Many hitherto unknown peptides are principal constituents of uremic "middle molecules".

The aim of the present investigation was to collect information on the molecular composition of uremic "middle molecules," the 500-5000 molecular-mass serum constituents assumed to be involved in the molecular etiology of uremic intoxication. For this purpose, three fractions of serum containing such molecules were separated by cation-exchange column chromatography. These fractions absorb at 240 nm and are characteristically increased in chronic uremia. By one-dimensional paper chromatography of these fractions it was demonstrated that each could be resolved into at least seven (altogether, 21) ninhydrin-positive subfractions. These, like the original fractions, yielded amino acids on acid hydrolysis. The qualitative amino acid composition of the subfractions differed substantially, both from each other and from that of 31 known peptides with which they were compared. We conclude that hitherto unknown peptides with only limited diffusibility through hemodialysis membranes constitute the main bulk of uremic middle molecules.