El-Daher, Samer Sami; (1992) The substrate specification of human α-mannosidases in relation to α-mannosidosis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The specificities of the lysosomal and neutral forms of human α-mannosidase towards natural substrates have been established. The substrates were isolated from the urine of cases of genetically and chemically induced α-mannosidosis, which excrete large amounts of mannose-containing oligosaccharides on account of a deficiency of lysosomal α-mannosidase. The oligosaccharides were purified to homogeneity and their structures determined by a combination of high performance liquid chromatography (hplc), high performance thin layer chromatography (hptlc) and acetolysis followed by hplc. To increase the sensitivity of chromatographic procedures, a method was developed to radiolabel the substrates by reduction with tritiated sodium borohydride. The chromatographic properties of the reduced substrates and reference standards were established. The lysosomal and the neutral forms were partially purified from human liver. The identity of each form was confirmed by its physico-chemical and kinetic properties. The pathways of hydrolysis of natural substrates by the lysosomal and the neutral human liver α-mannosidases in vitro were established. These pathways were specific, non-random and reproducible and were different depending on the enzyme form and starting material used. The pathways were established by analysing the mixtures of digestion products by hplc and hptlc. The structures of most intermediates were deduced by comparison with authentic standards. The structures of intermediates which did not correspond to these standards were elucidated by isolating these intermediates by hplc followed by acetolysis and further hplc analysis. The catabolism of the natural substrates by each form in vitro produced specific isomers indicating that the enzyme activity identifies the substrates used and cleaves specific mannosidic-residues. However when more than one mannosidic-linkage was hydrolysed the rate of hydrolysis showed that the enzyme although capable of hydrolysing all has a preference for one of these linkages. The lysosomal form was capable of completely hydrolysing all the substrates to Manβ(1,4)GlcNAc which is the final product expected to be found due to the action of the lysosomal form. The neutral form was capable of hydrolysing the high-mannose substrates to a limiting structure MangGlcNAc1 (5b). Further work using this limiting substrate as the starting material confirmed this observation. The substrate specificity of the lysosomal α-mannosidase in cells in culture was studied by using the potent reversible inhibitor, swainsonine, to induce a phenocopy of α-mannosidosis in normal human skin fibroblasts. On removal of the inhibitor, enzymic activity was restored permitting the pathway for the catabolism of the induced storage material products to be followed in situ. The pathway observed for the catabolism of the induced mannose-containing oligosaccharides by lysosomal α-mannosidase in situ was the same as that established in vitro for the liver lysosomal enzyme validating the detailed substrate specificity studies obtained in vitro. The storage material in genetic and chemically induced α-mannosidosis was shown to be localised in the lysosomes of fibroblasts in culture by sub-cellular fractionation on Percoll gradients and by electron microscopy. Evidence for the presence of another form of lysosomal α-mannosidase was obtained by growing human mannosidosis fibroblasts in the presence of swainsonine. The resultant change in the pattern of the storage material was consistent with the inhibition by swainsonine of a lysosomal α-mannosidase specific for α(1,6)-mannosidic linkages. The break-down of the induced storage material by the putative α(1,6) mannosidase was followed by the removal of the inhibitor. The results of this experiment showed that the induced storage material was partially removed to yield a mannose containing oligosaccharide lacking the peripheral α(1,6) linked mannose residue. Synthetic compounds which are inhibitors of α-mannosidases were studied. The synthetic amino sugars 1,4-dideoxy-1,4-imino-L-allitol (DIA) and 1,4-dideoxy-1,4-imino-D-mannitol (DIM) proved to be potentially good inhibitors of the various forms of α-mannosidases. Therefore their effect on 12 different lysosomal human liver glycosidases and on lysosomal, cytosolic and Golgi II α-mannosidases was determined. The effect of the substitution of the ring nitrogen and the addition of different compounds to the side chain of the pyrrolidine ring of the inhibitor were also investigated. It was shown that the ring substitution or the ring addition changed the degree of inhibition of the three forms of the enzyme α-mannosidase. In one case the specificity of inhibition changed from α-mannosidase to α-fucosidase. The effect of these compounds on human normal skin fibroblasts in culture and their ability to induce mannose-containing storage material were studied. It was observed that certain derivatives namely 6-deoxy-DIM and 6-deoxy-6-fluoro-DIM were better inhibitors than DIM itself of the lysosomal activity in cells in culture.

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