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: Lometrexol is an antifolate which inhibits glycinamide ribonucleotide formyltransferase ( GARFT ) , an enzyme essential for de novo purine synthesis .
### Output Text:
<span class="chemical">Lometrexol</span> is an antifolate which inhibits <span class="chemical">glycinamide ribonucleotide</span> formyltransferase ( GARFT ) , an enzyme essential for de novo <span class="chemical">purine</span> synthesis . 