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: Reactive oxygen species have been implicated in the pathogenesis of acute puromycin aminonucleoside ( PAN ) - induced nephropathy , with antioxidants significantly reducing the proteinuria .
### Output Text:
Reactive <span class="chemical">oxygen</span> species have been implicated in the pathogenesis of acute <span class="chemical">puromycin aminonucleoside</span> ( <span class="chemical">PAN</span> ) - induced nephropathy , with antioxidants significantly reducing the proteinuria . 