Ferritin from Reuber H-35 hepatomas was compared with ferritin from livers of ACI rats, the strain of rats in which the original H-35 hepatoma developed. The isoelectric points of these two proteins are 5.20 ± 0.02 (H-35 ferritin) and 4.95 ± 0.03 (ACI liver ferritin). The amino acid composition of apoferritin prepared from H-35 hepatoma ferritin differs significantly from that of ACI rat liver apoferritin. For the majority of amino acids, the differences are significant at p < 0.01. Ion-exchange chromatography on carboxymethylcellulose at pH 5.5 and 7.2 also gave significantly different results for the two ferritins. These findings, taken together, indicate that the primary sequence of amino acids in some or all protein subunits of the two ferritins (apoferritins) is not the same. However, by electron microscopy the two kinds of ferritin are similar in size, shape, and overall appearance. The sedimentation velocities of the two apoferritins are almost identical (18.6 S for ACI liver, 18.5 S for H-35 hepatoma apoferritin). Subunits of the apoferritins, prepared by treatment with sodium dodecyl sulfate, have identical mobilities on electrophoresis in polyacrylamide gel, presumably because the net charge densities of sodium dodecyl sulfate subunit complexes are due to sulfate.
This work was supported by USPHS Grant AM-12381.