Assessment of the performance of a capture immunoassay for the bone isoform of alkaline phosphatase in serum

Alkaline Phosphatases: Biochemistry, Functions, and Measurement

Alkaline phosphatases (ALPs) are a group of isoenzymes, situated on the external layer of the cell membrane; they catalyze the hydrolysis of organic phosphate esters present in the extracellular space. Zinc and magnesium are significant co-factors for the biological activity of these enzymes. Although ALPs are available in various body tissues and have distinct physiochemical properties, they are true isoenzymes since they catalyze a similar reaction. In the liver, ALP is cytosolic and present in the canalicular membrane of the hepatocytes. ALPs are available in placenta, ileal mucosa, kidney, bone, and liver. However, most of the ALPs in serum (over 80%) are delivered from liver and bone and in more modest quantities from the intestines. Despite the fact that alkaline phosphatases are found in numerous tissues all through the body, their exact physiological function remains largely unknown.

Bone alkaline phosphatase: An important biomarker in chronic kidney disease - mineral and bone disorder

Increased cardiovascular morbidity and mortality in chronic kidney disease (CKD) represents an emerging major health problem. Indeed, disturbances in mineral and bone metabolism occur frequently in CKD and are termed chronic kidney disease - mineral and bone disorder (CKD-MBD). These can lead to cardiovascular pathology, resulting in an increased cardiovascular risk. Bone alkaline phosphatase (BALP) is essential for biomineralization. Recent findings demonstrate a crucial role for BALP in the pathogenesis of vascular calcification and identified it as a promising predictor of mortality in CKD. In conjunction with parathyroid hormone (PTH), serum BALP has been suggested as a biomarker of bone turnover in CKD-MBD. In contrast to PTH, serum BALP demonstrates a lower variability and may thus be better suited for the diagnosis and longitudinal follow-up of bone turnover. The linear association with mortality, compared to the U-shaped curve for PTH, is an additional advantage, making BALP more suitable than PTH as a treatment target in CKD. Here we review the main characteristics of alkaline phosphatase isozymes/isoforms and the various assays currently used in clinical routine laboratories. We also discuss the role of BALP in both physiological and pathological mineralization, and the clinical benefit of BALP determination in CKD.

Study on immunocapture–chemiluminescence assay of lipase activity in a biological sample

A new approach for the determination of lipase (triacylglycerol lipase, EC.3.1.1.3) activity in a biological sample was investigated by combining an immunocapture technique with a chemiluminescence (CL) assay method in order to eliminate interference with CL detection. The proposed method consists of an immunocapture step to trap lipase and a subsequent step for CL detection of the activity of the captured lipase. The CL detection is based on the luminol-hydrogen peroxide (H(2)O(2))-horseradish peroxidase (HRP) reaction and utilizes a proenhancer substrate [a lauric acid ester of 2-(4-hydroxyphenyl)-4,5-diphenylimidazole (HDI)] which liberates an active enhancer, HDI, by enzymatic hydrolysis. A polyclonal antibody prepared with porcine pancreas lipase was used for the immunocapture. The proposed immunocapture-CL method effectively eliminated the interference with the CL reaction from biological components and enabled the determination of spiked porcine pancreas lipase activity in serum samples in the range 0.41-1.1 U(HDI) (1 U(HDI) corresponds to the amount which liberates 1 pmol HDI/min at 37 degrees C from the substrate). The method was further applied to the assay of the activity for human pancreas lipase in serum and the results showed good correlation (r = 0.871) with those by the conventional colorimetric method.

Conserved epitopes in human and mouse tissue-nonspecific alkaline phosphatase. Second report of the ISOBM TD-9 workshop

A panel of 19 monoclonal antibodies (MAbs) against human tissue-nonspecific (liver/bone/kidney) alkaline phosphatase (TNAP) was obtained through the ISOBM TD-9 workshop. In the present study, the reactivity of these MAbs has been characterized against mouse TNAP. A mouse embryonic stem cell line, frozen sections of long bones, alkaline phosphatase extracted from mouse bone, and serum were used as the source of TNAP for individual assays. Each MAb was tested for immunoreactivity to mouse TNAP by Western blot analysis, immunohistochemistry and enzyme immunoassay. Antibodies 314 and 315 reacted strongly with mouse TNAP in Western blots, while all other antibodies were negative. By immunohistochemistry, antibodies 314, 315 and 333 produced strong positive staining using frozen sections, while antibody 334 was moderately positive. Enzyme immunoassays indicated that MAb 333 was also able to bind to serum TNAP. These antibodies represent very useful reagents to study the pathophysiological expression of TNAP in mouse tissues and in mouse serum.

Interlaboratory Variation of Biochemical Markers of Bone Turnover

Background: Biochemical markers of bone metabolism are used to assess skeletal turnover, but the variability of marker assays is still an issue of practical concern. We describe the results of an international proficiency testing program for biochemical bone markers among clinical laboratories.

Methods: Two serum and two urine pools (normal and increased marker concentrations) were sent on dry ice to 79 laboratories for analysis within 2 weeks of receipt.

Results: Data were submitted by 73 laboratories. The within-method interlaboratory CVs (CVILs) were as follows: serum bone-specific alkaline phosphatase (n = 47 laboratories), 16–48%; serum osteocalcin (n = 31), 16–42%; urinary free deoxypyridinoline (n = 30), 6.4–12%; urinary total deoxypyridinoline and pyridinoline (n = 29), 27–28%; urinary N-terminal cross-linked telopeptide of type I collagen (n = 10), 39%; serum C-terminal cross-linked telopeptide of type I collagen (ICTP; n = 8), 22–27%; urinary hydroxyproline (n = 13), 12%. Analytical results showed both systematic and nonsystematic deviations. In identical samples, results obtained for the same marker by the same method differed up to 7.3-fold. In urine-based assays, correction for urinary creatinine slightly increased CVs.

Conclusion: Even with identical assays and methods, results for most biochemical markers of bone turnover differ markedly among laboratories.

Effect of chronic renal failure on bone turnover and bone alkaline phosphatase isoforms

BACKGROUND

Biochemical markers of bone turnover are used to monitor metabolic bone disease associated with renal failure. We have applied a comprehensive panel of markers to patients with chronic renal failure (CRF), with particular focus on the isoforms of bone alkaline phosphatase (BALP).

METHODS

Twenty CRF patients undergoing hemodialysis (N = 9) and peritoneal dialysis (N = 11) were measured for serum parathyroid hormone (PTH), osteocalcin, total ALP, and four BALP isoforms (B/I, B1x, B1, and B2) by high-performance liquid chromatography. These BALP isoforms were also compared with BALP measured by three commercial immunoassays (Alkphase-B, Tandem-R Ostase, and Tandem-MP Ostase). Type I collagen turnover was assessed by serum samples using the type I procollagen intact amino- and carboxy-terminal propeptides (PINP and PICP) and two fragments (ICTP and CrossLaps) derived from the carboxy-terminal telopeptide of mature matrix collagen by different degradative pathways.

RESULTS

Mean levels of bone turnover markers were elevated in CRF, with marked increases in those markers, osteocalcin, ICTP, and CrossLaps, cleared by the kidney. Total ALP activities were increased corresponding to elevated B/I and B2 isoform levels. The B1 isoform level was not significantly different from healthy controls. B1x was detected in 60% of the patients but was not resolved in healthy individuals. Kendall's tau rank correlation showed that B1x correlated significantly (P < 0.05) with B1 (0.53) and PINP (0.55), and was the only marker to correlate with PTH (0.49). B1x was not significantly correlated with any of the commercial BALP immunoassays. Interestingly, the immunoassay calibrators contained high activities of the B/I peak (39 to 80%) compared with human serum (4%).

CONCLUSION

There are selective differences between the BALP isoforms in CRF compared with healthy adults. The commercial BALP immunoassays are comparable with each other but are unable to distinguish the BALP isoform-specific differences in CRF patients.

Analytical and clinical performance characteristics of Tandem-MP Ostase, a new immunoassay for serum bone alkaline phosphatase

The performance characteristics of the Tandem®-MP Ostase® assay, a new microplate immunoassay for bone-specific alkaline phosphatase (bone ALP; EC 3.1.3.1) in human sera, are described. Bone ALP is bound to streptavidin-coated microwells by a single biotinylated anti-bone ALP monoclonal antibody. Antigen is detected by the addition of p-nitrophenyl phosphate. The assay is performed at room temperature in &lt;90 min. Imprecision was 2.3–6.1% with a detection limit of 0.6 μg/L. Method comparison of bone ALP measurements with the Tandem-MP Ostase assay and the mass-based Tandem-R Ostase assay (n = 285) indicated regression statistics of Tandem-MP Ostase = 1.03 Tandem-R Ostase + 0.22 μg/L, Sy‖x = 4.0 μg/L, r = 0.97. Serum bone ALP values in apparently healthy men and in pre- and postmenopausal women were also similar between the two Ostase assay formats. Liver ALP reactivity determined using the slope and heat inactivation methods was similar in both Ostase assays. Liver ALP reactivity ranged from 3 μg/L (heat inactivation) to 6 μg/L (slope method) per 100 U/L of liver ALP activity, whereas bone ALP reactivity was 37 μg/L per 100 U/L of bone ALP activity, indicating a liver ALP relative reactivity of 8.1–16.2%. Similar results were obtained with the Alkphase-B bone ALP immunoassay. The Tandem-MP Ostase bone ALP assay demonstrated increased concentrations of serum bone ALP in conditions where bone metabolism is increased and showed a rapid, temporal decrease in serum bone ALP in Paget disease patients on bisphosphonate therapy. In conclusion, the Tandem-MP Ostase assay for serum bone ALP is a rapid, simple, robust nonisotopic alternative to the Tandem-R Ostase immunoradiometric assay that provides an accurate and sensitive assessment of bone turnover.

Direct comparison of performance characteristics of two immunoassays for bone isoform of alkaline phosphatase in serum

A clinical need exists for a sensitive and specific assay for the quantitation of the bone isoform of alkaline phosphatase in serum. The majority of methods do not meet this requirement; however, the recent development of immunoassays for this isoform may provide a solution. In a detailed evaluation of two immunoassays, we found a degree of imprecision that enables the discrimination of changes within the reference range. The cross-reactivity of the liver isoform was found to be between 7.1% and 12.7% when two different methods of assessment were used. The comparison of results with an electrophoretic procedure showed that the immunocapture method recovered less of the bone isoform in samples from children than in samples from patients with Paget disease; no such difference was found with the immunometric method. This suggests that the immunocapture antibody may discriminate between different bone isoforms in children whereas the immunometric assay does not.