Critical examination for the presence of a low molecular weight fraction in serum iron

Native human pool serum and individual sera were ultrafiltered by Pellicon ultrafilters (Millipore) and the ultrafiltrates were extracted by an ammonium pyrrolidinedithiocarbamate/methylisobutylketone system after addition of different internal iron standards to three of four identical ultrafiltrates. The extracts were examined for iron content by atomic absorption spectrometry. During ultrafiltration pH 7.4 was maintained by a constant atmosphere of a CO2/air mixture. Low molecular weight iron in native human sera from normal, normal orally iron substituted and siderotic individuals was found to be less than 0.05 micrograms/100 ml. Elevating serum citrate to 3-fold normal had no effect on this result. More iron became ultrafiltrable if the serum pH were lowered by citric acid as compared with hydrochloric acid.