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: However , the involvement of a specific , terminal prostaglandin ( PG ) isomerase has not been evaluated .
### Output Text:
However , the involvement of a specific , terminal <span class="chemical">prostaglandin</span> ( <span class="chemical">PG</span> ) isomerase has not been evaluated . 