Procollagen type I N-terminal propeptide (PINP) as an indicator of type I collagen metabolism: ELISA development, reference interval, and hypovitaminosis D induced hyperparathyroidism

A sandwich enzyme-linked immunosorbent assay (ELISA) for quantification of the N-terminal propeptide of human procollagen type I (PINP) utilizing purified ∝ 1-chain specific rabbit antibodies is described. The ELISA measured the content of the ∝ 1-chain of PINP independent of the molecular form of the molecule. A parallelism was found between amniotic fluid (calibrator), normal and patient serum, and purified PINP (∝ 1), as well as the high and low molecular weight forms of PINP (∝ 1). The concentration of PINP in the calibrator (second trimester amniotic fluid) was determined to 25 μg/mL and the detection limit was 62 pg/mL measured in amniotic fluid, and 41 pg/mL measured in serum. The interassay coefficients of variation were 4.6% (low control) and 5.3% (high control), and the corresponding intraassay parameters were 2.9% and 4.9%. Recovery studies revealed an accuracy between 93 % and 105 %. The normal range (n = 57) for PINP was 56 ng/mL (median) the 10th and 90th Gentiles being 30 and 82 ng/mL, respectively. Patients with hyperparathyroidism due to hypovitaminosis D had median serum level of 168 ng/mL with a 10th centile of 44 ng/mL and a 90th centile of 450 ng/mL, these values being significantly different from the normal range (p< 0.001). The PINP-ELISA was superior to commercially available assays for PICP and osteocalcin in separation between healthy controls and patients with osteomalaci.