Molecular weight distribution, diffusibility and comparability of middle molecular fractions prepared from normal and uremic sera by different fractionation procedures.

Sephadex G-25 (SG-25) chromatography of normal and uremic (U) sera yielded three (A, B, C) optically active (280 nm) fractions. Only fraction B disclosed a substantial increase in uremia that could not be counteracted by routine hemodialysis. Although middle molecules (MM) might be included in all SG-25 fractions (B greater than C greater than A), none of them was exclusively composed of MM. Solutes in U fraction B and C were found to be (a) heat stable; (b) soluble in ethanol; (c) diffusible through a Visking membrane, and (d) adsorbed by activated charcoal. Solutes in U fraction B and C and those in a toxic ethanol U serum extract could be partially or entirely recovered in the same Dowex fractions which were obtained by Dowex chromatography of native U sera. Methodical pitfalls leading to misinterpretation of molecular weight data obtained by SG chromatography and to erroneous classification of MM are also thoroughly discussed in this paper.