Transgenic pigs as bioreactors: a comparison of gamma-carboxylation of glutamic acid in recombinant human protein C and factor IX by the mammary gland

The mammary gland of transgenic livestock can be used as a bioreactor for p roducing complex therapeutic proteins. However, the capacity for making a g iven post-translational modification upon any given polypeptide is uncertai n. For example, the efficiency of gamma-carboxylation of glutamic acid in t he amino terminal regions of recombinant human protein C (rhPC) and recombi nant human Factor IX (rhFIX;) is different at similar expression levels. At an expression level of about 200 mu g/ml in the milk of transgenic Figs, r hFIX is highly gamma-carboxylated as indicated by pro-coagulant activity an d amino acid sequencing. However. only about 30-35% of rhPC has a native, g amma-carboxyglutamic acid-dependent conformation and anti-coagulant activit y. Thus. this work provides an example of apparent differences in substrate specificity between two homologous proteins to the endogenous carboxylase of porcine mammary epithelium which leads to varying degrees of post-transl ational modification. (C) 1999 Elsevier Science B.V. All rights reserved.