Human desmocollin 3-specific IgG antibodies are pathogenic in a humanized HLA-class II transgenic mouse model of pemphigus

Pemphigus is a potentially lethal autoimmune bullous skin disorder, which is associated with IgG autoantibodies against desmoglein 3 (Dsg3) and Dsg1. Notably, a subset of pemphigus patients presents with a similar clinical phenotype in the absence of anti-Dsg IgG, suggesting the presence of serum IgG reactive with desmosomal components other than Dsg1 or Dsg3. We and others have previously shown that such patients have serum IgG autoantibodies against desmocollin 3 (Dsc3), a component of desmosomes, that induce loss of keratinocyte adhesion ex vivo. Moreover, Dsc3 hypomorphic mice show a severe blistering phenotype of the mucous membrane which is highly characteristic in pemphigus. These findings prompted us to study induction and regulation of anti-human Dsc3 IgG in humanized mice transgenic for HLA-DRB1*04:02, which is a highly prevalent haplotype in pemphigus. We show that IgG from sera of immunized mice induce acantholysis in a dispase-based keratinocyte dissociation assay via the activation of p38 mitogen-activated protein kinases and epidermal growth factor receptor. Passive IgG transfer from mice immunized with recombinant human Dsc3 into neonates did not induce intraepidermal loss of adhesion presumably due to the lack of homology between human and mouse Dsc3. Ex vivo stimulation of splenocytes from Dsc3-immunized mice with human Dsc3 leads to a significant proliferative interferon-γ and interleukin 4 T cell response, which is restricted by HLA-DR/DQ. These findings suggest that induction of pathogenic anti-Dsc3 IgG is associated with Dsc3-specific T cells that recognize Dsc3 in association with HLA-DRB1*04:02.

Susceptibility-Weighted Imaging Findings in Aspartylglucosaminuria

BACKGROUND AND PURPOSE

Aspartylglucosaminuria is a rare lysosomal storage disorder that causes slowly progressive, childhood-onset intellectual disability and motor deterioration. Previous studies have shown, for example, hypointensity in the thalami in patients with aspartylglucosaminuria on T2WI, especially in the pulvinar nuclei. Susceptibility-weighted imaging is a neuroimaging technique that uses tissue magnetic susceptibility to generate contrast and is able to visualize iron and other mineral deposits in the brain. SWI findings in aspartylglucosaminuria have not been reported previously.

MATERIALS AND METHODS

Twenty-one patients with aspartylglucosaminuria (10 girls; 7.4-15.0 years of age) underwent 3T MR imaging. The protocol included an SWI sequence, and the images were visually evaluated. Thirteen patients (6 girls, 7.4-15.0 years of age) had good-quality SWI. Eight patients had motion artifacts and were excluded from the visual analysis. Thirteen healthy children (8 girls, 7.3-14.1 years of age) were imaged as controls.

RESULTS

We found a considerably uniform distribution of decreased signal intensity in SWI in the thalamic nuclei in 13 patients with aspartylglucosaminuria. The most evident hypointensity was found in the pulvinar nuclei. Patchy hypointensities were also found especially in the medial and anterior thalamic nuclei. Moreover, some hypointensity was noted in globi pallidi and substantia nigra in older patients. The filtered-phase images indicated accumulation of paramagnetic compounds in these areas. No abnormal findings were seen in the SWI of the healthy controls.

CONCLUSIONS

SWI indicates accumulation of paramagnetic compounds in the thalamic nuclei in patients with aspartylglucosaminuria. The finding may raise the suspicion of this rare disease in clinical practice.

Regulation of cargo transfer between ESCRT-0 and ESCRT-I complexes by flotillin-1 during endosomal sorting of ubiquitinated cargo

Ubiquitin-dependent sorting of membrane proteins in endosomes directs them to lysosomal degradation. In the case of receptors such as the epidermal growth factor receptor (EGFR), lysosomal degradation is important for the regulation of downstream signalling. Ubiquitinated proteins are recognised in endosomes by the endosomal sorting complexes required for transport (ESCRT) complexes, which sequentially interact with the ubiquitinated cargo. Although the role of each ESCRT complex in sorting is well established, it is not clear how the cargo is passed on from one ESCRT to the next. We here show that flotillin-1 is required for EGFR degradation, and that it interacts with the subunits of ESCRT-0 and -I complexes (hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs) and Tsg101). Flotillin-1 is required for cargo recognition and sorting by ESCRT-0/Hrs and for its interaction with Tsg101. In addition, flotillin-1 is also required for the sorting of human immunodeficiency virus 1 Gag polyprotein, which mimics ESCRT-0 complex during viral assembly. We propose that flotillin-1 functions in cargo transfer between ESCRT-0 and -I complexes.

Influence of a HTR2B Stop Codon on Glucagon Homeostasis and Glucose Excursion in Non-Diabetic Men

Limited data are available about the role of the serotonin 2B (5-HT2B) receptor in the function of human islets. This study aimed to test whether the 5-HT2B receptor contributes to glucose, insulin, and glucagon homeostasis in humans, utilizing a hereditary loss-of-function gene mutation in the receptor, which causes a 50% reduction in the production of the receptor protein in heterozygotes. This clinical study enrolled participants recruited by newspaper advertisements and from mental status examinations. A cohort of participants from a young Finnish founder population composed of 68 non-diabetic males with a mean age of 30 was divided into groups for comparison based on being a 5-HT2B receptor loss-of-function gene mutation (HTR2B Q20*) heterozygote carrier (n=11) or not (n=57). Serum levels of glucose, insulin, and glucagon were measured in a 5 h oral glucose tolerance test using a 75 g glucose challenge. Insulin resistance, insulin sensitivity, and beta cell activity were calculated using the homeostasis model assessment (HOMA2) and whole body insulin sensitivity index (WBISI), as well as the ratio of glucagon to insulin was noted. The areas under the curves (AUCs) were also determined. Concentrations of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in cerebrospinal fluid (CSF). Covariate adjusted mean score comparisons were applied. Lower glucagon secretion and decreased glucose excursion were observed among HTR2B Q20* carriers as compared with individuals who were homozygotes for the wild-type Q20 allele (controls). No differences in insulin secretion, beta cell activity, insulin resistance, or insulin sensitivity were observed. The glucagon to insulin ratio differed between the HTR2B Q20* carriers and controls. CSF levels of 5-HIAA were similar between groups. Our findings indicate that the 5-HT2B receptor may contribute to the regulation of human glucagon and glucose homeostasis and the interplay between glucagon and insulin secretion.

Barebacking among men who have sex with men recruited through a Swedish website: associations with sexual activities at last sexual encounter

The research topic of barebacking emerged in the mid-1990s. Since then, a multitude of studies, largely from the United States, have produced invaluable knowledge of factors that help explain the behaviour among men who have sex with men (MSM), and that may contribute to HIV risk reduction programming and advice to counsellors working with barebackers. Given the scant empirical research about barebacking among European MSM, we conducted a survey among 3,634 MSM recruited through a web community in Nordic countries. The objectives of the study were twofold: to describe the sexual activities associated with barebacking behaviour at last sexual encounter, and to evaluate the relationship of barebacking with relevant variables. Men who reported barebacking (n=356) and men who did not (n=3,278) were compared. On the basis of the results of the analyses, the socio-sexual profile of barebackers drawn was one that is at increased risk of acquiring human immunodeficiency virus (HIV) and other sexually transmitted infections due to their sexual practices, particularly unprotected anal intercourse, but also group sex and rimming. In a multivariate logistic regression analysis, the likelihood of engaging in barebacking was higher for MSM who reported more frequent HIV testing (odds ratio (OR)=5.16), a higher number of female sex partners (OR=16.80), using gay cruising places (OR=1.51) and gay chat rooms (OR=2.11).

Barebacking among men who have sex with men recruited through a Swedish website: associations with sexual activities at last sexual encounter

The research topic of barebacking emerged in the mid-1990s. Since then, a multitude of studies, largely from the United States, have produced invaluable knowledge of factors that help explain the behaviour among men who have sex with men (MSM), and that may contribute to HIV risk reduction programming and advice to counsellors working with barebackers. Given the scant empirical research about barebacking among European MSM, we conducted a survey among 3,634 MSM recruited through a web community in Nordic countries. The objectives of the study were twofold: to describe the sexual activities associated with barebacking behaviour at last sexual encounter, and to evaluate the relationship of barebacking with relevant variables. Men who reported barebacking (n=356) and men who did not (n=3,278) were compared. On the basis of the results of the analyses, the socio-sexual profile of barebackers drawn was one that is at increased risk of acquiring human immunodeficiency virus (HIV) and other sexually transmitted infections due to their sexual practices, particularly unprotected anal intercourse, but also group sex and rimming. In a multivariate logistic regression analysis, the likelihood of engaging in barebacking was higher for MSM who reported more frequent HIV testing (odds ratio (OR)=5.16), a higher number of female sex partners (OR=16.80), using gay cruising places (OR=1.51) and gay chat rooms (OR=2.11).

The biological role of the Alzheimer amyloid precursor protein in epithelial cells

Proteolytic processing of the Alzheimer amyloid precursor protein (APP) results in the generation of at least two distinct classes of biologically relevant peptides: (1) the amyloid beta peptides which are believed to be involved in the pathogenesis of Alzheimer's disease and (2) the soluble N-terminal ectodomain (sAPP) which exhibits a protective but as yet ill-defined effect on neurons and epithelial cells. In this report we present an overview on the functions of sAPP as an epithelial growth factor. This function involves specific binding of sAPP to membrane rafts and results in signal transduction and various physiological effects in epithelial cells as different as keratinocytes and thyrocytes. At nanomolar concentrations sAPP induces a two to fourfold increase in the rate of cell proliferation and cell migration. Specific inhibition of APP expression by antisense techniques results in decreased sAPP release and in reduced proliferative and motogenic activities. Proliferation and migration are known to be part of complex processes such as wound healing which, therefore, might be facilitated by the growth factor function of sAPP.

The R-SNARE endobrevin/VAMP-8 mediates homotypic fusion of early endosomes and late endosomes

Endobrevin/VAMP-8 is an R-SNARE localized to endosomes, but it is unknown in which intracellular fusion step it operates. Using subcellular fractionation and quantitative immunogold electron microscopy, we found that endobrevin/VAMP-8 is present on all membranes known to communicate with early endosomes, including the plasma membrane, clathrin-coated pits, late endosomes, and membranes of the trans-Golgi network. Affinity-purified antibodies that block the ability of endobrevin/VAMP-8 to form SNARE core complexes potently inhibit homotypic fusion of both early and late endosomes in vitro. Fab fragments were as active as intact immunoglobulin Gs. Recombinant endobrevin/VAMP-8 inhibited both fusion reactions with similar potency. We conclude that endobrevin/VAMP-8 operates as an R-SNARE in the homotypic fusion of early and late endosomes.

Differences between Internet samples and conventional samples of men who have sex with men: implications for research and HIV interventions

The Internet is becoming a new erotic oasis for obtaining sex online or in person. We reviewed the literature on cybersex and compared differences in data from samples of homosexually active men obtained on identical questionnaires from a conventional written questionnaire, distributed through the mailing and contact lists of a large national gay organization in Sweden, and through the same organization's website and chat room. A total of 716 written questionnaires and 678 Internet questionnaires were obtained. The Internet sample was younger, more likely to live in small towns or cities, live with parents or a girlfriend, and have lower formal education. They are less likely to have previous sexual experience solely with other men (one in three of the Internet sample vs. 1 in 14 of the written sample defined themselves as bisexual) and more likely to visit erotic oases such as bathhouses, video clubs and erotic movie houses. They also visited Internet chat rooms more frequently (86% of the Internet sample vs. 50% of the written sample). One third of the Internet sample wanted the opportunity to talk with an expert about HIV compared with a quarter of the written sample. Sexual practices between the two samples were generally similar, although the Internet sample reported significantly less body contact, kissing, hugging, mutual masturbation, and more condom use for anal intercourse with steady partners. Over four times as many of the Internet samples reported sex with women in the past year as the written sample. These data indicate that Internet data collection is feasible and that this mode of data collection, despite the nonrandom and self-selected nature of both types of samples, is likely to be more significantly oriented toward the young, geographically more isolated, and more behaviorally and self-identified bisexual respondent than conventionally distributed written questionnaires.

The dileucine motif within the tail of MPR46 is required for sorting of the receptor in endosomes

The cytoplasmic tail of MPR46 carries a leucine-based motif that is required for the sorting of lysosomal enzymes by the receptor. In addition, it is one of three independent, but functionally redundant, internalization signals present in the cytoplasmic tail of MPR46. We have analyzed a mutant of MPR46, in which the dileucine pair was replaced by alanines (MPR46 LL/AA) with respect to its intracellular distribution and trafficking. Ultrastructural analysis of cells expressing the MPR46 LL/AA mutant revealed that the substitution of the dileucine pair causes a shift of the receptor distribution from the TGN, where it is packaged into AP1-containing vesicles, to vesicular structures distributed throughout the cytoplasm. The vesicles could be identified as early endosomes with internalized BSA-gold and rab5 as markers. By analyzing the receptor trafficking biochemically, we found that return of the LL/AA mutant receptor from the plasma membrane/endosome pool back to the TGN was impaired, while recycling from endosomes to the plasma membrane was enhanced. In conclusion, our data indicate that the dileucine motif in the MPR46 tail is required for a sorting event in endosomes.

Defective intracellular transport of CLN3 is the molecular basis of Batten disease (JNCL)

Batten disease [juvenile-onset neuronal ceroid lipofuscinosis (JNCL)], the most common progressive encephalopathy of childhood, is caused by mutations in a novel lysosomal membrane protein (CLN3) with unknown function. In this study, we have confirmed the lysosomal localization of the CLN3 protein by immunoelectron microscopy by co-localizing it with soluble and membrane-associated lysosomal proteins. We have analysed the intracellular processing and localization of two mutants, 461-677del, which is present in 85% of CLN3 alleles and causes the classical JNCL, and E295K [corrected], which is a rare missense mutation associated with an atypical form of JNCL. Pulse-chase labelling and immunoprecipitation of the two mutant proteins in COS-1-cells indicated that 461-677del is synthesized as an approximately 24 kDa truncated polypeptide, whereas the maturation of E295K [corrected] resembles that of the wild-type CLN3 polypeptide. Transient expression of the two mutants in BHK cells showed that 461-677del is retained in the endoplasmic reticulum, whereas E295K [corrected] was capable of reaching the lysosomal compartment. The CLN3 polypeptides were expressed further in mouse primary neurons where the wild-type CLN3 protein was localized both in the cell soma and in neuronal extensions, whereas the 461-677del mutant was arrested in the cell soma. Interestingly, co-localization of the wild-type CLN3 and E295K [corrected] proteins with a synaptic vesicle marker indicates that the CLN3 protein might participate in synaptic vesicle transport/transmission. The data presented here provide clear evidence for a cellular distinction between classical and atypical forms of Batten disease both in neural and non-neural cells.

Activation and oligomerization of aspartylglucosaminidase

Secretory, membrane, and lysosomal proteins undergo covalent modifications and acquire their secondary and tertiary structure in the lumen of the endoplasmic reticulum (ER). In order to pass the ER quality control system and become transported to their final destinations, many of them are also assembled into oligomers. We have recently determined the three-dimensional structure of lysosomal aspartylglucosaminidase (AGA), which belongs to a newly discovered family of homologous amidohydrolases, the N-terminal nucleophile hydrolases. Members of this protein family are activated from an inactive precursor molecule by an autocatalytic proteolytic processing event whose exact mechanism has not been thoroughly determined. Here we have characterized in more detail the initial events in the ER required for the formation of active AGA enzyme using transient expression of polypeptides carrying targeted amino acid substitutions. We show that His124 at an interface between two heterodimers of AGA is crucial for the thermodynamically stable oligomeric structure of AGA. Furthermore, the side chain of Thr206 is essential both for the proteolytic activation and enzymatic activity of AGA. Finally, the proper geometry of the residues His204-Asp205 seems to be crucial for the activation of AGA precursor polypeptides. We propose here a reaction mechanism for the activation of AGA which could be valid for homologous enzymes as well.

Several cooperating binding sites mediate the interaction of a lysosomal enzyme with phosphotransferase

Lysosomal targeting of soluble lysosomal hydrolases is mediated by mannose 6-phosphate receptors, which recognize and bind mannose 6-phosphate residues in the oligosaccharide chains of proteins destined for delivery to lysosomes. This recognition marker is generated by the sequential action of two enzymes, the first of which, UDP-N-acetylglucosamine phosphotransferase, recognizes lysosomal enzymes on the basis of a structural determinant in their polypeptide chains. This recognition event is a key step in lysosomal targeting of soluble proteins, but the exact nature of the recognition determinant is not well understood. In this study we have characterized the phosphotransferase recognition signals of human lysosomal aspartylglucosaminidase (AGA) using transient expression of polypeptides carrying targeted amino acid substitutions. We found that three lysine residues and a tyrosine residing in three spatially distinct regions of the AGA polypeptide are necessary for phosphorylation of the oligosaccharides. Two of the lysines are especially important for the lysosomal targeting efficiency of AGA, which seems to be mostly dictated by the degree of phosphorylation of the alpha subunit oligosaccharide. On the basis of the results of this and previous studies we suggest a general model for recognition of lysosomal enzymes by the phosphotransferase.

Primary folding of aspartylglucosaminidase. Significance of disulfide bridges and evidence of early multimerization

Aspartylglucosaminidase (AGA) is a lysosomal enzyme involved in the degradation of N-linked glycoproteins in lysosomes. AGA is synthesized as an inactive precursor molecule, which is rapidly activated in the endoplasmic reticulum by a proteolytic cleavage into alpha- and beta-subunits. We have recently determined the three-dimensional structure of AGA and shown that it is a globular molecule with a heterotetrameric (alphabeta)2 structure. On the basis of structural and functional analyses, AGA seems to be the first mammalian protein belonging to a newly described protein family, the N-terminal nucleophile hydrolases. Because the activation of the prokaryotic members of the N-terminal nucleophile hydrolase family seems to be triggered by the assembly of the subunits, we have studied the initial folding and oligomerization of AGA and provide evidence that dimerization of two precursor molecules in the endoplasmic reticulum is a prerequisite for the activation of AGA. To gain further information on the structural determinants influencing the early folding of AGA, we used site-specific mutagenesis of cysteine residues to define the role of intrachain disulfide bridges in the folding and activation of the enzyme. The N-terminal disulfide bridges in both the alpha- and beta-subunits seem to have only a stabilizing role, whereas the C-terminal disulfide bridge in both subunits evidently plays an important role in the early folding and activation of AGA.

Functional analyses of active site residues of human lysosomal aspartylglucosaminidase: implications for catalytic mechanism and autocatalytic activation

Aspartylglucosaminidase (AGA) is a lysosomal asparaginase that participates in the breakdown of glycoproteins by cleaving the amide bond between the asparagine and the oligosaccharide chain. Active AGA is an (alphabeta)2 heterotetramer of two non-identical subunits that are cleaved proteolytically from an enzymatically inactive precursor polypeptide. On the basis of the three-dimensional structure recently determined by us, we have here mutagenized the putative active site amino acids of AGA and studied by transient expression the effect of targeted substitutions on the enzyme activity and catalytic properties of AGA. These analyses support the novel type of catalytic mechanism, suggested previously by us, in which AGA utilizes as the nucleophile the N-terminal residue of the beta subunit and most importantly its alpha-amino group as a base that increases the nucleophilicity of the OH group. We also provide evidence for autocatalytic activation of the inactive AGA precursor and putative involvement of active site amino acids in the proteolytic processing. The data obtained on the structure and function of AGA would indicate that AGA is a member of a recently described novel class of hydrolytic enzymes (amidohydrolases) sharing a common structural determinant in their three-dimensional structure and whose catalytic mechanisms with an N-terminal nucleophile seem basically to be similar.

Ser72Pro active-site disease mutation in human lysosomal aspartylglucosaminidase: abnormal intracellular processing and evidence for extracellular activation

Aspartylglucosaminuria (AGU) is a lysosomal storage disease caused by deficient activity of aspartylglucosaminidase (AGA). We report here a T214C mutation leading to a Ser72Pro substitution in four Arab families. This is the first naturally occurring AGU mutation involving an active-site amino acid of this recently crystallized hydrolase and it seems to represent the second most common AGU mutation worldwide. The intracellular consequences of the Ser72Pro mutation were analyzed by transient expression in COS-1 cells and we were able to demonstrate that this active-site mutation most probably does not destroy the enzyme activity per se, but specifically prevents the proteolytic activation cleavage of AGA in the endoplasmic reticulum (ER). The mutant enzyme is, however, folded correctly enough to allow mannose-6-phosphorylation and targeting to lysosomes. The overexpressed mutant enzyme remained inactive intracellularly, but the secreted mutant precursor was proteolytically activated extracellularly, resulting in a similar subunit composition to that in the wild-type AGA in the ER. The partially activated mutant enzyme was endocytosed further by the recipient cells. These data demonstrate that the proteolytic activation of AGA can also occur extracellularly and suggest that the driving mechanism of AGA precursor cleavage is autocatalytic.

Large-scale purification and preliminary x-ray diffraction studies of human aspartylglucosaminidase

Aspartylglucosaminidase (AGA) is a lysosomal asparaginase that takes part in the ordered degradation of glycoproteins and a deficiency of which results in a lysosomal accumulation disease aspartylglucosaminuria in human. The mature enzyme consists of 24-kDa and 17-kDa subunits, which are both heterogeneously glycosylated. Activation of the enzyme from a single precursor polypeptide into two subunits is accomplished in the endoplasmic reticulum (ER). The relative lack of this proteolytic capacity in several tested high-producing expression systems has complicated the production of active recombinant enzyme in high quantities, which would be an alternative for purification of this molecule for crystallization. Consequently, the AGA enzyme has to be purified directly from cellular or tissue sources for crystallographic analysis. Here we describe a large-scale purification method to produce milligram amounts of homogeneous AGA from human leukocytes. The purified AGA enzyme represents a heterogeneous pool of molecules not only due to glycosylation, but also heterogeneity at the polypeptide level, as demonstrated here. We were able to isolate a homogeneous peptide pool that was successfully crystallized and preliminary X-ray data collected from the crystals. The crystals diffract well to 2.0 angstroms and are thus suitable for determination of the crystal structure of AGA.

Three-dimensional structure of human lysosomal aspartylglucosaminidase

The high resolution crystal structure of human lysosomal aspartylglucosaminidase (AGA) has been determined. This lysosomal enzyme is synthesized as a single polypeptide precursor, which is immediately post-translationally cleaved into alpha- and beta-subunits. Two alpha- and beta-chains are found to pack together forming the final heterotetrameric structure. The catalytically essential residue, the N-terminal threonine of the beta-chain is situated in the deep pocket of the funnel-shaped active site. On the basis of the structure of the enzyme-product complex we present a catalytic mechanism for this lysosomal enzyme with an exceptionally high pH optimum. The three-dimensional structure also allows the prediction of the structural consequences of human mutations resulting in aspartylglucosaminuria (AGU), a lysosomal storage disease.

Intracellular sorting of aspartylglucosaminidase: the role of N-linked oligosaccharides and evidence of Man-6-P-independent lysosomal targeting

Aspartylglucosaminidase (AGA, E.C. 3.5.1.26) is a soluble lysosomal hydrolase that participates in the degradation of glycoproteins. Here we analyzed the special features in the intracellular targeting of this dimeric amidohydrolase, especially the role of N-linked sugars and their phosphorylation in transport and activity of heterodimeric aspartylglucosaminidase, using in vitro mutagenesis and transient expression of mutant polypeptides in COS cells. The single N-glycosylation sites of both the alpha and beta subunits were destroyed individually and in combination. Just one remaining N-glycosylation site on either subunit was sufficient for normal processing into subunits and lysosomal transport, but the totally nonglycosylated enzyme, although active and processed into subunits, was not transported into lysosomes and became trapped in the endoplasmic reticulum (ER) or secreted. The intracellular targeting of AGA was partially disturbed by the lack of glycosylation in the beta subunit, resulting in accumulation of dimeric, active polypeptides in the ER, whereas lack of oligosaccharides in the alpha subunit did not affect the intracellular targeting of AGA. N-glycans in the beta subunit were found to be essential for the long-term stability of the polypeptide in the cell, but not for initial folding or subunit processing into the active dimeric molecule. Both subunits have two glycosylation isoforms. Both forms of the alpha subunit were found to be phosphorylated, whereas only one of the two glycosylation isoforms of the beta subunit is phosphorylated. The mutant enzyme with nonglycosylated alpha subunit and nonphosphorylated beta subunit is transported into lysosomes, suggesting that AGA is capable of using an alternative, mannose-6-phosphate receptor-independent routing into lysosomes.

Immediate interaction between the nascent subunits and two conserved amino acids Trp34 and Thr206 are needed for the catalytic activity of aspartylglucosaminidase

Aspartylglucosaminidase (AGA, EC 3.5.1.26) is a dimeric lysosomal hydrolase involved in the degradation of glycoproteins. The synthesized precursor polypeptide of AGA is rapidly activated in the endoplasmic reticulum by proteolysis into two subunits. Expression of the alpha- and beta-subunits of AGA in separate cDNA constructs showed that independently folded subunits totally lack enzyme activity, and even when co-expressed in vitro they fail to produce an active heterodimer of the enzyme. Both of the subunits are required for the enzyme activity, and the immediate interaction of the subunits in the endoplasmic reticulum is necessary for the correct folding of the dimeric enzyme molecule. The specific amino acid residues essential for the active site of the AGA enzyme were further analyzed by site-directed mutagenesis and in vitro expression of mutagenized constructs. Replacement of Thr206, the most amino-terminal residue of the beta-subunit, with Ser resulted in a complete loss of enzyme activity without influencing intracellular processing or transport of the mutant polypeptide to the lysosomes. Analogously, replacement of the most amino-terminal tryptophan, Trp34 with Phe or Ser in the alpha-subunit, resulted in a totally inactive enzyme without influencing the intracellular processing or stability of the polypeptide. These results suggest that the catalytic center of this amidase is formed by the interaction of the amino-terminal parts of two subunits and requires both Trp34 in the alpha-subunit and Thr206 in the beta-subunit.

Comparative bioavailability of two commercial preparations of carbamazepine tablets

A comparative bioavailability study was performed using two commercially available types of carbamazepine tablets, by statistical analysis of serum levels and other bioavailability parameters. After single oral dose the extent of absorption from the two preparations was similar, although a statistically significant difference in absorption rates was observed. In a multiple-dose study no significant difference in serum carbamazepine levels was found, so the tablets could be considered as bioequivalent drug products.

Effect of food, food constituents and fluid volume on the bioavailability of sotalol

The effect of food, some food constituents, and large volumes of fluid taken with the drug on the relative bioavailability of sotalol has been examined in five healthy volunteers. Each subject received an oral 160 mg dose in six different experimental schedules. Venous blood samples were drawn 1, 2, 3, 4, 6 and 8 hrs after the dosing, and sotalol concentrations in serum were determined fluorometrically. The results indicate that large volumes of fluid delay but do not affect the extent of sotalol absorption. Food, especially milk, decreases the bioavailability of the drug and an interaction with calcium seems to be the major reason for the reduced absorption.