Where can I find a website/database that provides a list of all the natural chemicals involved in regulating a receptor both directly and indirectly (through the pathway)?
For example, given the Histamine H1 receptor, i want to know it's antagonist and agonist molecules/receptors.
I have tried Kegg and Biosystems but I am not receiving a full list.
AFAIK, Pathway databases like KEGG as such don't report agonist or antagonist information. You may look in to IUPHAR database for literature curated agonist, antagonist information for receptors. Here is the link for the Histamine H1 receptor.
Other potential resources are STITCH which offers interaction networks of proteins and small molecules. A quick search is giving me the following figure that depicts the interaction of Histamine H1 receptor with various ligands.
Here also to understand a given interactant is agonist or antagonist you may need to read the background literature of each connection. You may also consult PubChem / ChEBI if you are interested in downloading the chemical structure of these agonists / antagonists.
I just looked up the SwissProt entry for Histamine H1 receptor in human. There is a link to databases called BindingDB and DrugBank. The SwissProt entry lists the following from DrugBank:
The DrugBank entry for Aceprometazine has too much info for me to put here. You need to check it to see if this is helpful. It may also be worth to take a look at BRENDA or other links for your protein directly on the expasy (SwissProt) website.