N-terminal Amino Acid Sequences and C-terminal Residues of Rat Alpha-fetoprotein Electrophoretic Variants, ‘Fast’ and ‘Slow’

Rat alpha-fetoproteins ‘fast’ (FRAFP) and ‘slow’ (SRAFP) were prepared to homogeneity from amniotic fluid by preparative polyacrylamide disc gel electrophoresis. Automated Edman degradation revealed that both proteins had the same sequence for residues 1–18. Hydrazinolysis yielded glycine and valine as the C-termini of FRAFP and SRAFP, respectively. SRAFP was refractile to C-terminal sequence determination with carboxypeptidases A, B and Y under various enzyme/substrate ratios. S-carboxyamido methylation and subsequent digestion either in the presence or absence of 6M urea yielded no improvements; these methods work on rat serum albumin. Amino terminal sequences of rat AFP show a high degree of similarity (only seven differences, eight base changes) with the N-terminal sequence of human AFP. There were few sequence similarities between the N-termini of rat AFP's and rat or bovine serum albumin or pro- or preproalbumin. The C-terminus of SRAFP is identical to human AFP. The degree of homology between AFP and albumin is discussed.