Alkaline phosphatase glycoprotein

Golgi, in early neuroanatomical studies (1898) staining neurones by silver impregnation, observed a reticular apparatus which was crescent shaped and appeared to be linked through canaliculi. The structure was also seen in secretory cells. Between 1949 and 1954, Baker reported the presence of similar systems in unfixed cells examined by phase contrast. The structures could be stained by osmium tetroxide and probably contained lipid. They also stained for glycoprotein and alkaline phosphatase. Baker s confirmation of the existence of the... 

As markers of osteogenie differentiation, high activity of alkaline phosphatase, production of collagen 1 and non-collagenous calcium-binding ECM glycoproteins osteocalcin and osteopontin are currently evaluated [14-17,24-27,37,47],... 

Hessle L, Johnson KA, Anderson HC, Narisawa S, Sali A, Coding JW, Terkeltaub R, Millan JL. 2002. Tissue-nonspecific alkaline phosphatase and plasma cell membrane glycoprotein-1 are central antagonistic regulators of bone mineralization. Proc Natl Acad Sci USA 99 9445—9. 

The blood-brain barrier is a biochemical as well as a physical barrier. Brain endothelial cells create an enzymatic barrier composed of secreted proteases and nucleotidases, as well as intracellular metabolizing enzymes such as cytochrome P-450. Furthermore, y-glutamyl transpeptidase, alkaline phosphatase, and aromatic acid decarboxylase are more prevalent in cerebral microvessels than in nonneuronal capillaries. The efflux transporter P-glycoprotein and other extrusion pumps are present on the membrane surface of endothelial cells, juxtaposed toward the interior of the capillary. Furthermore, CNS endothelial cells display a net negative charge at the interior of the capillaries and at the basement membrane. This provides an additional selective mechanism by impeding passage of anionic molecules across the membrane. 

COS cells have been used for the production of recombinant proteins, including HSV-1 glycoprotein B, ricin B chain, placental alkaline phosphatase, thrombomodulin, CD7, von Willebrand factor, human dihydropteridine reductase, human P-glucuronidase, interleukin 5 and human interferon-... 

Many enzymes are glycoproteins, and variations in carbohydrate side chains are a common cause of nonhomogeneity of preparations of these enzymes. Some carbohydrate moieties, notably h/-acetylneuraminic acid (sialic acid), are strongly ionized and consequently have a profound effect on some properties of enzyme molecules. For example, removal of terminal sialic acid groups from human liver and/or bone alkaline phosphatase with neuraminidase greatly reduces the electrophoretic heterogeneity of the enzyme. 

A physiologic phosphate concentration is required for bone mineralization. Lowering the concentration prevents mineralization, but raising it does not ensure precipitation because pyrophosphate is present to inhibit precipitation. The concentration of PPi in cartilage and bone is controlled by three enzymes, two on the outer surface of matrix vesicles (Fig. 9.5b). One is tissue-nonspecific alkaline phosphatase (TNAP), which decreases stromal pyrophosphate and the other is NTP-PPi hydrolase (also called plasma cell membrane glycoprotein-1), which increases it. The progressive ankylosis gene product (ANK protein) is expressed by osteoblasts to add to the pyrophosphate of the osteoid matrix from osteoblast cytosol. 

Fig.9.8 Removal of pyrophosphate is necessary for precipitation. Pyrophosphate (PPi) inhibits the precipitation of calcium phosphate. In the bone matrix, PC-1 (red) is the major producer of PPi from nucleotide triphosphates (NTPs, thick arrow on left) and ANK is a minor producer by transporting it from the cytosol of osteoblasts. TNAP (green) causes mineralization by its phosphatase activity converting PPi to two molecules of Pi. TNAP also generates Pi directly from NTPs and PPi, but most Pi and most Ca2+ are derived directly from the diet (thick arrow on right) (Slightly modified from Fig. 4 in Hessle L et al. (2002) Tissue-nonspecific alkaline phosphatase and plasma cell membrane glycoprotein-1 are central antagonistic regulators of bone mineralization. Proceedings of the National Academy of Sciences 99 9445-9449. Copyright (2002) National Academy of Sciences, U.S.A)...

A biochemical evalution of human alkaline phosphatase is postponed until the above considerations have been presented. In our view, the most reasonable analytical approach is based on the measurement of L-phenyl-alanine-sensitive and -insensitive moieties along with their respective heat stabilities. To this may be added information gathered from starch-gel electrophoresis with native and heated serum and from the presence of L-phenylalanine-sensitive bands on the gels following electrophoresis. Experiments of a different type can be included, in which the serum is incubated with neuraminidase and susceptibility of the glycoprotein is established following electrophoresis. Finally, the data on L-phenyl-alanine inhibition of heat-sensitive and -insensitive moieties appear to make sense, if the population of normal subjects is divided into one with the slow-moving intestinal band and one without it. It is from this consideration and other indirect and direct inferences that the intestine is... 

It is a fact that, in a normal adult healthy individual, the serum alkaline phosphatase remains relatively constant. This event is indistinguishable from the constancy of other serum proteins such as albumin, glycoproteins, etc. Therefore the considerations that apply to the homeostasis of serum proteins logically apply to serum enzymes. These have been discussed elsewhere (F5). 

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