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: Proteinuria is a side effect of captopril treatment in hypertensive patients .
### Output Text:
Proteinuria is a side effect of <span class="chemical">captopril</span> treatment in hypertensive patients . 