Defining 'full-length' recombinant factor VIII: a comparative structural analysis

Coagulation factor VIII (FVIII) is an important glycoprotein co-factor involved in haemostasis, functioning to accelerate activation of factor X by activated factor IX. Insertion of expression vectors containing the full-length cDNA sequence of human FVIII into mammalian cell lines results in the production of recombinant factor VIII (rFVIII), typically referred to as 'full-length' rFVIII (FLrFVIII). Both FLrFVIII and plasma-derived FVIII exist primarily as heterodimeric proteins, consisting of a heterogenous light and heavy chain. The objectives of this study were to compare the structural heterogeneity of high-purity FVIII preparations and further define the term 'full length' as it refers to rFVIII protein structure. Five commercially available FVIII concentrates were characterized based on SDS-PAGE, N-terminal sequencing, and peptide and domain mapping coupled to mass spectrometry. The major heavy chain species identified in FLrFVIII included various B-domain-truncated forms of FVIII, with the predominant species terminating at Arg(1313). This study demonstrates that the use of full-sequence FVIII cDNA for the production of rFVIII does not result in a homogeneous FLrFVIII protein product. Rather, commercially available FLrFVIII represents a heterogenous mixture of various B-domain-truncated forms of the molecule, with no evidence of a contiguous, intact B-domain.