Quantification of fetal antigen 2 (FA2) in supernatants of cultured osteoblasts, normal human serum, and serum from patients with chronic renal failure

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Abstract
Fetal antigen 2 (FA2) was found within the cytoplasm of osteoblasts and in osteoid material, in a bone biopsy with morphological changes of renal osteodystrophy. An ELISA technique for FA2 quantification was developed, and the specificity confirmed by comparison with electroimmunoassay. The intraand interassay coefficient of variations (%) were 8.3 and 9.7 respectively, and the detection limit 0.0004 arbitrary units FA2/1 using second-trimester human amniotic fluid as reference (1 AU FA2/1). FA2 was detected in serum-free supernatants from osteoblast cultures. Following size chromatography, the FA2 distribution (two peaks eluted corresponding to Mw 30 kDa and 100 kDa) in serum from a patient with chronic renal failure complicated with secondary hyperparathyroidism and in human amniotic fluid were identical. Probably due to the detection limit, only one peak fraction (30 kDa) was seen in normal human serum. Significantly greater FA2 concentrations were found in sera (n=14) from patients with chronic renal failure (median: 11.9 mAU FA2/1; range: 5.2–49.0 mAU FA2/1) compared to normal healthy individuals (n=23) (median: 4.1 mAU FA2/1; range 2.4–9.4 mAU FA2/1) (P<0.00001). A close correlation was found between serum FA2 and alkaline phosphatase R(s) = 0.761; P=0.006), c-terminal fraction of PTH (R(s)=0.872; P=0.003) and intact PTH(l-84) (R(s) = 0.904; P=0.011) in the haemodialysis patients. These data indicate that FA2 is synthesized by osteoblasts and may represent a new marker for metabolic bone changes.

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