Ultrastructural features of mucopeptide concretions from the lacrimal sac help in better understanding of their etiopathogenesis and tissue interactions. Further exploration of different stages of a concretion is needed to understand the potential factors that trigger its genesis and evolution.
Lysosomal enzymes undergo phosphorylation on their mannose residues in the Golgi apparatus and are recognized by two distinct type I transmembrane glycoproteins designated as the mannose 6-phosphate receptors; MPR300, (Mr 300 kDa) and MPR46, (Mr 46 kDa) that internally transport them to the lysosomes. In humans, absence of this recognition system leads to severe lysosomal storage disease, emphasizing their essential role in the biogenesis of lysosomes. Among the two receptors only MPR46 shows an absolute requirement for divalent metal ions. Only MPR300 is known to be a multifunctional protein that also binds many other ligands such as the human IGF-II, thyroglobulin, retinoic acid, granzyme A and B. In mammals, the extracytoplasmic domain of MPR300 protein is comprised of 15 repetitive cassettes which share significant similarity with each other and also with the single cassette that constitutes the extracytoplasmic domain of MPR46. Therefore it became necessary to understand the evolution of these receptors. Homologous proteins were affinity purified from different non-mammalian vertebrates such as birds, reptiles, amphibians, fish and also from the invertebrates, echinodermates (starfish) and molluscs (unio). Cloning and sequencing of both receptors from different mammals, chicken, fish and MPR46 from starfish revealed that these proteins exhibit similar structural domains as the mammalian receptors. alpha-fucosidase characterized from the molluscs exhibits specific interaction with the putative MPR300 protein from the same species. Available evidence suggests evolutionary conservation of both receptors from molluscs, as below these species no receptors that bind phosphomannan have been identified.
The galactose‐specific lectin present in the seeds of snake gourd (Trichosanthes anguina) was purified in high yield by affinity chromatography on cross‐linked guar gum. The purified snake gourd seed lectin (SGSL) yielded a single symmetrical peak on gel filtration with an Mr of 62 kDa and gave a single band in PAGE under non‐denaturing conditions. In SDS‐PAGE, SGSL gave a single band of Mr 53 kDa in the absence of β‐mercaptoethanol, and two bands of Mr 32 and 23 kDa in its presence, indicating that the lectin is a heterodimer in which the subunits are linked by a disulphide bridge. The lectin gave a single precipitin line in immunodiffusion experiments with antiserum raised against the purified SGSL. No cross‐reactivity was found between SGSL and antiserum raised against the Momordica charantia lectin and vice versa, suggesting that the two lectins are antigenically dissimilar. Haemagglutination‐inhibition data show that MeβD‐Gal is the best monosaccharide inhibitor of SGSL and indicate that an equatorial hydroxyl at C‐2 and axial hydroxyl at C‐4 in the pyranose form are important binding loci for the lectin.
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