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: EDRF ( endothelium - derived relaxing factor ) / nitric oxide does not seem to play an important role in the peptide - produced hypotension .
### Output Text:
EDRF ( endothelium - derived relaxing factor ) / <span class="chemical">nitric oxide</span> does not seem to play an important role in the peptide - produced hypotension . 