Nature of the heterogeneity within genetic variants of bovine serum transferrin.

A comparison is made of the four main components of an homozygous variant (A or D2D2) of bovine serum transferrin. These are designated I-IV in order of increasing mobility in electrophoresis at pH 7.5. Components I, II, III and IV have 2,2,3 and 3 residues of sialic acid per transferrin molecule and appear to correspond to components 2a, 2b, 3a and 3b respectively of Stratil & Spooner (1971). The difference between components I and II and between III and IV does not reside in sialic acid differences. On the basis of peptide maps of reduced carboxamidomethylated components, urea-starch gel electrophoresis and quantitative sequence studies, it is concluded that components II and IV have a scission in the peptide chain. By homology with the sequency of MacGillivray et al. (1977) for human serum transferrin it is suggested that the scission occurs between residues 55 and 54 from the C-terminus and this portion of the chain has a 'molecular' weight of ca. 6000. The implications are briefly discussed.