Lowell police have arrested two teenagers who are accused of stealing a purse from a 58-year-old woman in a wheelchair last week. Elvin Alvarez, 18, of Lowell, pleaded not guilty in Lowell District Court on Wednesday to charges of receiving stolen property, operating a vehicle without a license, and unarmed robbery, said Stephanie Chelf Guyotte, a spokeswoman for Middlesex District Attorney Gerard T. Leone Jr. Details about a 16-year-old suspect were not available because he is being tried as a minor, Guyotte said. On March 27, one of the boys allegedly stood as lookout while the other ran up behind the woman as she was making her way down South Street and ripped the purse out of her hands, a Lowell police statement said.Todd Feathers can be reached at firstname.lastname@example.org. Follow him on Twitter at @ToddFeathers.