Purpose of adding zinc to insulin in commercial preparations for treatment of diabetes mellitus is:-

Insulin exists in two forms:- Hexamer Monomer Monomer is the active form of Insulin, so it will sta producing action --> decreasing blood sugar immediately and in no time the action will be finished. We want Insulin to stay as Hexamer and gradually it is released into the monomer forms so that duration of action can be increased. This is achieved by adding zinc to the Insulin (on adding zinc, the insulin stas getting crystallized and those crystals are deposited and act as a depot preparation and slowly-slowly from that site, monomers are released and insulin keep on acting) Hence, zinc is added to make insulin long acting.