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Biochemical and Immunohistochemical Evidence That in Cartilage an Alkaline Phosphatase Is a Ca2+-binding Glycoprotein

By Benedetto De Bernard, Paolo Bianco, Ermanno Bonucci, Maurizio Costantini, Gian Carlo Lunazzi, Paolo Martinuzzi, Chiara Modricky, Luigi Moro, Enrico Panfili, Piero Pollesello, Nicola Stagni and Franco Vittur


Abstract. A glycoprotein that exhibits alkaline phosphatase activity and binds Ca 2 ÷ with high affinity has been extracted and purified from cartilage matrix vesicles by fast protein liquid chromatography. Antibodies against this glycoprotein were used to analyze its distribution in chondrocytes and in the matrix of calcifying cartilage. Under the light microscope, using immunoperoxidase or immunolhorescenee techniques, the glycoprotein is localized in cbondrocytes of the resting zone. At this level, the extracellular matrix does not show any reaction. In the cartilage plate, between the proliferating and the hypertrophic region, a weak immune reactivity is seen in the cytoplasm, whereas in the intercolumnar matrix the collagen fibers appear clearly stained. Stained granular structures, distributed with a pattern similar to that of THOUGH the molecular mechanism of tissue calcification still awaits full elucidation, undisputed evidence points to the involvement of alkaline phosphatase in this process. First, it has been repeatedly reported that the enzyme is present at significant levels in precalcifying matrices. Second, in cartilage and in other mineralizing tissues, an intense alkaline phosphatase activity is detected in matrix vesicles, where the earliest crystals of calcium phosphate are formed (5). In contrast, matrix vesicles of the elastic cartilage of the epiglottis, a tissue that does not calcify, show no alkaline phosphatase activity (24). Studies on alkaline phosphatase extracted from variou

Year: 1986
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