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: RESULTS : The ' long - term ' group had significant glomerular and tubular lesions , depressed activities of mtDNA - encoded NADH dehydrogenase and cytochrome - c oxidase ( COX ) and increased citrate synthase activity .
### Output Text:
RESULTS : The ' long - term ' group had significant glomerular and tubular lesions , depressed activities of mtDNA - encoded NADH dehydrogenase and cytochrome - c oxidase ( COX ) and increased <span class="chemical">citrate</span> synthase activity . 