A method for producing a recombinant polypeptide, comprising: (a) providing a 100 L to 25,000 L fed-batch bioreactor comprising (i) cells capable of expressing the recombinant polypeptide asfotase alfa (SEQ ID NO: 1), and (ii) a culture medium suitable for conducting such expression the culture medium comprising about 25 μM to about 300 μM zinc； (b) culturing the cells under conditions suitable to express the recombinant asfotase alfa wherein the pH of the culture medium is about 6.7 to about 7.1, and wherein zinc is added into said culture medium such that the zinc concentration in the culture medium is maintained at a concentration of about 25 μM to about 300 μM of zinc.
