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: Debrisoquine phenotype and the pharmacokinetics and beta - 2 receptor pharmacodynamics of metoprolol and its enantiomers .
### Output Text:
<span class="chemical">Debrisoquine</span> phenotype and the pharmacokinetics and beta - 2 receptor pharmacodynamics of <span class="chemical">metoprolol</span> and its enantiomers . 