I'm looking for the table in which Activity assignee contact information is stored in mysql.
In most cases you would not use direct sql queries to look up or manipulate CiviCRM data.
Activity contacts, like most CiviCRM data, has an api which you can use to read, write, update and delete contacts in an activity.
You can browser this api on your own site using the Api Explorer:
Depends on your version. From CiviCRM 4.4 the activity assignee can be found in the table civicrm_activity_contact, where record_type_id = 1 is 1 for source contact, 2 for assignee contact and 3 for target contact.