Passage Oxidative Stress Occurs in Cells When Reactive Oxygen Species (ROS)

Passage
Oxidative stress occurs in cells when reactive oxygen species (ROS) such as H₂O₂, superoxide (O₂⁻) , and hydroxyl radicals (^•OH) interact destructively with biological molecules.  Several studies have indicated that histidine and certain peptides that contain histidine can decrease oxidative stress.A common marker of oxidative stress in cells is leakage of the enzyme lactate dehydrogenase (LDH) into the extracellular space.  Researchers incubated cultured rat cells (PC12 cells) with H₂O₂ in the presence of different tripeptides, each containing one alanine, one glycine, and one histidine residue.  Every possible sequence was tested, but the researchers found that the sequence GAH was the most effective in limiting extracellular LDH activity.  After obtaining these preliminary results, the following experiments were conducted.Experiment 1PC12 cells were incubated with or without 5 mM H₂O₂ and with varying concentrations of GAH for 4 hours.  Cells were centrifuged, and the supernatant (ie, extracellular material) was extracted and assayed for LDH activity (Figure 1) .
Figure 1  Extracellular LDH activity from PC12 cells exposed to H₂O₂ and varying concentrations of GAH.  p-values are relative to 5 mM H₂O₂ and 0 mg/mL GAH.Experiment 2LDH was purified and assayed for activity in the presence of 5 mM H₂O₂ and varying concentrations of GAH (Figure 2) .
Figure 2  Activity of purified LDH with varying concentrations of GAHExperiment 3PC12 cells were incubated with or without 5 mM H₂O₂ in the presence and absence of 1 mg/mL GAH.  Cells were then stained with the fluorescent marker TUNEL, which binds strongly to apoptotic cells.  The percentage of apoptotic cells in each condition is shown in Figure 3.
Figure 3  Percentage of apoptotic cells in the presence or absence of H₂O₂ and GAH
-The results of Experiment 3 best support which of the following statements?

A) 5 mM H₂O₂ induced over 70% of all PC12 cells to undergo apoptosis.
B) The effect of GAH on H₂O₂-induced apoptosis is dose dependent.
C) GAH decreases oxidative stress and apoptosis by reacting with H₂O₂.
D) 1 mg/mL of GAH fully counteracts induction of apoptosis by 5 mM H₂O₂.

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