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 lowest myocardial VIP concentration was found in the hearts of patients with coronary disease ( one patient receiving a transplant and three receiving mitral prostheses ) ( 6 . 3 + / - 1 . 9 pg / mg protein ) .
### Output Text:
The lowest myocardial VIP concentration was found in the hearts of patients with coronary disease ( one patient receiving a transplant and three receiving mitral prostheses ) ( 6 . 3 + / - 1 . 9 pg / mg protein ) . 