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: Liver toxicity was evaluated based on serum levels of alpha - glutathione S - transferase ( alpha - GST ) and by histology .
### Output Text:
Liver toxicity was evaluated based on serum levels of alpha - <span class="chemical">glutathione</span> S - transferase ( alpha - GST ) and by histology . 