can i use music that i have purchased from itunes to play as a mobile dj?

Most likely not. You need  to read the "license" you agreed to when you downloaded the music from iTunes which details how you can "use" the downloaded music. The Terms of Use for iTunes can be found at http://www.apple.com/legal/itunes/us/terms.html#SALE, and the terms specifically say the following under the section "Usage Rules": "(i) You shall be authorized to use iTunes Products only for personal, noncommercial use." For you to play the downloaded content publicly for your commercial DJ use would violate these terms, which iTunes says in another part of the Terms of Use could be copyright infringement. For you to publicly play the same music you have downloaded from iTunes as a mobile DJ, you will need a license from a performing rights society, such as ASCAP or BMI, depending on whether the copyright owner of the music engaged one of them to administer the performance rights.