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: Western blotting studies have indicated that the increase in the P450 enzymes could be due to the increase in the expression of P450 1A1 / 1A2 , 2B1 / 2B2 and 2E1 isoenzymes .
### Output Text:
Western blotting studies have indicated that the increase in the P450 enzymes could be due to the increase in the expression of P450 1A1 / 1A2 , 2B1 / 2B2 and 2E1 isoenzymes . 