Heterogeneity of the antigens related to the aminoterminal propeptide of type III procollagen in human serum.

Inhibition curves that are considerably less steep than the reference peptide curve are a constant finding when human serum samples are studied with the radioimmunoassay for the aminoterminal propeptide Col 1-3 of type III procollagen. This is due to the presence in the serum of three main peptide forms which differ in their antigenic properties and can be separated by gel filtration. Their molecular sizes are, respectively, larger than, equal to and smaller than the peptide Col 1-3. The proportions of these forms were different in a number of serum samples tested. An elevated value in the Col 1-3 radioimmunoassay need not reflect increased deposition of type III collagen in the liver, but could also be due to increased degradation of a newly-synthesized type III procollagen or degradation of a tissue form still containing the aminoterminal propeptide. This should be considered when interpreting elevated serum values.