Purification and characterization of altered cystic fibrosis liver alpha-L-fucosidase

Development of a UHPLC-MS method for inhibitor screening against α-L-1,3-fucosidase

α-L-Fucosidase (AFU) is a promising therapeutic target for the treatment of inflammation, cancer, cystic fibrosis, and fucosidosis. Some of the existing analytical methods for the assessment of AFU activity are lacking in sensitivity and selectivity, since most of them are based on spectrofluorimetric methods. More recently, mass spectrometry (MS) has evolved as a key technology for enzyme assays and inhibitor screening as it enables accurate monitoring of the conversion of substrate to product in enzymatic reactions. In this study, UHPLC-MS has been utilized to develop a simple, sensitive, and accurate assay for enzyme kinetics and inhibition studies of AFU3, a member of the AFU family. A reported method for analyzing saccharide involving a porous graphitic carbon column, combined with reduction by NaBH₄/CH₃OH, was used to improve sensitivity. The conversion of saccharide into alditol could reach nearly 100% in the NaBH₄ reduction reaction. In addition, the bioanalytical quantitative screening method was validated according to US-FDA guidance, including selectivity, linearity, precision, accuracy, stability, and matrix effect. The developed method displayed a good accuracy, high sensitivity (LOD = 0.05 mg L^-1), and good reproducibility (RSD < 15%). The assay accurately measured an IC₅₀ value of 0.40 μM for the known AFU inhibitor, deoxyfuconojirimycin, which was consistent with results reported in the literature. Further validation of the assay was achieved through the determination of a high Z'-factor value of 0.89. The assay was applied to screen a marine-derived chemical library against AFU3, which revealed two marine-oriented pyrimidine alkaloids as potential AFU3 inhibitors. Graphical abstract.

Increased monocyte oxidase activity in cystic fibrosis heterozygotes and homozygotes

Freshly isolated monocytes from cystic fibrosis (CF) heterozygotes and homozygotes had significantly increased oxygen uptake and superoxide formation after surface glycoprotein stimulation than did monocytes from age- and sex-matched controls. Lack of differences among the genotypes in inhibition by simple sugars of the concanavalin A-stimulated superoxide production and lack of differences in concanavalin A-binding surface proteins suggested that different regulation of the oxidase pathway produced the increased oxygen uptake and superoxide formation in CF patients and carriers. This regulatory role is consistent with the predicted structure of the CF gene product. The results support the hypothesis that the mononuclear phagocytes of CF heterozygotes have a significantly increased ability to kill intracellular microbes and may confer a selective advantage to the host.

Dolichyl phosphate-mannosyltransferase and dolichyl phosphate-N-acetylglucosaminyltransferase activities in liver preparations from normal controls and patients with cystic fibrosis and diabetes mellitus

Optimal assay conditions have been determined in human liver preparations for the catalytic transfer of mannose and N-acetylglucosamine from GDP-mannose and UDP-N-acetylglucosamine, respectively, to dolichyl phosphate. Both enzymatic reactions have an absolute requirement for divalent cation (5 mmol/l Mn2+ optimal), detergent (Triton X-100 or Nonidet P-40) and dolichyl phosphate (as acceptor substrate) and both reactions have optimal activity at a pH value of 7.8. Preliminary characterization of the glycolipid products for both enzymatic reactions indicates that phosphorylated dolichol is the major acceptor substrate for radiolabeled mannose and N-acetylglucosamine. The activity levels and specific activities of dolichyl phosphate-mannosyltransferase are comparable in liver homogenates from normal controls and patients with cystic fibrosis and diabetes mellitus. The activity levels and specific activities of dolichyl phosphate-N-acetylglucosaminyltransferase are comparable in liver homogenates from normal controls and patients with cystic fibrosis and diabetes mellitus but considerably lower than the activity levels of dolichyl phosphate-mannosyltransferase. It appears that two of the initial steps of the lipid-mediated glycosylation pathway are normal in livers from patients with cystic fibrosis and diabetes mellitus.

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.

alpha-L-Fucosidase in cystic fibrosis: analysis of skin fibroblasts and liver

The lysosomal enzyme alpha-L-fucosidase has been examined by thin layer gel and column isoelectric focusing in skin fibroblasts and liver from patients with cystic fibrosis and controls. All three common phenotypes of the enzyme were observed in both control and CF fibroblasts. When individuals of the same alpha-L-fucosidase phenotype were compared, no major differences between the isozyme profiles of cystic fibrosis patients and controls were detected in either fibroblasts or liver tissue.

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).

Decreased sialic acid and altered binding to lectins of purified alpha 2-macroglobulin from patients with cystic fibrosis

Purified preparations of plasma alpha 2-macroglobulin from patients with cystic fibrosis are shown to have normal amounts of total hexose but as much as 40% decrease in their sialic acid content. The binding of these preparations to concanavalin A and wheat-germ agglutinin was markedly reduced as compared to normal values in controls. Intermediate values were found in obligate heterozygotes. These results suggest a possible alteration in the carbohydrate moiety of alpha 2-macroglobulin in cystic fibrosis, presumably due to a defective posttranslational process.

Increased resistance to influenza as a possible source of heterozygote advantage in cystic fibrosis

Cystic fibrosis is the most common lethal or semi-lethal genetic disease in Caucasians of Central European origin, among whom it is inherited as an autosomal recessive trait at a frequency approximately 10 times that expected from recurrent mutation alone. A decreased sialic acid content has been observed in cell surface glycoproteins on cystic fibrosis fibroblasts and in numerous soluble glycoprotein preparations from cystic fibrosis homozygotes. Sialic acid residues on cell surface glycoconjugates play an essential role in the binding and infectivity of myxoviruses and paramyxoviruses, including those causing pandemic influenza. It is suggested that increased resistance to these viruses conferred by similar but quantitatively smaller alterations in sialoglycoconjugate structure in cystic fibrosis heterozygotes may have provided a selective advantage to maintain the high frequency of the cystic fibrosis gene in Caucasian populations.

Lysosomal hydrolases in cystic fibrosis livers

The specific activity of several lysosomal hydrolases in normal and cystic fibrosis (CF) livers has been investigated. Neuraminidase activity with respect to two natural substrates (fetuin and bovine submaxillary mucin) is normal in CF livers. No significant differences were found between CF and normal livers in 4-methylumbelliferyl-alpha-D-mannosidase specific activities determined at three different pH values (4.3, 5.65 and 6.5) corresponding to lysosomal, golgi and cytosolic components of the enzyme, respectively. The specific activities of p-nitrophenyl-beta-D-mannosidase and 4-methylumbelliferyl-beta-N-acetylglucosaminidase were also both similar in CF and normal livers.