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: Oxidized glutathione ( GSSG ) increased significantly following 30 and 60 min of reperfusion .
### Output Text:
<span class="chemical">Oxidized glutathione</span> ( <span class="chemical">GSSG</span> ) increased significantly following 30 and 60 min of reperfusion . 