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: These observations indicate that puromycin aminonucleoside alters plasma lipoproteins by lowering VLDL and increasing HDL .
### Output Text:
These observations indicate that <span class="chemical">puromycin aminonucleoside</span> alters plasma lipoproteins by lowering VLDL and increasing HDL . 