Thermal instability of the trimeric structure of the N-terminal propeptideof human procollagen type I in relation to assay technology

The N-terminal propeptide of procollagen type I (PINP) appeared in two peak s after size chromatography. The high-molecular weight form was transformed to the low-molecular weight form during incubation at 37 degrees C, wherea s the low-molecular weight form remained unchanged. The PINP concentrations in amniotic fluid and sera remained unchanged during 37 degrees C incubati on when measured using an ELISA; however, concentrations decreased by 89-93 % when measured using an RIA. The ELISA:RIA ratio varied from 1.1 to 2.9 in these fluids because of different size distributions and the inability of the RIA to measure the low-molecular weight form. Thermal transition of the high-molecular weight form caused a change in its elution volume but did n ot change its migration in sodium dodecyl sulfate-polyacrylamide gel electr ophoresis. Mass spectrometry revealed identical results for both forms. We reached the following conclusions: (a) the trimeric structure of PINP is un stable at 37 degrees C; (b) the two molecular forms represent intact arl ch ains in trimeric and monomeric forms; (c) thermal transition is an ongoing in vivo process; and cn, this is important in the choice of assay technolog y.