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: Pretreatment of mice with alpha - methyltyrosine ( 20 mg / kg i . p . , one hour ) , an inhibitor of tyrosine hydroxylase , significantly decreased the activity - increasing effects of morphine .
### Output Text:
Pretreatment of mice with <span class="chemical">alpha - methyltyrosine</span> ( 20 mg / kg i . p . , one hour ) , an inhibitor of <span class="chemical">tyrosine</span> hydroxylase , significantly decreased the activity - increasing effects of <span class="chemical">morphine</span> . 