Subunit composition of human liver alpha-L-fucosidase

Defective expression of alpha-L-fucosidase by lymphoid cells of a fucosidosis patient

Fucosidosis is an inherited lysosomal storage disease due to a deficiency of alpha-L-fucosidase activity. Exponentially growing lymphoid cell cultures from a fucosidosis patient (JH) had 16-fold lower extracellular alpha-L-fucosidase protein and 72-fold lower intracellular alpha-L-fucosidase protein with negligible catalytic activity as compared with the mean of 19 control cultures. The percentage of total alpha-L-fucosidase protein released extracellularly by JH cells was 71% as compared with 35% +/- 9% for control cells. During a 1.5 h pulse with 35S-methionine, alpha-L-fucosidase was synthesized by JH cells as an intracellular doublet with Mr of 58,000 and 56,000 and by control cells as an intracellular form with Mr = 58,000. During a subsequent 21 h chase with unlabeled methionine, JH alpha-L-fucosidase was entirely secreted. In contrast, only 25%-30% of control enzyme was secreted with the remainder retained intracellularly. Thus, JH lymphoid cells synthesized a reduced amount of alpha-L-fucosidase that was catalytically inefficient and was hypersecreted. Treatment of JH alpha-L-fucosidase with N-glycanase produced polypeptide chains with Mr of 52,000 and 54,000. Previously, treatment of control alpha-L-fucosidase with N-glycancase produced a single polypeptide chain with Mr of 52,000 (Biochem Genet 1988; 26: 401-20). The doublet polypeptide chains of alpha-L-fucosidase in JH cultures may represent expression of two distinct allelic forms of mutant alpha-L-fucosidase.

Mammalian alpha-L-fucosidases

Mammalian alpha-L-fucosidases are a ubiquitous group of relatively large multimeric lysosomal glycosidases involved in the degradation of a diverse group of naturally-occurring fucoglycoconjugates. These enzymes are closely related structurally as indicated by immunochemical cross-reactivity and cloning studies. Mammalian fucosidases are sialoglycoproteins and the carbohydrate, particularly sialic acid, contributes to producing multiple isoforms which can differ in various species as well as in different tissues within a given species. alpha-L-Fucosidases exhibit maximal activity at pH values between 4 and 7, have similar kinetic properties with synthetic substrates (PNP-fucoside and 4-MU-fucoside), and exhibit broad substrate specificity on natural substrates. Numerous linkages (alpha 1-2, alpha 1-3, alpha 1-4, alpha 1-6), primarily to galactose and N-acetylglucosamine, can be hydrolyzed but preference is often seen for small mol. wt water-soluble substrates with fucose in alpha 1-2 linkage to galactose. The importance of alpha-L-fucosidase in mammalian metabolism is evidenced by deficiency or absence of its enzymatic activity leading to a fatal genetic disease, at least in humans and English Springer Spaniels.

Hydrolysis of fucosyl-GM1 ganglioside by purified pellet-associated human brain and human liver alpha-L-fucosidases without activator proteins or detergents

Pellet-associated human brain alpha-L-fucosidase was solubilized with 0.5% (w/v) Triton X-100 and purified by affinity chromatography on agarose-6-aminohexanoyl-fucosamine resin. The procedure resulted in a 290,000-fold purification, a 58% yield and a final specific activity of 11,500 nmol/min per mg of protein. Isoelectric focusing indicated that all six major isoforms (with pI values between 4.1 and 5.3) present in crude brain pellet preparations were purified by the affinity technique. SDS/PAGE indicated the presence of one subunit (54 kDa) and a minor protein band at 67 kDa, which presumably is a contaminant since it was not immunoreactive on Western blotting. The pH optimum of the brain enzyme and its apparent Km for the synthetic substrate 4-methylumbelliferyl alpha-L-fucopyranoside were 5.5 and 0.07 mM respectively. Pellet-associated human brain and liver alpha-L-fucosidases were both capable of hydrolysing fucosyl-GM1 ganglioside without activator proteins or detergents. Linear hydrolysis rates were found only for short incubation times (1-5 min). Optimal enzymic activity at 37 degrees C was found at pH 3.4 for both alpha-L-fucosidases, with no activity at pH values above 4.0.

Structural studies on the carbohydrate moieties of human liver alpha-L-fucosidase

alpha-L-Fucosidase was purified from human liver to apparent homogeneity and subjected to exhaustive digestion with Pronase. The resulting glycopeptides were isolated by gel filtration on Sephadex G-50 and further fractionated by Bio-Gel P-4 chromatography. Five glycopeptide fractions were obtained. The structures of the carbohydrate portions of all glycopeptide components were fully characterized by a combination of 500-MHz 1H NMR spectroscopy and carbohydrate composition analysis. Fraction I contained disialyl diantennary glycopeptides of the N-acetyllactosamine type. Fractions II and III contained predominantly mono(sialyl-N-acetyllactosaminyl) diantennary glycopeptides with the NeuAc alpha(2----6)Gal beta(1----4)GlcNAc beta(1----2) branch attached to alpha(1----3)-linked Man in II and to alpha(1----6)-linked Man in III. The N-acetyllactosamine-type glycopeptides in fractions I to III have a small portion (10-15%) of their Asn-linked GlcNAc residues substituted by additional alpha(1----6)-linked Fuc. Also, a minor portion of the NeuAc residues appeared to be attached to Gal in alpha(2----3) rather than alpha(2----6) linkage. Fraction IV contained a mixture of larger-size oligomannoside-type glycopeptides with a variable number (6 to 9) of Man residues. Smaller-size oligomannoside-type glycopeptides were found in fraction V, containing 3 or 5 Man residues; a small portion (10%) of the Man3GlcNAc2Asn component appeared to contain in addition a Fuc residue in alpha(1----6) linkage to the Asn-bound GlcNAc. The overall ratio of oligomannoside-type to N-acetyllactosamine-type carbohydrate structures was found to be 5:4. This article is the first account of the complete characterization of the oligomannoside-type structures in alpha-L-fucosidase; furthermore, the occurrence in alpha-L-fucosidase of mono(sialyl-N-acetyllactosaminyl) structures, Fuc-containing oligosaccharides, and NeuAc alpha(2----3) linked to Gal are reported for the first time.

Further studies on the catalytic mechanism of human liver alpha-L-fucosidase

Radiolabeling of human liver alpha-L-fucosidase (alpha-L-fucoside fucohydrolase, EC 3.2.1.51) with [1-3H]conduritol C trans-epoxide revealed that there are four active sites per tetrameric enzyme complex. Solvent isotope effect experiments give evidence for a proton transfer at the rate-limiting step in catalysis. Transglycosylase activity was observed using methanol as an alternative glycone acceptor to produce methyl alpha-L-fucoside, suggesting that alpha-L-fucose is formed when water is the acceptor. Initial burst kinetics experiments suggest that a glycosyl-enzyme intermediate is formed, although the magnitude of the burst is not stoichiometric with the number of active sites. These data, along with previous results, suggest a general acid-general base catalytic mechanism involving double inversion of stereochemistry at C-1 of fucose, as well as the formation of either a covalent glycosyl-enzyme intermediate or a tight ion pair between a charged active-site residue and a hypothetical fucosyl oxocarbonium ion intermediate.

Purification and kinetic studies of an alpha-L-fucosidase of Venus mercenaria

An alpha-L-fucosidase activity has been isolated from the liver (hepatopancreas) of the common edible clam, Venus mercenaria, and has been purified approximately 300-fold (11% yield) by affinity chromatography on agarose-epsilon-amino-caproylfucosamine. Isoelectric focusing profiles were heterogeneous, revealing several isoenzymes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated the presence of a single subunit of Mr 50,000. The purified enzyme preparation contained only trace amounts of other alpha- and beta-D-glycosidases tested. In addition to p-nitrophenyl alpha-L-fucopyranoside, the enzyme hydrolyzed natural substrates such as fucose-containing milk pentasaccharides, thyroglobulin glycopeptides, human salivary glycoproteins, and blood-group-active glycosphingolipids. The enzyme preparation had a broad pH optimum range between 4.5 and 5.5. The apparent Km value with respect to p-nitrophenyl alpha-L-fucopyranoside was 0.26mM.

Purification and properties of two forms of human alpha-L-fucosidase

High (100 000) and low (50 000) molecular weight forms of alpha-L-fucosidase (alpha-fucosidase I and II) were purified to apparent homogeneity from human spleen, liver brain and kidney on the basis of differential affinity for epsilon-amino-caproyl fucosamine-agarose bead columns. Alpha-fucosidase I (the "bound" form) consisted of two 50 000 dalton monomers; however, both forms can aggregate to tetramer and hexamer forms. Most previous studies on alpha-fucosidase have been carried out on this form of human alpha-fucosidase although the bound and unbound forms of the enzyme were present in equal amounts in human spleen. The bound (100 000) form is a sialoglycoprotein whereas the unbound (50 000) form, is a neutral mannose-rich glycoprotein. Other differences with respect to amino acid composition, pH optimum, electrophoretic mobility, KM, thermal stability, and natural substrate specificities were observed. All preparations hydrolysed 4-methylumbelliferyl-alpha-L-fucoside, flucosyllactose, and lacto-N-fucopentaose I, but the unbound fraction (alpha-fucosidase II) preferentially hydrolyused lacto-N-fucopentaose II. The unbound (mannose-rich) alpha-fucosidase II was taken up by human skin fibroblasts with higher affinity (5% per 2 h per 2 times 10(5) cells) than the bound (sialo-) alpha-fucosidase I (<1% per 2 times 10(5) cells). Uptake was inhibited by other lysosomal hydrolases, fetal calf serum, mannose 6-phosphate and phosphomannans and to a lesser extent by heparins. Our studies suggest that alpha-fucosidase I is not a simple dimer of alpha-fucosidase II and represents a less-biologically active form of the enzyme.