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: An increase in GSSG within hepatic tissue during intestinal reperfusion suggests exposure of hepatocytes to an oxidant stress .
### Output Text:
An increase in <span class="chemical">GSSG</span> within hepatic tissue during intestinal reperfusion suggests exposure of hepatocytes to an oxidant stress . 