A reappraisal of the electrophoretic patterns obtained from ferritin and apoferritin in the presence of denaturants

Abstract

1. Introduction Some years ago we demonstrated by polyacrylamide gel electrophoresis that the detergent sodium dodecyl sulphate (SDS) was capable of denaturing ferritin and apoferritin into subunits and that the molecular weight of these subunits was of the order of 18 000-19 000 [l] . This finding was later confirmed using a variety of techniques [2,3] and is now the generally accepted value for the molecular weight of the protein subunit. Since that time a number of groups, using SDS- polyacrylamide gel electrophoresis, have also demonstrated the presence of two smaller polypeptide species, B and C, of mol. wt 11 000 and 7000-8000, respectively [3-61 . Although at present there is no definitive evidence as to the nature of the origin of peptides B and C, it is clear that they are derived from the subunit of mol. wt 18 000-19 000 [4-61. Peptides B and C have been purified and it has been shown that the sum of their amino acid compositions agrees well with the amino acid composition of the subunit [5,6] . It has also been shown that a tryptic fingerprint of peptide C yields a ninhydrin-negative arginine-positive peptide characteristic of the N-acetyl blocked serine of the N-terminus of apoferritin. Whilst the origin of the two lower molecular weight peptides from the subunit is of considerable interest to us, we also felt that these peptides could be used to good advantage in the protein primary sequence studies provided they could be isolated in high yields in a purified form. In the course of such peptide isola- tion and purification we observed, on deliberate over-