Studies on the Biosynthesis of the Hydroxylysine-linked Disaccharide Unit of Basement Membranes and Collagens

Glycosylation Modulates the Structure and Functions of Collagen: A Review

Collagens are fundamental constituents of the extracellular matrix and are the most abundant proteins in mammals. Collagens belong to the family of fibrous or fiber-forming proteins that self-assemble into fibrils that define their mechanical properties and biological functions. Up to now, 28 members of the collagen superfamily have been recognized. Collagen biosynthesis occurs in the endoplasmic reticulum, where specific post-translational modification-glycosylation-is also carried out. The glycosylation of collagens is very specific and adds β-d-galactopyranose and β-d-Glcp-(1→2)-d-Galp disaccharide through β-O-linkage to hydroxylysine. Several glycosyltransferases, namely COLGALT1, COLGALT2, LH3, and PGGHG glucosidase, were associated the with glycosylation of collagens, and recently, the crystal structure of LH3 has been solved. Although not fully understood, it is clear that the glycosylation of collagens influences collagen secretion and the alignment of collagen fibrils. A growing body of evidence also associates the glycosylation of collagen with its functions and various human diseases. Recent progress in understanding collagen glycosylation allows for the exploitation of its therapeutic potential and the discovery of new agents. This review will discuss the relevant contributions to understanding the glycosylation of collagens. Then, glycosyltransferases involved in collagen glycosylation, their structure, and catalytic mechanism will be surveyed. Furthermore, the involvement of glycosylation in collagen functions and collagen glycosylation-related diseases will be discussed.

Identification of Regulatory Molecular "Hot Spots" for LH/PLOD Collagen Glycosyltransferase Activity

Hydroxylysine glycosylations are post-translational modifications (PTMs) essential for the maturation and homeostasis of fibrillar and non-fibrillar collagen molecules. The multifunctional collagen lysyl hydroxylase 3 (LH3/PLOD3) and the collagen galactosyltransferase GLT25D1 are the human enzymes that have been identified as being responsible for the glycosylation of collagen lysines, although a precise description of the contribution of each enzyme to these essential PTMs has not yet been provided in the literature. LH3/PLOD3 is thought to be capable of performing two chemically distinct collagen glycosyltransferase reactions using the same catalytic site: an inverting beta-1,O-galactosylation of hydroxylysines (Gal-T) and a retaining alpha-1,2-glucosylation of galactosyl hydroxylysines (Glc-T). In this work, we have combined indirect luminescence-based assays with direct mass spectrometry-based assays and molecular structure studies to demonstrate that LH3/PLOD3 only has Glc-T activity and that GLT25D1 only has Gal-T activity. Structure-guided mutagenesis confirmed that the Glc-T activity is defined by key residues in the first-shell environment of the glycosyltransferase catalytic site as well as by long-range contributions from residues within the same glycosyltransferase (GT) domain. By solving the molecular structures and characterizing the interactions and solving the molecular structures of human LH3/PLOD3 in complex with different UDP-sugar analogs, we show how these studies could provide insights for LH3/PLOD3 glycosyltransferase inhibitor development. Collectively, our data provide new tools for the direct investigation of collagen hydroxylysine PTMs and a comprehensive overview of the complex network of shapes, charges, and interactions that enable LH3/PLOD3 glycosyltransferase activities, expanding the molecular framework and facilitating an improved understanding and manipulation of glycosyltransferase functions in biomedical applications.

Collagen glycosylation

Despite the ubiquity of collagens in the animal kingdom, little is known about the biology of the disaccharide Glc(α1-2)Gal(β1-O) bound to hydroxylysine across collagens from sponges to mammals. The extent of collagen glycosylation varies by the types of collagen, with basement membrane collagen type IV being more glycosylated than fibrillar collagens. Beyond true collagens, proteins including collagen domains such as the complement protein 1Q and the hormone adiponectin also feature glycosylated hydroxylysine. Collagen glycosylation is initiated in the endoplasmic reticulum by the galactosyltransferases COLGALT1 and COLGALT2. Mutations in the COLGALT1 gene cause cerebral small vessel abnormality and porencephaly, which are common in collagen type IV deficiency. Beyond the strongly conserved Glc(α1-2)Gal(β1-O) glycan, additional forms of collagen glycosylation have been described in the deep-sea worm Riftia pachyptila and in the giant virus Mimivirus, thereby suggesting that further forms of collagen glycosylation are likely to be identified in the future.

Glycosylation modulates melanoma cell α2β1 and α3β1 integrin interactions with type IV collagen

Although type IV collagen is heavily glycosylated, the influence of this post-translational modification on integrin binding has not been investigated. In the present study, galactosylated and nongalactosylated triple-helical peptides have been constructed containing the α1(IV)382-393 and α1(IV)531-543 sequences, which are binding sites for the α2β1 and α3β1 integrins, respectively. All peptides had triple-helical stabilities of 37 °C or greater. The galactosylation of Hyl(393) in α1(IV)382-393 and Hyl(540) and Hyl(543) in α1(IV)531-543 had a dose-dependent influence on melanoma cell adhesion that was much more pronounced in the case of α3β1 integrin binding. Molecular modeling indicated that galactosylation occurred on the periphery of α2β1 integrin interaction with α1(IV)382-393 but right in the middle of α3β1 integrin interaction with α1(IV)531-543. The possibility of extracellular deglycosylation of type IV collagen was investigated, but no β-galactosidase-like activity capable of collagen modification was found. Thus, glycosylation of collagen can modulate integrin binding, and levels of glycosylation could be altered by reduction in expression of glycosylation enzymes but most likely not by extracellular deglycosylation activity.

Rare and unusual glycosylation of peptides and proteins

Glycosylation represents the most complex co- and post-translational modification of proteins. In addition to N- and O-glycans, almost all combinations, including the nature of the carbohydrate moiety and the amino-acid involved, but also the type of the chemical linkage, can be isolated from natural glycoconjugates. This diversity correlates with the importance and the variety of the biological processes (and consequently the diseases) glycosides are involved in. This review focuses on rare and unusual glycosylation of peptides and proteins.

Application of topologically constrained mini-proteins as ligands, substrates, and inhibitors

Protein-protein interactions are governed by a variety of structural features. The sequence specificities of such interactions are usually easier to establish than the "topological specificities," whereby interactions may be classified based on recognition of distinct three-dimensional structural motifs. Approaches to explore topological specificities have been based primarily on assembly of mini-proteins with well defined secondary, tertiary, and/or quarternary structures. The present chapter focuses on three approaches for constructing topologically well defined mini-proteins: template-assembled synthetic proteins (TASPs), disulfide-stabilized structures, and peptide-amphiphiles (PAs). Specific examples are given for applying each approach to explore topologically-dependent protein-protein interactions. TASPs are utilized to identify a metastatic melanoma receptor that binds to the alpha1(IV)1263-1277 region of basement membrane (type IV) collagen. A disulfide-stabilized structure incorporating a sarafotoxin (SRT) 6b model was examined as a matrix metalloproteinase (MMP)-3 inhibitor. PAs were developed as (a) fluorogenic triple-helical or polyPro II substrates for MMPs and aggrecanase members of the a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family and (b) glycosylated and nonglycosylated ligands for metastatic melanoma cells. Topologically constrained mini-proteins have proved to be quite versatile, helping to define critical primary, secondary, and tertiary structural elements that modulate enzyme and receptor functions.

Melanoma cell CD44 interaction with the alpha 1(IV)1263-1277 region from basement membrane collagen is modulated by ligand glycosylation

Invasion of the basement membrane is believed to be a critical step in the metastatic process. Melanoma cells have been shown previously to bind distinct triple-helical regions within basement membrane (type IV) collagen. Additionally, tumor cell binding sites within type IV collagen contain glycosylated hydroxylysine residues. In the present study, we have utilized triple-helical models of the type IV collagen alpha1(IV)1263-1277 sequence to (a) determine the melanoma cell receptor for this ligand and (b) analyze the results of single-site glycosylation on melanoma cell recognition. Receptor identification was achieved by a combination of methods, including (a) cell adhesion and spreading assays using triple-helical alpha1(IV)1263-1277 and an Asp(1266)Abu variant, (b) inhibition of cell adhesion and spreading assays, and (c) triple-helical alpha1(IV)1263-1277 affinity chromatography with whole cell lysates and glycosaminoglycans. Triple-helical alpha1(IV)1263-1277 was bound by melanoma cell CD44/chondroitin sulfate proteoglycan receptors and not by the collagen-binding integrins or melanoma-associated proteoglycan. Melanoma cell adhesion to and spreading on the triple-helical alpha1(IV)1263-1277 sequence was then compared for glycosylated (replacement of Lys(1265) with Hyl(O-beta-d-galactopyranosyl)) versus non-glycosylated ligand. Glycosylation was found to strongly modulate both activities, as adhesion and spreading were dramatically decreased due to the presence of galactose. CD44/chondroitin sulfate proteoglycan did not bind to glycosylated alpha1(IV)1263-1277. Overall, this study (a) is the first demonstration of the prophylactic effects of glycosylation on tumor cell interaction with the basement membrane, (b) provides a rare example of an apparent unfavorable interaction between carbohydrates, and (c) suggests that sugars may mask "cryptic sites" accessible to tumor cells with cell surface or secreted glycosidase activities.

Protein glycosylation: nature, distribution, enzymatic formation, and disease implications of glycopeptide bonds

Formation of the sugar-amino acid linkage is a crucial event in the biosynthesis of the carbohydrate units of glycoproteins. It sets into motion a complex series of posttranslational enzymatic steps that lead to the formation of a host of protein-bound oligosaccharides with diverse biological functions. These reactions occur throughout the entire phylogenetic spectrum, ranging from archaea and eubacteria to eukaryotes. It is the aim of this review to describe the glycopeptide linkages that have been found to date and specify their presence on well-characterized glycoproteins. A survey is also made of the enzymes involved in the formation of the various glycopeptide bonds as well as the site of their intracellular action and their affinity for particular peptide domains is evaluated. This examination indicates that 13 different monosaccharides and 8 amino acids are involved in glycoprotein linkages leading to a total of at least 41 bonds, if the anomeric configurations, the phosphoglycosyl linkages, as well as the GPI (glycophosphatidylinositol) phosphoethanolamine bridge are also considered. These bonds represent the products of N- and O-glycosylation, C-mannosylation, phosphoglycation, and glypiation. Currently at least 16 enzymes involved in their formation have been identified and in many cases cloned. Their intracellular site of action varies and includes the endoplasmic reticulum, Golgi apparatus, cytosol, and nucleus. With the exception of the Asn-linked carbohydrate and the GPI anchor, which are transferred to the polypeptide en bloc, the sugar-amino acid linkages are formed by the enzymatic transfer of an activated monosaccharide directly to the protein. This review also deals briefly with glycosidases, which are involved in physiologically important cleavages of glycopeptide bonds in higher organisms, and with a number of human disease states in which defects in enzymatic transfer of saccharides to protein have been implicated.

Galactosyltransferases: physical, chemical, and biological aspects

Galactosyltransferases (GTs) are one of the members of a family of enzymes called glycosyltransferases involved in the biosynthesis of complex carbohydrates. These enzymes catalyze the transfer of galactose from UDP-galactose to an acceptor (glycoprotein, glycolipid) containing terminal N-acetylglucosamine or N-acetylgalactosamine residue. GTs occur in soluble (milk, serum, effusions, etc.) and insoluble (membrane) forms. The GT activities on the outer surface of the cells have been correlated with a host of cellular interactions, including fertilization, cell migration, embryonic induction, chondrogenesis, contact inhibition of growth, cell adhesion, hemostasis, intestinal cell differentiation, and immune recognition. GTs have been purified to homogeneity using affinity chromatography. Most GTs are found active in the pH range 6 to 8 and at temperatures between 35 to 40 degrees C. Manganese is an essential co-factor for GT activity. Isoenzymes of GT have been recognized, especially in tumor tissues, malignant effusions, and sera of cancer patients using polyacrylamide gel electrophoresis in the presence and absence of SDS. Depending on the source of the enzyme, the molecular weights of GTs range between 40,000 to 80,000 daltons. Carcinoma-associated GT isoenzyme has been reported to have a higher molecular weight than the normal GT isoenzyme. Development of monoclonal antibody against the cancer-specific GT isoenzyme will provide help in the development of an immunoassay for the measurement of this isoenzyme in the sera and an aid in the radioimmunolocalization of the tumors in cancer patients.

Diabetic nephropathy: clinical and histological study in 22 patients

Twenty-two patients with insulin-dependent diabetes mellitus and renal involvement were submitted to renal biopsy. Mean age was 42 years; 10 were males, 12 females. The mean interval between clinical manifestation of nephropathy and biopsy was about 2 years. At the time of biopsy, 4 groups were distinguished according to clinical conditions, depending on the presence or absence of nephrotic syndrome and renal failure. Renal lesions were semiquantitatively evaluated, a separate score being considered for glomerular and vascular lesions. Immunofluorescence most frequently showed a pattern of faint linear IgG deposits along glomerular basement membranes. Severity of histological lesions and pattern of urinary abnormalities were not correlated with the duration of diabetes or the patients' age. Both glomerular and vascular lesions were correlated with the presence of renal failure, while no relationship with the pattern of urinary abnormalities was found. Fourteen patients were followed for more than one year after biopsy: 5 had normal renal function, 4 were in chronic renal insufficiency and 5 in end-stage renal failure (3 were in dialysis, 2 died). There was no correlation between the 3 above-mentioned types of evolution and glomerular histological findings. Nevertheless a higher score of vascular impairment at biopsy was observed among patients who subsequently were found to have a more unfavorable prognosis. Therefore renal biopsy, by providing information on the degree of renal vascular damage, may have some value in predicting the clinical course of diabetic nephropathy.

[Diabetic glomerulosclerosis: current status of its morphology and pathogenesis (author's transl)]

The diabetic glomerulosclerosis as a consequence of the abnormal metabolic state is characterized by an uniform thickening of the glomerular basement membrane (GBM) and an augmentation of the mesangial matrix. Both alterations begin already few years after the onset of the diabetes and are observed in all glomeruli to the same extent (=diffuse type). Later on, nodular deposits of glycoproteins are additionally found in the mesangium (=nodular type). Only these nodules are morphologically specific for the diabetic disorder. In association with both the diffuse and the nodular glomerulosclerosis, insudation of plasma can be seen in the afferent and efferent vessels and the glomerular capillary loops. The mechanism of the increase in the amount of GBM-material is not known, since contradictory data have been reported with regards to both the chemical composition and metabolism of the GBM in human and experimental diabetes. Some postulate that the abnormal deposition of GBM-mateiral is due to an excessive synthesis (anabolic disorder), others argue that a further decrease in the normally slow breakdown and disposal (catabolic disorder) might be responsible. This review presents the different pathogenetic concepts of the glomerulosclerosis and attemps to explain the possible causes for the current discrepancies.

Human glomerular basement membrane. Heterogeneity of antigenic determinants

Human glomerular basement membrane was solubilized by digestion with proteolytic enzymes and immunoreactive components were quantitated and characterized by using rabbit antibodies raised against the particulate membrane. A number of antigens were demonstrated but they did not separate on gel filtration. However, two antigenic components in a collagenase digest of the membrane could be separated and isolated by Sepharose 6B chromatography. Chemical characterization suggests that both fragments are noncollagenous glycopeptides (molecular weights approx. 1,000,000 and 60,000--200,000, respectively).

Glycoprotein biosynthesis in calf kidney. Glycoprotein sialyltransferase activities towards serum glycoproteins and calf Tamm-Horsfall glycoprotein

CMP-AcNeu:glycoprotein sialyltransltransltransltransltransferase of calf kidney cortex was characterized using serum glycoproteins and Tamm-Horsfall glycoprotein, obtained from calf urine, as acceptors. Native calf Tamm-Horsfall glycoprotein showed the best acceptor properties, followed by desialylated calf fetuin and desialylated human alpha 1-acid glycoprotein exhibiting V values of, respectively, 114, 63 and 41 nmol/h per g wet wt. of kidney cortex and Km values of 0.12, 0.16 and 0.26 mM glycoprotein acceptor. Desialylated ovine submaxillary mucine appeared to be a very poor acceptor. Tamm-Horsfall glycoprotein sialyltransferase could be distinguished from serum glycoprotein sialyltransferase by competition studies. In addition the two glycoprotein sialyltransferase activities showed different distributions over the three regions of the calf kidney: the ratios of the Tamm-Horsfall to serum glycoprotein sialyltransferase activities decreased from 3.3 in the cortex to 0.8 and 0.4 in the medulla and the papilla, respectively. It was concluded that in calf kidney at least two different sialyltransferases exist. The high cortical Tamm-Horsfall glycoprotein sialyltransferases activity corresponds markedly to the origin of the urinary Tamm-Horsfall glycoprotein, namely the distal part of the kidney tubule. Inactivation of glycoprotein sialyltransferase activity by preincubation at various temperatures and during storage at 0 degree C, could be reduced by the addition of CMP-AcNeu. The possible relevance towards the in vivo sialylation of this finding is discussed.

Glucosyltransferase activity in kidney fractions of normal and streptozotocin-diabetic rats

Glucosyltransferase (UDPglucose: galactosylhydroxylysine-basement membrane glucosyltransferase), an enzyme specifically involved in collagen synthesis, was measured in various kidney fractions of normal, diabetic and underfed rats, using as basis the incorporation of radioactivity into protein during incubation with UDP[U-14C]glucose and alkali-soluble fetal calf-skin collagen. Three criteria of enzyme activity were compared: A, total radioactivity of the washed protein precipitate; B, this figure minus activity incorporated in the absence of the collagen acceptor; and C, radioactivity incorporated into the mixed amino acid fraction, collected by elution with dilute NH4OH from a Dowex 50 resin column after alkaline hydrolysis of the protein. Method A was found satisfactory using whole medulla or isolated glomeruli, since the average proportions of total protein radioactivity recovered in the NH3 fraction were 0.81 and 0.87, respectively, and the deviations were small. There was a larger and variable proportion of nonspecific incorporation using whole cortex. Incubation of a control set of sample without added collagen was found to be unnecessary (Method B). Per mg protein, medulla and glomeruli had more enzyme than did whole cortex. In diabetes, activity was enhanced in the 10,000 X g supernatant fraction of cortex, as previously reported. However, the increase associated with diabetes was even more consistent in the medulla, averaging 3-fold in the 10,000 X g pellet fraction. No increase was found in isolated glomeruli in diabetes. Also, no increase was seen in the kidneys of non-diabetic rats with body weight similar to that of the diabetics.

Diabetic regulation and its relationship to microangiopathy

This review addresses the question of the effectiveness of rigid regulation of the blood glucose in delaying or preventing the vascular complications of diabetes mellitus. Morphometric, pathologic, biochemical, and clinical evidence on both sides of this question are presented. It is concluded that because of present inadequate methods of completely returning glucose homeostasis to normal in diabetics, the question remains moot.

Diabetic microangiopathy. I. Current status of the chemistry and metabolism of the glomerular basement membrane

In recent years, the nature of the renal glomerular basement membrane has been the subject of numerous investigations. In diabetes mellitus, the renal glomerulus is characterized primarily by thickening of the basement membrane and excessive accumulation of basement membrane-like material in the mesangial region. Compositional analyses have shown that basement membranes are glycoprotein in nature. Studies of the glomerular basement membranes in diabetes have indicated a change from normal chemical composition. Furthermore, studies of the metabolism of diabetic kidneys in experimental animals, using cortical homogenates and isolated glomeruli, have demonstrated higher anabolic and lower catabolic enzyme activities. However, contradictory data have been reported with regard to both the chemical composition and metabolism of the kidney in human and experimental diabetes. This review attempts to examine these reports in detail and discuss the possible causes for these discrepancies.

Developmental changes in collagen glycosyltransferase activities in chick embryos

The developmental pattern of collagen galactosyltransferase and collagen glucosyltransferase activities was determined in chick embryos between the 4th and 21st day of growth. Both enzyme activities increased up to the 16th day and decreased thereafter in whole chick embryos and in most tissues studied. The highest collagen glycosyltransferase activities were found in the leg tendons of the 16-day-old embryos, and the activities found in cartilage were higher than those noted in either skin or skull, indicating that the the activities of the collagen glycosyltransferases may play a part in the regulation of the carbohydrate content of the collagen synthesized by a given tissue. The changes observed in the collagen glycosyltransferase activities agree with previous data on the development of prolyl and lysyl hydroxylase activities and also with findings on collagen turnover in the developing chick embryo.

Age-related changes in human skin collagen galactosyltransferase and collagen glucosyltransferase activities

Collagen galactosyltransferase and collagen glucosyltransferase activities were assayed in human skin specimens of about 100 mg wet weight. The assay of the glucosyltransferase activity was found to be highly specific. The assay of the galactosyltransferase activity was somewhat less specific, but there was no difference in specificity between the foetal and adult human skin samples. The activities of the two collagen glycosyltransferases in human skin extract were found to vary with age, being highest in foetal skin, and higher in the skin of young children that in that of adults. The galactosyltransferase and glucosyltransferase activities in foetal skin were respectively about 4 times and 6 times those in adult skin. The magnitudes of the changes with age in the two collagen glycosyltransferase activities were smaller than those occurring in the activities of the two other intracellular enzymes of collagen biosynthesis namely prolyl and lysyl hydroxylase. This difference suggests that the four intracellular enzyme activities of collagen biosynthesis are not regulated in an identical manner.

Biochemical basis of skeletal defects induced by hydralazine: inhibiton of collagen synthesis and secretion in embryonic chicken cartilage in vitro

Hydralazine (1-hydrazinophthalazine) produces skeletal defects resembling those observed in experimentally induced manganese deficiencies. Since glycosylation of collagen, a step preceding its secretion, requires Mn2+, the effect of hydralazine on collagen secretion and the formation hydroxylysine glycosides was examined in explants of embryonic chicken long-bone rudiments. Auto-radiographic studies showed that hydralazine blocked collagen secretion. Secretion was restored by Fe2+ alone or Fe2+ + Mn2+ but not by Mn2+ alone, suggesting that a Fe2+-requiring step was involved. Biochemical analyses showed that hydralazine inhibited the formation of hydroxylysine, a step requiring Fe2+, but it did not inhibit the formation of hydroxylysine-glocosides by Mn2+-requiring steps, although the reaction was inhibited in vitro. Hydralazine also failed to inhibit intracellular mucopolysaccharide synthesis which involves several Mn2+-requiring enzymes. These observations suggest that the deleterious effects of hydralazine on bones are caused by its inhibition of hydroxylation steps in collagen synthesis.

Biochemical characteristics and biological significance of the genetically-distinct collagens

In recent years it has become evident that genetic polymorphism is dramatically expressed in the structural protein, collagen. Current information on the biochemical properties, biosynthesis, and tissue distribution of Type I, II, and III collagens is summarized with special reference to possible unique functional roles fulfilled by each of these collagens.

Affinity chromatography of collagen glycosyltransferases on collagen linked to agarose

Denatured citrate-soluble collagen was coupled to agarose by the cyanogen bromide activation technique, and columns prepared from this material were studied for affinity chromatography of collagen glycosyltransferases. Both collagen glycosyltransferases became bound to the column, the degree of binding and the capacity of the column being higher with the glucosyltransferase than with the galactosyltransferase than with the galactosyltransferase. The addition of Mn2+ enhanced the binding, especially with the glucosyltransferase. The enzymes were eluted from the column with small peptides prepared from collagen, and they were separated from the peptides by gel filtration. With this procedure a collagen glucosyltransferase purification of about 5000-fold and a collagen galactosyltransferase purification of about 1000-fold was obtained from chick embryo extract by relatively simple steps.

Search for a biochemical basis of diabetic microangiopathy

Diabetic microangiopathy, particularly as seen in the renal glomerulus, is characterized by morphological and biochemical alterations of the capillary basement membrane. Observations from a number of disciplines have indicated that the microvascular disease is not a separately inherited entity but a true consequence or "complication" of insulin deficiency. An evaluation of the biochemical events which could be responsible for the basement membrane lesions of diabetes indicates that the hyperglycemia or plasma somatotropin elevation of this disease alone, or in combination, may play an important role.

Collagen glucosyltransferase. Partial purification and characterization of the enzyme from whole chick embryos and chick-embryo cartilage

A purification of over 2000-fold is reported for collagen glucosyltransferase from Triton X-100 extract of whole chick embryos and one of about 160-fold from similar extract of chick embryo cartilage. The addition of the detergent more than doubled the enzyme activity in the homogenates. The purified enzyme preparations from whole chick embryos showed one major band and two or three minor bands in polyacrylamide gel electrophoresis and were entirely free of collagen galactosyltransferase activity. The molecular weight of collagen glucosyltransferase from both sources was about 52000 -- 54000, as determined by gel filtration. In some enzyme preparations an additional form was observed, with an elution position corresponding to a molecular weight of about 130000. Manganese was the most effective metal co-factor for the purified enzyme, but partial replacement could be obtained with Co2+, Mg2+ and Ca2+, whereas no replacement was found with other metals. The activity of the purified enzyme was stimulated by the addition of dithiothreitol to the incubation system and inhibited by preincubation with p-mercuribenzoate. UDP-glucose or the collagen substrate partially protected the enzyme against p-mercuribenzoate inactivation in the presence of Mn2+ but not in its absence. Some protection was also noted with Mn2+ alone.