newbie question: XmlDocument not serializable?

Hello all!
I have an orchestration which does the following:

Receives a request as type System.Xml.XmlDocument (Doc1)
Calls a C# class to parse this document to get the message subject
Uses a decision shape to determine what to do based on the message
subject.
For the path I am having problems with, it then:
Passes the OuterXml from the request to a C# class to process. This
calls a DB2 database using HIS, then builds an XML document to return
to the orchestration. (Doc2)
Maps Doc2 to the Oracle adapter schema for update.
Sends the message to an Oracle port
Receives OracleResponse from port
Assigns response to an Xml Document and returns it to calling
application.

When I try to run this, it gives me the error that
System.Xml.XmlDocument is not marked as serializable. From what I’ve
been able to find, Biztalk will persist the orchestration after the
send to the Oracle port, which is where the problem is coming in since
the XmlDocument is not serializable.

How can I get around this? Is there any way to make the XmlDocument
serializable? Or, is there a way to destroy the messages Doc1 and Doc2
before attempting to send to the Oracle port?

Thanks in advance,

Amy