Serum ferritin contains minimal iron and its iron core resembles the liver ferritin core structure. (A) Equal amounts of liver and serum ferritin were immunoprecipitated and iron content was determined by the ferrozine assay. Ferritin protein concentration was determined by ELISA before precipitation and loss of ferritin during precipitation was calculated by quantitative Western blot of immunoprecipitated and nonimmunoprecipitated ferritin. The average iron atoms/ferritin molecule ratio was calculated. An average of 2 measurements is shown. (B) Visualization of ferritin–iron core from serum ferritin by transmission electron microscopy (TEM) reveals similar core size as in liver ferritin. TEM analysis of liver ferritin and serum was performed at room temperature on a Jeol 2000 FXII microscope using a 120 000× magnification. Images were captured using a Gatan Model 694 CCD camera with Gatan Digital Micrograph software. Images were processed with Adobe Photoshop. Size bar indicates 20 nm. (C) Temperature dependence of the in-phase susceptibility of the mouse serum and isolated mouse liver ferritin. Paramagnetic tail in serum is visible at the left end of curve. (D) Temperature dependence of the out-of-phase susceptibility of the mouse serum and isolated mouse liver ferritin.