Multidirectional Changes in Parameters related to Sulfur Metabolism in Frog Tissues exposed to Heavy Metal-related Stress.

The investigations showed changes of the cystathionine gamma-lyase (CTH), 3-mercaptopyruvate sulfurtransferase (MPST) and rhodanese (TST) activity and gene expression in the brain, heart, liver, kidney, skeletal muscles and testes in frogs Pelophylax ridibundus, Xenopus laevis and Xenopus tropicalis in response to Pb2 +, Hg2+ and Cd2+ stress. The results were analyzed jointly with changes in the expression of selected antioxidant enzymes (cytoplasmic and mitochondrial superoxide dismutase, glutathione peroxidase, catalase and thioredoxin reducatase) and with the level of malondialdehyde (a product of lipid peroxidation). The obtained results allowed for confirming the role of sulfurtransferases in the antioxidant protection of tissues exposed to heavy metal ions. Our results revealed different transcriptional responses of the investigated tissues to each of the examined heavy metals. The CTH, MPST and TST genes might be regarded as heavy metal stress-responsive. The CTH gene expression up-regulation was confirmed in the liver (Pb2+, Hg2+, Cd2+) and skeletal muscle (Hg2+), MPST in the brain (Pb2+, Hg2+), kidney (Pb2+, Cd2+), skeletal muscle (Pb2+, Hg2+,Cd2+) and TST in the brain (Pb2+) and kidney (Pb2+, Hg2+, Cd2+). Lead, mercury and cadmium toxicity was demonstrated to affect the glutathione (GSH) and cysteine levels, the concentration ratio of reduced to oxidized glutathione ([GSH]/[GSSG]) and the level of sulfane sulfur-containing compounds, which in case of enhanced reactive oxygen species generation can reveal their antioxidative properties. The present report is the first to widely describe the role of the sulfane sulfur/H2S generating enzymes and the cysteine/glutathione system in Pb2+, Hg2+ and Cd2+ stress in various frog tissues, and to explore the mechanisms mediating heavy metal-related stress.