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: Injury was positively correlated with superoxide production and the activities of nucleus - encoded mitochondrial or cytoplasmic enzymes .
### Output Text:
Injury was positively correlated with <span class="chemical">superoxide</span> production and the activities of nucleus - encoded mitochondrial or cytoplasmic enzymes . 