The Subunit Structure of Horse Spleen Apoferritin

The subunit molecular weight of horse spleen apoferritin has been determined by polyacrylamide gel electrophoresis in the presence of the anionic detergent sodium dodecyl sulfate, gel filtration on columns of agarose in 6 m guanidine hydrochloride, and by sedimentation equilibrium under dissociating conditions in the analytical ultracentrifuge. The results obtained indicate that the polypeptide chain of horse spleen apoferritin has a molecular weight of 18,500, which is in marked contrast to the previously determined value. The tryptophan content of the protein was estimated spectrophotometrically and, after modification, as 2-(2-nitrophenylsulfenyl)tryptophan. The value of 2.08 residues per 18,500 daltons is also in disagreement with literature values. These observations suggest that the undissociated horse spleen apoferritin molecule may consist of 23 to 25 subunits. For reasons of symmetry a value of 24 would seem most probable. The molecular weight of human and horse transferrin was also determined and found to be 76,600.