Sedimentation values of thyroglobulin and ferritin containing iron are 19S [1] and 51S [2], respectively. But, by density gradient centrifuge, it seems thyroglobulin (19S) migrates faster than ferritin (51S). Could anyone explain this?

Figure 3 in this article shows a migration pattern of GE High molecular weight marker containing ferritin and thyroglobulin. Thyroglobulin comes almost at the bottom but ferritin, in the middle, which I did not expect.

PS I asked GE and confirmed ferritin in the marker contains Fe ions.


1 Answer 1

  1. Ferritin and iron-containing ferritin are different and the latter would of course be heavier; it will therefore sediment faster.
  2. Figure 3 of the third article uses thyroglobulin and ferritin as molecular weight markers. For your information thyroglobulin is 650kDa whereas ferritin is 440kDa. Also note that apoferritin i.e. ferritin that is not bound with iron has a sedimentation rate of 18S (from the same article that measures the sedimentation rate of iron bound ferritin - ref#2).

There seems to be some confusion regarding the sedimentation rate of ferritin. From this article:

One molecule of ferritin can bind up to 4500 Fe2+ ions. When fully loaded with iron, the ferritin can increase in mass up to ca 900 kDa; it then has a sedimentation coefficient of 24S (Chasteen & Harrison 1999).

55S for ferritin seems dubious.

  • $\begingroup$ I confirmed that ferritin they used contains iron. I directly asked GE because the marker is their product. (I accidentally sent wrong message before.) $\endgroup$
    – 243
    Commented May 31, 2015 at 5:56
  • $\begingroup$ @243 There is some confusion with the sedimentation rate measurement of ferritin. See the edit. $\endgroup$
    Commented Jun 7, 2015 at 5:50

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