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: Treatment with AG prevented the impairment of renal vascular bed responses and reduced both urine nitrite levels and apoptosis to control levels .
### Output Text:
Treatment with <span class="chemical">AG</span> prevented the impairment of renal vascular bed responses and reduced both urine <span class="chemical">nitrite</span> levels and apoptosis to control levels . 