Web API Accept XML Request and pass to another Web API as JSON

We have 2 Web API Layers one for public facing and another one is for internal facing which handles more complex logic etc. All request received by public facing are intercepted,transformed and passed to internal Web API Layer methods.

The Internal Web API only accepts JSON, the public facing can receive XML or JSON request. Can you help/guide how to transform the request posted from public web api into JSON format before it passed to internal facing web api for processing.