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: A correlation was found between serum acid protease activity and prothrombin time , and the increase in cathepsin D activity was sustained over several days compared with aspartate aminotransferase , which showed a sharp early peak and then a fall .
### Output Text:
A correlation was found between serum acid protease activity and prothrombin time , and the increase in cathepsin D activity was sustained over several days compared with <span class="chemical">aspartate</span> aminotransferase , which showed a sharp early peak and then a fall . 