Purification of a novel glycosylated ferritin from horse heart

We have previously shown that mRNA coding for ferritin L subunit is present on both cytosolic ribosomes and endoplasmic reticulum-bound ribosomes in rat heart tissue [Campbell et al. (1989) Arch Biochem Biophys 273:89-98]; from this we infer that heart tissue is capable of making a secreted ferritin. We now report the purification from horse heart, of a ferritin that specifically binds to Concanavalin A-Sepharose and is immunologically cross-reactive with antibodies raised against both horse cellular ferritin and horse serum ferritin. Where cellular ferritin is 10 nm in diameter and contains primarily 21-kDa subunits (as determined by gel exclusion chromatography and electron microscopy), the glycosylated heart ferritin is smaller with diameters of 3-5 nm. Antisera raised against serum ferritin cross-reacted with the glycosylated heart ferritin did but did not show significant cross-reactivity with cellular ferritin thus indicating that serum ferritin and glycosylated heart ferritin have antigenic determinants which may not be present on cellular ferritin. The glycosylated ferritin also differs from cellular ferritin in subunit composition, with subunits of 66, 60.5, 53.5, 43.5, and 29.5 kDa, as shown by SDS-PAGE and Western blot analysis. Interestingly, ferritin purified from horse serum contains subunits of similar size.