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: Phenylalanine , a large neutral amino acid ( LNAA ) competing with levodopa for transport across the blood - brain barrier , reduced the hypotensive and antiparkinsonian effects of levodopa .
### Output Text:
<span class="chemical">Phenylalanine</span> , a large neutral <span class="chemical">amino acid</span> ( LNAA ) competing with <span class="chemical">levodopa</span> for transport across the blood - brain barrier , reduced the hypotensive and antiparkinsonian effects of <span class="chemical">levodopa</span> . 