Carbohydrates are thought to function as tags that mark circulatory glycoproteins for rapid clearance. To examine the role of the mannose receptor (MR) in glycoprotein clearance, we generated mice genetically deficient in MR. MR-/- mice were defective in clearing proteins bearing accessible mannose and N-acetylglucosamine residues and had elevated levels of eight different lysosomal hydrolases. Proteomic analysis of MR-/- and control mouse sera showed that an additional 4 out of 52 proteins identified were elevated in MR-/- serum. Each of these is up-regulated during inflammation and wound healing. Thus, MR appears to operate as an essential regulator of serum glycoprotein homeostasis.
We developed a radioimmunoassay (RIA) for the carboxy-terminal telopeptides of type I collagen (ICTP), cross-linked with the helical domain of another type I collagen molecule, after isolation from human femoral bone. The cross-linked peptide was liberated by digesting insoluble, denatured bone collagen either with bacterial collagenase or with trypsin, and purified by two successive reversed-phase separations on HPLC, with monitoring of pyridinoline-specific fluorescence. The purity of the peptide was verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and its origin in the type I collagen fibers was determined by amino-terminal amino acid sequencing. Polyclonal antibodies and a separation reagent containing second antibody and polyethylene glycol are used in the RIA. An immunologically identical, somewhat larger antigen is present in human serum; its concentration increases in multiple myeloma and in rheumatoid arthritis. The ICTP antigen seems to be cleared from the circulation by the kidneys, because glomerular filtration rates that are two-thirds of normal or less are associated with increased circulating ICTP concentrations. The CVs of the method are between 3% and 8% for a wide range of concentrations. The analysis of 40 serum samples can be completed in 4 h.
In this article we review the biochemical basis for markers of bone metabolism and comment on their bone specificity and representativeness for bone tissue. Major developments have recently taken place particularly with respect to markers of bone collagen metabolism; accordingly, they are in the focus of this review. We also attempt to relate the various collagenous and non-collagenous markers to each other and to the phases of the osteoblast phenotype.
Type I collagen is the most abundant collagen type in soft tissues and the only type found in mineralized bone. We established a rapid equilibrium radioimmunoassay for the carboxyterminal propeptide of human type I procollagen (PICP), to be used as an indicator of the synthesis of type I collagen. We isolated type I procollagen from the medium of primary cultures of human skin fibroblasts, digested the protein with highly purified bacterial collagenase, and purified PICP by lectin-affinity chromatography, gel filtration, and ion-exchange separation on HPLC. The purity of the protein was verified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and by N-terminal amino acid sequencing of its component chains. The final radioimmunoassay was established with polyclonal rabbit antibodies. Material antigenically related to PICP is readily detected in human serum. There is only one form of the serum antigen, its molecular size and affinity to the antibodies being similar to those of the isolated propeptide. Intra- and interassay CVs are 3% and 5%, respectively. Preliminary reference intervals for healthy adults (18 to 61 years of age) are 38-202 micrograms/L for men and 50-170 micrograms/L for women: in men the concentration is inversely related to age. The serum antigen is stable during storage and after repeated thawing.
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