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Sedimentation equilibrium experiments of folate-binding protein from cow's milk performed in the analytical ultracentrifuge at pH 5 9 showed a single molecular species with a molecular weight of 30,000 9 In contrast, at pH 7.4 sedimentation equilibrium indicated molecular weights ranging from about 30,000 to at least 300,000. OSedimentation velocity experiments at pH 5.0 showed a single symmetrical peak with s20,w = 2.8 S in agreement with a molecular weight of about 30,000. At pH 6.0 was seen a faster moving boundary (s20,w~ 6S) in addition to the slow one. At higher pH two peak maxima were seen: a slow one with sedimentation coefficients of about 5S and a fast one increasing from 9S to 1 IS with increasing pH. Addition of folate further increased the sedimentation rates. By assuming that G.LBERT'S rapid monomer-n-mer equilibrium theory was valid also for a lower polymer in equilibrium with a higher polymer our data can be interpreted as an equilibrium between a tetramer and a polymer consisting of more than 16 monomers. In the case of folate addition the results may similarly be interpreted as an equilibrium between an octamer and a polymer composed of more than 32 monomers.

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