After qualifying as such neither the commissioner nor any of the deputies, clerks or employees of the department shall be interested in any mineral, oil or gas business, mineral, oil or gas brokerage firm, real estate company or any real estate brokerage firm, as director, stockholder, officer, member, agent or employee, or act as a broker or salesman, or act as a co-partner or agent for any broker or brokers, salesman or salesmen.