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: Peak urine NAG activity and a change in NAG isoenzyme pattern coincided with both the peak proteinuria and the reduction in intracellular protein and NAG droplets ( day six onwards ) .
### Output Text:
Peak urine NAG activity and a change in NAG isoenzyme pattern coincided with both the peak proteinuria and the reduction in intracellular protein and NAG droplets ( day six onwards ) . 