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: The present data indicate that the determination of malondialdehyde , oxypurines , and nucleosides in peripheral blood , may be used to monitor the metabolic alterations of tissues occurring during ischemic phenomena . ( ABSTRACT TRUNCATED AT 250 WORDS )
### Output Text:
The present data indicate that the determination of <span class="chemical">malondialdehyde</span> , <span class="chemical">oxypurines</span> , and <span class="chemical">nucleosides</span> in peripheral blood , may be used to monitor the metabolic alterations of tissues occurring during ischemic phenomena . ( ABSTRACT TRUNCATED AT 250 WORDS ) 