Radiometric assays of N-acetylglucosaminylphosphotransferase and alpha-N-acetylglucosaminyl phosphodiesterase with substrates labeled in the glucosamine moiety

Clinical variability in mucolipidosis III (pseudo-Hurler polydystrophy)

Mucolipidosis III (MLIII) is caused by a deficiency of UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine 1-phosphotransferase (phosphotransferase) activity, an enzyme responsible for the formation of the recognition marker on most lysosomal enzymes. The consequences of this defect are impairment of many lysosomal catabolic processes. A deficiency of phosphotransferase activity causes two phenotypically different diseases: mucolipidosis II and a rare form, mucolipidosis III (pseudo-Hurler polydystrophy). The purpose of this article is to report three patients with ML III, presenting quite different clinical courses: Patient 1 is a 13-year-old girl in whom the only symptoms of ML III were joint stiffness of the hands. Patients 2 and 3 are sibs: a 5-year-old boy with a severe form of ML III and his 2-year-old sister, who is less affected than her brother at the same age. A comparison of biochemical results and the clinical picture of our patients with cases in the literature is presented.

Role of free radicals in the pathogenesis of lipid-induced glomerulosclerosis in rats

BACKGROUND

We examined whether a high-cholesterol (HC) diet causes glomerulosclerosis in rats, and investigated the role of free radicals and lipid peroxidation in lipid-induced glomerulosclerosis.

METHODS

The rats were given a normal diet, a HC diet, or a HC diet with antioxidants and radical scavengers. Serum levels of lipid, lipid peroxide (LOOH), urinary excretion of protein (UP), and urinary norepinephrine excretion (UNE) were measured. The glomerular sclerosing score was used to evaluate the renal injury.

RESULTS

Blood pressure, total cholesterol, and LOOH were increased by a HC diet, as were UP and UNE. The HC diet induced renal injury. Treatment with superoxide dismutase, dimetylthiourea as a scavenger of hydroxyl radical (OH.), defferoxamine masilate as an iron chelator, or vitamin E inhibited the increases in blood pressure, LOOH, UP, and UNE, whereas total cholesterol was not affected. The production of superoxide anion (O2-.) by neutrophil and LOOH in the kidney was increased, and superoxide dismutase and hydrogen peroxide in the kidney were decreased. Almost all of these changes were attenuated by vitamin E; however, the O2-. production was not inhibited. OH. was increased by the HC diet, and it was normalized with the treatments. Furthermore, the sclerosing score was partially suppressed by the treatments. Ferric iron was stained in the proximal tubulus, and it was not observed in the treated rats.

CONCLUSIONS

The data suggest that lipid peroxidation is involved in the pathogenesis of lipid-induced glomerulosclerosis and that O2-. and OH. may play a role in the process.

Lysosomal phospholipase activity is decreased in mucolipidosis II and III fibroblasts

Mucolipidosis (ML) II and III are rare autosomal recessively inherited diseases characterized by deficiency of multiple lysosomal enzymes and, as a result, a generalized storage of macromolecules in lysosomes of cells of mesenchymal origin. In ML II and ML III fibroblasts, most, but not all, newly synthesized lysosomal enzymes are secreted into the medium instead of being targeted correctly to lysosomes. Defects in the enzyme UDP-N-acetylglucosamine:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase underlie this effect. It is unknown how lysosomal phospholipases are targeted to the lysosomes of fibroblasts. In the present study lysosomal phospholipase activity was determined in delipidated fibroblast homogenates and plasma from ML II and ML III patients and controls using a [3H]choline-labeled phosphatidylcholine. After incubation, residual phosphatidylcholine and its labeled degradation products (lysophosphatidylcholine, glycerophosphorylcholine and choline phosphate) were quantified. We found that ML II and ML III fibroblasts are deficient in lysosomal phospholipase A and C activity. These enzymes were present in elevated amounts in plasma of ML II and ML III patients. These data indicate that phospholipases, like most other lysosomal enzymes in these diseases, are secreted into the blood instead of being targeted specifically to lysosomes. Thus, the mannose-6-phosphate receptor pathway is needed for proper delivery of lysosomal phospholipases to lysosomes. We also found that production of labeled choline phosphate was mainly due to the activity of acid sphingomyelinase instead of phospholipase C under the assay conditions used. Other active lipolytic enzymes were phospholipase A and lysophospholipase. No evidence for phospholipase D activity was found.

Spontaneous mucolipidosis in a cat: an animal model of human I-cell disease

A 7-month-old female cat was seen for abnormal facial features and abnormality of gait. Facial dysmorphism, large paws in relation to body size, dysostosis multiplex, and poor growth were noted, and mucopolysaccharidosis was suspected. A negative urine test for sulfated glycosaminoglycans and extreme stiffness of skin indicated a mucolipidosis hitherto unknown in animals. Deficiency of UDP-N-acetylglucosamine: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase, EC 2.7.8.17) activity was demonstrated in leukocytes and cultured fibroblasts, which had the appearance of inclusion cells (I-cells). Activities of a set of lysosomal hydrolases were abnormally low in fibroblasts and excessive in blood plasma. Postmortem morphology revealed lysosomal inclusions predominantly in fibroblasts but also in endothelial cells and chondrocytes, i.e., in cells of mesenchymal origin. Storage lysosomes contained oligosaccharides, mucopolysaccharides, and lipids. Tissues most affected were bones, cartilage, skin, and other connective tissues such as those in heart valves, aortic wall, and vocal cords. Parenchymal cells of liver and kidney were unaffected, as was skeletal muscle. Only a few of the cerebral cortical neurons had lipid inclusions; in sciatic nerve some axons were affected, but other peripheral nerves were normal. There were striking clinical, biochemical, and morphologic similarities between the disorder in this cat and the human I-cell disease.

A mild form of mucolipidosis type III in four Baluch siblings

Four Baluch siblings with mucolipidosis type III (pseudo-Hurler polydystrophy) are described. The patients had features commonly found in mucolipidosis III, including claw hands, joint stiffness, aortic valve involvement and radiological dysostosis multiplex. However, intelligence was normal, there were no eye abnormalities on slit-lamp examination and skin elasticity was normal. Many lysosomal enzymes were elevated in serum and diminished in cultured fibroblasts, although the findings for beta-galactosidase were atypical. Assays for the two enzymes involved in formation of the phosphomannose recognition marker revealed normal activity of the phosphotransferase with alpha-methylmannoside as an acceptor, and normal activity of the phosphodiester glycosidase. Metabolic labelling of fibroblasts with 32P followed by immunoprecipitation of cathepsin D, electrophoresis and fluorography showed that this enzyme was not labelled in the patients' cells, although some label was detected in the secreted precursor polypeptide. The data are consistent with the assumption that activity of the phosphotransferase is low towards lysosomal enzymes as substrates, and that the patients belong to complementation group C.

The synthesis of substrates and two assays for the detection of N-acetylglucosamine-1-phosphodiester alpha-N-acetylglucosaminidase (uncovering enzyme)

A method for the synthesis and purification of large quantities of four radiolabeled substrates for quantitation of uncovering enzyme is described. Four substrates, [3H]GlcNAc-alpha-P-Man alpha Me, [3H]GlcNAc-alpha-P-uteroferrin, [3H]GlcNAc alpha-P-Man alpha 1-2Man-O-Me, and [3H]GlcNAc alpha-P-Man9GlcNAc, were enzymatically synthesized using GlcNAc-phosphotransferase from Acanthamoeba castellanii and uridine diphosphate N-acetyl-[3H]glucosamine and, as acceptor, methyl-alpha-D-mannopyranoside (Man alpha Me), uteroferrin, Man alpha 1-2Man-O-methyl, or Man9GlcNAc. The isolation of the [3H]GlcNAc-P-modified product of each reaction is detailed. Two assays for the detection of uncovering enzyme activity using [3H]GlcNAc-alpha-P-uteroferrin and [3H]GlcNAc-alpha-P-Man alpha Me are outlined. The ability to easily synthesize four relevant substrates for uncovering enzyme offers flexibility in assaying uncovering enzyme.

Elevated carbohydrate phosphotransferase activity in human hepatoma and phosphorylation of cathepsin D

To determine the cause of the increased content of carbohydrate-bound phosphate in tumour lysosomal hydrolases, the activity and kinetics in human hepatocellular carcinoma of two enzymes involved in the formation of mannose-6-phosphate in lysosomal hydrolases UDP-GlcNAc: lysosomal enzyme GlcNAc alpha l-phosphotransferase (GlcNAc-phosphotransferase) and phosphodiester glycosidase were studied. The activity level of the phosphotransferase with artificial and natural substrates was elevated (P less than 0.025 and P less than 0.001, respectively) in hepatoma compared to that in uninvolved tissue, while the phosphodiester glycosidase of hepatoma was at a level similar to that of the uninvolved tissue. To verify a previous observation that cathepsin D of human hepatoma contained increased GlcNAc-phosphomannose, the protease was examined for carbohydrate phosphorylation by the GlcNAc-phosphotransferase. The protease from normal human liver was much more phosphorylated than hepatoma protease, confirming the previous observation. The predominant phosphorylation of the protease occurred in one of two major heavy subunits, with some phosphorylation in one of two minor light subunits.

Prenatal diagnosis of I-cell disease in the first and second trimesters

First trimester prenatal diagnosis of I-cell disease (1 case) was based on demonstration of profound deficiency of N-acetylglucosamine 1-phosphotransferase in chorionic villi and in cultured trophoblasts derived from the chorionic villus specimen. Deficiency of this enzyme in cultured amniotic fluid cells obtained via amniocentesis was the basis for prenatal diagnosis of I-cell disease in the second trimester (2 cases). In both procedures, the diagnosis was corroborated by the finding of intracellular deficiency and extracellular elevation of multiple lysosomal enzymes in the fetal cell cultures (trophoblasts and amniotic fluid cells), as well as a significant increase in several lysosomal enzyme activities in the maternal serum.

Altered molecular size of N-acetylglucosamine 1-phosphotransferase in I-cell disease and pseudo-Hurler polydystrophy

The size of the mutant N-acetylglucosamine 1-phosphotransferase in Golgi membranes from fibroblasts of patients with I-cell disease and classical pseudo-Hurler polydystrophy, which comprised one complementation group characterized by deficiency towards both artificial and natural acceptor substrates, was significantly smaller than the normal enzyme, 151-174 kDa compared with 225-278 kDa. The size of the mutant enzyme from cell lines of patients with variant forms of pseudo-Hurler polydystrophy, which comprised another complementation group characterized by normal activity towards mono- and oligo-saccharide substrates, was significantly larger than the normal enzyme, ranging from 321 to 356 kDa in two families and from 528 to 547 kDa in a third family. These findings suggest that the mutations in I-cell disease and classical pseudo-Hurler polydystrophy result in a missing enzyme component, which renders the enzyme catalytically inefficient toward any type of acceptor substrate. In contrast, the mutations in the variant forms of pseudo-Hurler polydystrophy produce a larger enzyme molecule which is active toward small substrates but is incapable of binding natural lysosomal glycoprotein substrates.

UDP-N-acetylglucosamine: lysosomal enzyme precursor N-acetylglucosamine-1-phosphate transferase activities in human ovarian tumor tissue and some transformed cell lines

Uridine diphosphate-N-acetylglucosamine: lysosomal enzyme precursor N-acetyl-glucosamine-1-phosphate transferase, is a key enzyme involved in the intracellular targeting of lysosomal enzymes. This enzyme is elevated fourfold in primary ovarian tumor microsomes with respect to normal ovarian microsomes. This elevation is associated with significant increases in the specific activity of multiple lysosomal hydrolases, including beta-D-hexasaminidase, alpha-L-fucosidase, and beta-D-galactosidase. The activity of the phosphotransferase was also documented in several cell lines derived from human tumors. The possible role of this enzyme in tumor-associated phosphorylation is discussed.

Lysosomal enzyme activities in fresh and frozen chorionic villi and in cultured trophoblasts

Fifteen lysosomal enzyme activities were compared in 14 presumed normal chorionic villus specimens that were each divided, processed and analyzed as fresh tissue, tissue frozen for 1 week, and cultures established from minced whole villi. Most of the activities determined in the chorionic villus tissue were not affected significantly by freezing. However, activities for most enzymes were significantly different from those determined in the cultured cells. Our experience with first trimester prenatal evaluations for several lysosomal disorders showed that the limited amount of tissue obtained is not always sufficient for thorough analysis and thus, cultured trophoblasts derived from the tissue specimen should also be examined. The results of this study stress the importance of using appropriate tissue-type and cell-type controls to establish the normal range in the respective analyses.

Molecular size of N-acetylglucosaminylphosphotransferase and alpha-N-acetylglucosaminyl phosphodiesterase as determined in situ in Golgi membranes by radiation inactivation

The radiation inactivation method was used to determine the molecular size of the two enzymes that participate in the synthesis of the phosphomannosyl recognition marker of lysosomal proteins. The determinations were carried out in situ, in Golgi membranes isolated from normal human placenta and cultured skin fibroblasts. A molecular size of 228 +/- 29 kDa was found for placental N-acetylglucosaminyl-phosphotransferase, and 129 +/- 11 kDa for placental alpha-N-acetylglucosaminyl phosphodiesterase. The values for the fibroblast enzymes were about 20% higher, 283 +/- 27 kDa and 156 +/- 14 kDa for the transferase and phosphodiesterase respectively. Triton X-100 had no effect on the molecular size of these enzymes.

Synthesis of methyl 6-(ammonium 2-acetamido-2-deoxy-alpha-D-glucopyranosyl phosphate)-alpha-D-mannopyranoside and use of this compound for the determination of N-acetylglucosamine-1-phosphotransferase

Methyl 6-(ammonium 2-acetamido-2-deoxy-alpha-D-glucopyranosyl phosphate)-alpha-D-mannopyranoside was synthesized and identified by 1H-n.m.r. and 13C-n.m.r. data, acid hydrolysis, and elemental analysis. It was utilized for the determination of UDP-N-acetylglucosamine-1-phosphotransferase in an assay procedure that employed methyl alpha-D-mannopyranoside as an acceptor. The assay product was identified and characterized by thin-layer chromatography with the title reference compound. The present technique does not require [32P]UDP-N-acetylglucosamine, but effectively uses commercially available UDP-[14C]GlcNAc.

I-cell disease and pseudo-Hurler polydystrophy: heterozygote detection and characteristics of the altered N-acetyl-glucosamine-phosphotransferase in genetic variants

The human disorders I-cell disease and pseudo-Hurler polydystrophy (also known as mucolipidosis II and III, respectively) are caused by an inherited deficiency of UDP-GlcNAc: lysosomal enzyme precursor GlcNAc-P transferase activity. The most common genetic variants of these diseases (complementation group A) can be identified in homozygotes and heterozygotes using a GlcNAc-P transferase assay with artificial acceptors and commercially available radiochemicals. The kinetic characteristics of the residual GlcNAc-P transferase activity in complementation group A fibroblasts indicates that the low activity is due to a low Vmax. The measured Michaelis-Menten constants for the substrates UDP-GlcNAc and alpha-methyl mannoside are in the normal range. Homozygotes and heterozygotes of another less common variant of pseudo-Hurler polydystrophy (complementation group C) have normal activity and normal kinetic characteristics with this assay using alpha-methyl mannoside as the acceptor substrate. Several PHP variants with unusual characteristics are discussed.