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: Metabolic studies suggested a block in the kynurenine pathway of tryptophan metabolism .
### Output Text:
Metabolic studies suggested a block in the <span class="chemical">kynurenine</span> pathway of <span class="chemical">tryptophan</span> metabolism . 