In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The data of the present study suggest that S . virgaurea extract exerts its protective effect by decreasing MDA level and increasing the antioxidant status in isoproterenol - treated rats .
### Output Text:
The data of the present study suggest that S . virgaurea extract exerts its protective effect by decreasing <span class="chemical">MDA</span> level and increasing the antioxidant status in <span class="chemical">isoproterenol</span> - treated rats . 