Difference between total and intact assays for N-terminal propeptide of type I procollagen reflects degradation of pN-collagen rather than denaturation of intact propeptide

Abstract

Background: The concentration of N-terminal propeptide of type I procollagen (PINP) in the serum reflects the rate of type I collagen formation. Intact PINP assay measures the trimeric propeptide while total P1NP assay measures both trimeric and monomeric forms. In this study we compared these two assays emphasizing the possible differences. Methods: Intact and total PINP were measured from serum in healthy Finnish blood donors (n ¼ 34) and in the patients with chronic renal failure before and after haemodialysis (n ¼ 39). In addition, the serum of a normal man, pooled hospital serum samples and the serum of a patient with haemodialysis treatment were fractioned by gel filtration and trimeric and monomeric forms were located. Fractions were lyophilized and intact and total PINP were measured in each fraction. Samples from bedridden geriatric patients (n ¼ 173) were also measured using intact and total PINP assays and a degradation marker of type I collagen (ICTP). Results: The correlation between intact and total PINP in controls was 0.89 and their PINP concentrations were similar. In haemodialysis or bedridden geriatric patients, the PINP methods gave significantly different results. In gel filtration studies, intact PINP hardly measured monomeric form even if its concentration was disproportionately increased in haemodialysis patients. In bedridden geriatric patients, the difference of total and intact PINP correlated significantly to degradation marker ICTP. Conclusions: Difference between total and intact assays for PINP seem to reflect degradation of pN-collagen rather than denaturation of intact propeptide.

IDS iSYS automated intact procollagen-1-N-terminus pro-peptide assay: method evaluation and reference intervals in adults and children

“…230 μg/L) would allow the analysis of specimens from adults without a need for sample dilution, but makes it necessary to dilute most paediatric specimens before analysis. Repeated freezing and thawing of specimens did not have a significant influence on measured PINP values, and specimens stored at −20°C for up to 133 days gave PINP values that were not significantly different from the intact assay appears to be its specificity for PINP and its lack of cross-reactivity with the so-called monomeric or other forms [5,12]. The identities of these molecules are unknown; they may be either pro-α1 chains or fragments of PINP, but they have also been reported to have a larger molecular weight than PINP [7].…”

“…The identities of these molecules are unknown; they may be either pro-α1 chains or fragments of PINP, but they have also been reported to have a larger molecular weight than PINP [7]. The retention of some of these molecules in blood of patients with renal impairment can make the interpretation of some total PINP results difficult [5]. It has been suggested that the monomeric form detected by the total PINP assay may be a breakdown product of collagen, and in breast cancer patients, for example, PINP has been shown to be higher by the Roche E170 total PINP assay as compared with the IDS iSYS method [13].…”

“…Intact PINP assays detect solely this form, whereas total PINP assays also detect monomers or possibly fragments of PINP, although these molecules have not been characterised well [5]. An automated total PINP immunoassay (Roche Diagnostics) has been available for some years [6], and in 2009, an automated chemiluminometric immunoassay for intact PINP was made available by Immunodiagnostic Systems Ltd (IDS) on their iSYS analyser.…”

Comparison of total and intact aminoterminal propeptide of type I procollagen assays in patients with breast cancer with or without bone metastases

“…7 However, especially in haemodialysis patients, the results were very different. 9 We have found out that the difference was due to a large increase of the monomeric forms, which accumulate in the serum when the kidneys are not functioning. The accumulation of the breakdown products caused by decreased kidney function is a normal phenomenon.…”

Bone turnover markers in serum and urine as diagnostic, prognostic and monitoring biomarkers of bone metastasis

“…Corresponding to their origin, the propeptides are called C-terminal or N-terminal propeptides of type I procollagen (PICP; PINP). PINP occurs in the serum in two forms: as intact, trimeric peptide corresponding to the native separation product of procollagen during the synthesis of type I collagen and as monomeric peptide which is rather a degradation product of procollagen [35][36][37]. In contrast to this view, there has been the assumption that the released intact PINP is rapidly degraded to the monomeric form when it is released into circulation and would make it necessary to determine both forms [38].…”