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Technical Briefs |
a address correspondence to this author at: Fundación Jiménez Díaz, Laboratorio de Bioquímica, Sección Metabolismo Mineral y Óseo, Avda Reyes Católicos 2, 28040 Madrid, Spain
Many fragments derived from collagen I degradation come to serum
during bone resorption. The characterization of these fragments and the
development of assays that can measure them are the basis of most of
the biochemical markers of bone resorption. One of these fragments
consists of two cross-linked amino acid sequences derived from the
C-terminal telopeptide region of type I collagen
1 chains. Fragments containing the sequence
EKAH-DGGR (CTX), with a DG site being either nonisomerized (-CTX) or
ß-isomerized (ß-CTX) were identified. Pyridinoline,
deoxypyridinoline, and other cross-links linked both telopeptides (Fig. 1
) (1). Recent reports have suggested that the
proportions of nonisomerized and ß-isomerized fragments depend on
bone age (1).
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There are two available assays that can measure the ß-isomer form of
the CTX epitope: urinary CrossLapsTM ELISA
(2), and Serum CrossLaps One Step ELISA (3), both
from Osteometer (Denmark). The urinary assay is a polyclonal (rabbit)
antibody assay for which the minimal requirement for detection is the
presence of the amino acid sequence EKAHßDGGR. According to the
manufacturer, only one strand is necessary for recognition. Thus, among
the products in Fig. 1
, this assay would be able to detect compounds 2
and 3. The serum assay is based on two highly specific monoclonal
antibodies, also directed against the amino acid sequence EKAHßDGGR;
however, according to the manufacturer, the minimal requirement of this
assay is the presence of two chains of ß-CTX, which must be
cross-linked. In this case, among the products in Fig. 1
, the serum
assay would be able to detect only compound 3.
The urinary version of this assay has provided a sensitive and specific index of bone resorption that is able to predict the outcome of antiresorptive therapy and bone loss rate in postmenopausal women (2). Recent works have demonstrated that Serum CrossLaps can also be used to predict women with the highest risk of developing osteoporosis and to monitor antiresorptive therapy (4).
Based on the different antigenicity of the two assays, the aim of the present work was to compare the usefulness of urinary ß-CTX (CrossLaps ELISA; Osteometer) and serum ß-CTX (Serum CrossLaps One Step ELISA), in the detection of changes of bone resorption between females 16–19 years of age and females 21–24 years of age. Urine (second morning void) and serum samples were collected in the morning on the same day.
Variations in the serum concentrations of the bone isoenzyme of alkaline phosphatase (Tandem®-R Ostase® IRMA; Hybritech) were used as control. Thirty-one healthy control females, age 19 ± 2.3 years (range, 16–24 years) were studied. None of them were receiving any treatment known to affect calcium metabolism. The study was approved by the Ethics Committee of our hospital.
The concentrations of the biochemical markers studied are shown in
Table 1
. As expected, bone alkaline phosphatase (bAP) was significantly
lower in the females >19 years of age than in the females 
19 years
of age. Serum ß-CTX also decreased significantly with age. However,
urinary ß-CTX in the older group was similar to that of the younger
group. We observed a significant linear correlation between serum
ß-CTX and serum bAP (P = 0.0296); however, we did not
find a significant correlation between urinary ß-CTX and serum bAP
(P = 0.2297), although serum ß-CTX correlated with
urinary ß-CTX (P = 0.0211).
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One possible explanation of the differences found between serum and urinary assays of ß-CTX could be the different antigenicity of both assays. Accordingly, the serum assay seems to be more restrictive than the urinary assay for detecting products derived from the telopeptide region of collagen type I.
The decrease in bone remodeling that occurs in older girls with respect to younger girls is a generally accepted fact. In accordance with the results found in this work in ß-CTX and bAP concentrations, it would be possible suggest that serum ß-CTX assay (Serum CrossLaps) reflects better than urinary ß-CTX assay (CrossLaps) the changes in bone remodeling that take place in young girls. It is possible that, independent of the differences attributable to renal clearance, the serum assay can be more specific for bone than the urinary assay because of the different antigenicity of the assays.
More work is necessary to investigate this point, but the observed differences between the two ß-CTX methods constitute an interesting point to consider when selecting a serum or a urinary assay to perform a particular study.
Acknowledgments
We thank the students of Ntra. Sra. de las Maravillas School and the Nursery School of the Fundación Jiménez Díaz (Madrid) for their kind collaboration.
Footnotes
Fundación Jiménez Díaz, Biochemistry Laboratory, Bone and Mineral Metabolism Section, 28040 Madrid, Spain
fax 34-91-5494764, e-mail cpiedra{at}fjd.es
References
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