A practical method for uniform isotopic labeling of recombinant proteins in mammalian cells

A method to obtain uniformly isotopically labeled (N-15 and N-15/C-13) protein from mammalian cells is described. The method involves preparation of isotopically labeled media consisting of amino acids isolated from bacterial and algal extracts supplemented with cysteine and enzymatically synthesized glutamine. The approach is demonstrated by producing N-15-labeled and N-15/C-13-labeled urokinase from Sp2/0 cells and successfully growing Chinese hamster ovary (CHO) cells on the labeled media. Thus, using the procedures described, isotopically labeled proteins that have been expressed in mammalian cells can be prepared, allowing them to be studied by heteronuclear multidimensional NMR techniques.