Stronger prooxidative effects of chromium(VI) comparing to chromium(III) in endocrine and non-endocrine tissues with the thyroid being completely resistant to antioxidant protection

Chromium (Cr) is a harmful heavy metal pollutant. Cr(VI) is a group 1 carcinogen (carcinogenic to humans), whereas Cr(III) is a group 3 carcinogen (not classifiable as to its carcinogenicity to humans). Cr is also documented to be an endocrine disrupting chemical. The study aimed to check whether Cr(VI) compound (K₂Cr₂O₇) or Cr(III) compound (CrCl₃·6H₂O) induce oxidative damage to membrane lipids (lipid peroxidation, LPO) in homogenates of two endocrine (the thyroid and the ovary) and two non-endocrine (the kidney and the liver) tissues, and whether antioxidants, such as melatonin, indole-3-propionic acid (IPA) and 17β-estradiol, reveal protective effects. Of note, the healthy thyroid gland is characterized by relatively high oxidative stress. Porcine tissue homogenates were incubated in the presence of Cr(VI) (0.05-10.0 mM) or Cr(III) (5.0-200.0 mM) with/without melatonin (5 mM) or IPA (5 mM) or 17β-estradiol (1 mM). The malondialdehyde+4-hydroxyalkenals (MDA + 4-HDA) concentration (LPO index) was measured spectrophotometrically. Cr(VI) (≥0.1-1.25 mM) significantly increased LPO in all tissues but these damaging effects were not prevented by any antioxidant tested. In turn, Cr(III) (≥25 mM) also significantly increased LPO in all examined tissues. All antioxidants reduced Cr(III)-induced LPO in the ovary, kidney, and liver but had no protective effects in the thyroid. Our findings highlight chromium's strong prooxidative effects, especially in the thyroid. The inability of antioxidants to prevent Cr-induced damage, especially in the thyroid, underscores the need for further research to identify effective protective strategies.