increased oxidation of re duced porphyrins in the kidney and that this action may be mechanistic ally linked to oxidative stress elicited by these chemicals. Increased urinary porphyrin levels may, therefore, represent a sensitive indica tor of oxidative stress in the kidney in vivo.