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: Several routine indicators of nephrotoxicity , the enzymes alkaline phosphatase and N - acetyl - beta - glucosaminidase , and the molecular weight of protein excretion were determined on urine samples .
### Output Text:
Several routine indicators of nephrotoxicity , the enzymes alkaline phosphatase and N - acetyl - beta - glucosaminidase , and the molecular weight of protein excretion were determined on urine samples . 