Abnormal distribution of alpha-L-fucosidase in cystic fibrosis: decreased activity in serum

Identification and characterization of a core fucosidase from the bacterium Elizabethkingia meningoseptica

All reported α-l-fucosidases catalyze the removal of nonreducing terminal l-fucoses from oligosaccharides or their conjugates, while having no capacity to hydrolyze core fucoses in glycoproteins directly. Here, we identified an α-fucosidase from the bacterium Elizabethkingia meningoseptica with catalytic activity against core α-1,3-fucosylated substrates, and we named it core fucosidase I (cFase I). Using site-specific mutational analysis, we found that three acidic residues (Asp-242, Glu-302, and Glu-315) in the predicted active pocket are critical for cFase I activity, with Asp-242 and Glu-315 acting as a pair of classic nucleophile and acid/base residues and Glu-302 acting in an as yet undefined role. These findings suggest a catalytic mechanism for cFase I that is different from known α-fucosidase catalytic models. In summary, cFase I exhibits glycosidase activity that removes core α-1,3-fucoses from substrates, suggesting cFase I as a new tool for glycobiology, especially for studies of proteins with core fucosylation.

Cell surface human alpha-L-fucosidase

The acid alpha-L-fucosidase is usually found as a soluble component of lysosomes where fucoglycoconjugates are degraded. In the present investigation, we have demonstrated the existence of a cell surface protein with enzymatic alpha-L-fucosidase activity that crossreacts specifically with a rabbit anti-(alpha-L-fucosidase) Ig. By different approaches, this alpha-L-fucosidase, which represents 10-20% of the total cellular fucosidase activity, was detected in all the tested human cells (hemopoietic, epithelial, mesenchymal). Two bands of approximately 43-49 kDa were observed, although theoretical data support the possibility of having the same genetic origin that the known 50 to 55-kDa Mr alpha-L-fucosidase. We speculate about an alternative traffic pathway for the plasma membrane alpha-L-fucosidase to work on the rapid turnover of glycoproteins.

Terminal glycosylation in cystic fibrosis

Cystic fibrosis (CF) is a common genetic disease for which the gene was identified within the last decade. Pulmonary disease predominates in this ultimately fatal disease and current therapy only slows the progression. CF transmembrane regulator (CFTR), the gene product, is an integral membrane glycoprotein that normally functions as a chloride channel in epithelial cells. The most common mutation, deltaF508, results in mislocalization and altered glycosylation of CFTR. Altered fucosylation and sialylation are hallmarks of both membrane and secreted glycoproteins in CF and the focus here is on these investigations. Oligosaccharides from CF membrane glycoproteins have the Lewis x, selectin ligand in terminal positions. In addition, two major bacterial pathogens in CF, Pseudomonas aeruginosa and Haemophilus influenzae, have binding proteins, which recognize fucose in alpha1,3 linkage and asialoglycoconjugates. We speculate that the altered terminal glycosylation of airway epithelial glycoproteins in CF contributes to the chronic infection and robust inflammatory response in the CF lung. Understanding the effects of mutant CFTR on glycosylation may provide further insight into the regulation of glycoconjugate processing as well as therapy for CF.

Variations in serum alpha-L-fucosidase activity during childhood and pregnancy

Variations in serum alpha-L-fucosidase activity (AFU) have been studied during childhood and pregnancy. 994 children, ages 1 day to 15 yr, were examined; no sex-linked difference was found, but significant variations according to age were demonstrated. AFU activity rose during the first 10-15 days after birth, remained high during the second month then decreased till the end of the first year, thereafter no significant changes were observed. In pregnancy, AFU activity rose and dropped quickly after delivery; neither hypertension nor fetal distress led to AFU activity changes during pregnancy. Thus, in addition to the great variability of AFU linked to the genetic polymorphism, the physiological factors such as age or pregnancy have to be taken into account to establish the significance of AFU variations in pathological situations.

Colorimetric assay for free and bound L-fucose

A novel, rapid, and reliable colorimetric method for measuring L-fucose has been developed. This method utilizes NADH formed from the interaction of L-fucose with fucose dehydrogenase and NAD to generate color in a reaction involving CuSO4 and neocuproine. NADH reduces Cu2+ to Cu1+ and the latter interacts with neocuproine to yield a complex with a maximal absorption at 455 nm. The reaction of NADH with copper-neocuproine is immediate and under the conditions of the assay the color formed remains stable for at least 2 h. When the assay is used to determine levels of L-fucose, the absorbance is found to be linearly proportional to exogenously added fucose concentrations from 16 to 179 nmol with resulting molar extinction coefficient of 13,660. Using this procedure, L-fucose released by acid hydrolysis from porcine submaxillary mucin, and by alpha-L-fucosidase from p-nitrophenyl-alpha-L-fucopyranoside, was quantitated.

Lack of proteolytic processing of alpha-L-fucosidase in human skin fibroblasts

Acid hydrolases are synthesized as precursors that undergo several posttranslational modifications including proteolytic processing to a smaller mature enzyme. The amount of proteolytic processing varies for different acid hydrolases, and many details of the intracellular pathways are not known. The processing of alpha-L-fucosidase was distinguished from that of other acid hydrolases reported when studied in systematic pulse-chase labeling experiments. Only one form of alpha-L-fucosidase, Mr 56,000-57,000, was demonstrated intra- and extracellularly. Under the same conditions, N-acetyl-beta-D-glucosaminidase was shown to be processed with several forms, as previously reported by Hasilik and Neufeld (1980a). To obtain these results, human skin fibroblasts were labeled metabolically with L-[3H]leucine for periods of 20 min to 8 hr with varying periods of chase from 1 to 96 hr with nonradioactive L-leucine. alpha-L-Fucosidase was immunoprecipitated by a polyclonal antibody from material extracted from cells and ammonium sulfate precipitated medium and was examined by polyacrylamide gel electrophoresis under denaturing conditions. N-Acetyl-beta-D-glucosaminidase was examined with similar procedures and served as a control for the methods. Tunicamycin treatment of the cells was used to show that glycosylation did not obscure proteolytic processing because, again, only one form of the intra- and extracellular enzyme was observed, although of smaller size, Mr 52,000-53,000. In addition, separation of the cells into prelysosomal and lysosomal fractions showed only one form of the enzyme. It is concluded that alpha-L-fucosidase does not undergo proteolytic processing in human skin fibroblasts in the usual manner described for other acid hydrolases.

Serum lysosomal hydrolases in cystic fibrosis

The activities of a number of lysosomal hydrolases were determined in sera from 100 patients with cystic fibrosis (CF), age 2-35 yr, and age-matched controls: beta-hexosaminidase activity was significantly elevated (p less than 0.005) in CF patients from all age groups. alpha-Mannosidase activity was increased only in the older CF patients (greater than 13 yr). The following enzyme activities were not altered in CF serum: alpha-fucosidase, beta-glucuronidase and acid phosphatase. The abnormal patterns of serum alpha-mannosidase and beta-hexosaminidase in CF cannot be explained by pancreatic disease or undernutrition, since serum values of these hydrolases in patients with anorexia nervosa or acute pancreatitis were not altered. However, the altered activities of the alpha-mannosidase and beta-hexosaminidase were proportional to the degree of pulmonary insufficiency in the CF group, indicating that these changes are probably a secondary consequence of the primary disease process. Except for beta-hexosaminidase, because differences in the serum hydrolases in CF do not become apparent until the second decade of life, determinations of lysosomal enzyme activities in serum will probably be of little diagnostic value.

Urinary alpha-L-fucosidase in cystic fibrosis

The binding of the urinary lysosomal enzyme alpha-L-fucosidase to free- and Sepharose 4B-bound concanavalin A has been compared in cystic fibrosis (CF) patients and normal controls. The concentration of methyl-alpha-D-mannoside necessary to prevent 50% of total alpha-L-fucosidase activity to bind to free and bound concanavalin A (Ki, 50%) was similar for CF (0.68 +/- 0.20 and 1.3 +/- 0.3 mmol/l, respectively) and normal controls (0.53 +/- 0.18 and 1.9 +/- 0.5 mmol/l, respectively). The CF and normal urinary alpha-L-fucosidase also showed similar pH optima (4.8), Km, app (0.071 and 0.074 mmol/l, respectively) and thermodenaturation curves at 44 degrees C (t1/2 = 108 min). We report that the kinetic and the concanavalin A-binding affinity of alpha-L-fucosidase are similar from urine of cystic fibrosis patients and controls.

Activity levels and properties of acid alpha-glucosidase from liver and neutral alpha-glucosidase from sera of cystic fibrosis patients and controls

The average activity levels of acid alpha-glucosidase are comparable in liver supernatant fluids for 15 cystic fibrosis patients and 12 controls (401 +/- 131 and 347 +/- 109 nmol/h/mg protein, respectively) and no significant differences were found for the cystic fibrosis and control liver acid alpha-glucosidases in their (a) apparent Km values for the 4-methylumbelliferyl substrate (1.1 mmol/l), (b) pH optima (4.2) and thermostability curves and (c) isoelectric profiles (one form with an isoelectric point of 4.5 +/- 0.2). In contrast, average neutral alpha-glucosidase activity levels were significantly increased (p less than 0.0002) in sera from 21 cystic fibrosis patients compared to 15 controls (10.7 and 2.7 nmol/h/ml). This increased activity is not due to (a) different stability upon storage at --20 degrees C, (b) the presence of activators in cystic fibrosis sera or inhibitors in normal sera (as determined by mixing studies), (c) altered Km values or (d) altered pH optima curves. Cystic fibrosis serum neutral alpha-glucosidase appears to be more thermostable and has a consistently altered isoelectric profile (greater percentage of activity above pI 4.8) when compared to the normal serum enzyme. This altered isoelectric composition may reflect changes in neutral alpha-glucosidase which contribute to its increased activity in cystic fibrosis sera.

Susceptibility to neuraminidase of alpha-L-fucosidase and N-acetyl-beta-D-glucosaminidase of cystic fibrosis, I-cell and neuraminidase-deficient fibroblasts

Intracellular alpha-L-fucosidase and hexosaminidase showed similar isoelectro-focusing patterns in control, cystic fibrosis and neuraminidase-deficient fibroblasts and were unaffected by neuraminidase treatment. An I-cell strain excreted these two enzymes at 3-4 times the rate of the three other cell types. I-cell and neuraminidase-deficient cells excreted more of the electronegative forms of these enzymes than control and cystic fibrosis cells. Extracellular hexosaminidase A and B were both sensitive to neuraminidase for the four cell types. Extracellular alpha-L-fucosidase consisted of a pH 6.1 form insensitive to neuraminidase and other forms that were sensitive and changed to a pI 7.0-7.1 form. Cystic fibrosis extracellular alpha-L-fucosidase and hexosaminidase behaved as for control fibroblasts.

Cystic fibrosis serum alpha-l-fucosidase: confirmation of normal activity levels and normal kinetic and isoelectric focusing properties

Comparable alpha-L-fucosidase activity was found for normal and cystic fibrosis sera, 7.8 +/- 3.3 and 9.8 +/- 4.7 nmol/min/ml, respectively. Isoelectric focusing of normal and cystic fibrosis sera alpha-L-fucosidase revealed similar isoelectric profiles with most activity between isoelectric points of 4.7--5.4. Kinetic analysis of normal and cystic fibrosis sera alpha-L-fucosidase revealed identical apparent Michaelis constants for the 4-methylumbelliferyl substrate (51 +/- 6 mumol/l and 50 +/- 8 mumol/l, respectively), similar pH optima curves (both have broad optima between pH 4.8 and 6.5) and identical thermostability curves at three different preincubation temperatures (37 degrees, 45 degrees and 55 degrees C).