Xylosyl transfer to an endogenous renal acceptor. Purification of the transferase and the acceptor and their identification as glycogenin

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

The Release of a Soluble Glycosylated Protein from Glycogen by Recombinant Lysosomal α-Glucosidase (rhGAA) In Vitro and Its Presence in Serum In Vivo

In studies on the degradation of glycogen by rhGAA, a glycosylated protein core material was found which consists of about 5-6% of the total starting glycogen. There was an additional 25% of the glycogen unaccounted for based on glucose released. After incubation of glycogen with rhGAA until no more glucose was released, no other carbohydrate was detected on HPAEC-PAD. Several oligosaccharides are then detectable if the medium is first boiled in 0.1 N HCl or incubated with trypsin. It is present in serum either in an HCl extract or in a trypsin digest. The characteristics of the in vivo serum material are identical to the material in the in vitro incubation medium. One oligosaccharide cannot be further degraded by rhGAA, from the incubation medium as well as from serum co-elute on HPAEC-PAD. Several masked oligosaccharides in serum contain m-inositol, e-inositol, and sorbitol as the major carbohydrates. The presence of this glycosylated protein in serum is a fraction of glycogen that is degraded outside the lysosome and the cell. The glycosylated protein in the serum is not present in the serum of Pompe mice not on ERT, but it is present in the serum of Pompe disease patients who are on ERT, so it is a biomarker of GAA degradation of lysosomal glycogen.

Purification and some properties of UDP-xylosyltransferase of rat ear cartilage

UDP-xylosyltransferase (UDP-D-xylose:proteoglycan core protein beta-D-xylosyltransferase EC 2.4.2.26) initiates the formation of chondroitin sulfate in the course of proteoglycan biosynthesis. The enzyme catalyzes the transfer of D-xylose from UDP-D-xylose to specific serine residues in the core protein. A procedure for purification of xylosyltransferase from rat ear cartilage was developed which includes ammonium sulfate fractionation, chromatography on heparin-agarose, on Sephacryl S300 and finally a substrate affinity chromatography applying the dodeca peptide Q-E-E-E-G-S-G-G-G-Q-G-G. The specific activity of the purified enzyme was about 420 mU per mg protein. The purification factor was about 26.000 with 27% yield. In SDS-polyacrylamide gel electrophoresis, the highly purified enzyme is homogeneous and yields only a single distinct band of 78 kDa. An apparent molecular mass of 71 kDa was determined for the native enzyme. These data suggest a monomeric structure for the enzyme. Xylosyltransferase activity was found to depend essentially on the presence of divalent metal ions. The K(m) value for UDP-D-xylose was determined to 6.5 micromol/l and for the dodeca peptide Q-E-E-E-G-S-G-G-G-Q-G-G as xylose acceptor to 8 micromol/l.

Manganese sulfate-dependent glycosylation of endogenous glycoproteins in human skeletal muscle is catalyzed by a nonglucose 6-P-dependent glycogen synthase and not glycogenin

Glycogenin, a Mn2+-dependent, self-glucosylating protein, is considered to catalyze the initial glucosyl transfer steps in glycogen biogenesis. To study the physiologic significance of this enzyme, measurements of glycogenin mediated glucose transfer to endogenous trichloroacetic acid precipitable material (protein-bound glycogen, i.e., glycoproteins) in human skeletal muscle were attempted. Although glycogenin protein was detected in muscle extracts, activity was not, even after exercise that resulted in marked glycogen depletion. Instead, a MnSO4-dependent glucose transfer to glycoproteins, inhibited by glycogen and UDP-pyridoxal (which do not affect glycogenin), and unaffected by CDP (a potent inhibitor of glycogenin), was consistently detected. MnSO4-dependent activity increased in concert with glycogen synthase fractional activity after prolonged exercise, and the MnSO4-dependent enzyme stimulated glucosylation of glycoproteins with molecular masses lower than those glucosylated by glucose 6-P-dependent glycogen synthase. Addition of purified glucose 6-P-dependent glycogen synthase to the muscle extract did not affect MnSO4-dependent glucose transfer, whereas glycogen synthase antibody completely abolished MnSO4-dependent activity. It is concluded that: (1) MnSO4-dependent glucose transfer to glycoproteins is catalyzed by a nonglucose 6-P-dependent form of glycogen synthase; (2) MnSO4-dependent glycogen synthase has a greater affinity for low molecular mass glycoproteins and may thus play a more important role than glucose 6-P-dependent glycogen synthase in the initial stages of glycogen biogenesis; and (3) glycogenin is generally inactive in human muscle in vivo.

Characterization of human glycogenin-2, a self-glucosylating initiator of liver glycogen metabolism

Glycogenin-2 is a recently described self-glucosylating protein potentially involved in the initiation of glycogen biosynthesis (Mu, J., Skurat, A. V., and Roach, P. J. (1997) J. Biol. Chem. 272, 27589-27597). In human liver extracts, most of the glycogenin-2 was only detectable after treatment with alpha-amylase. Similarly, purifed high Mr glycogen was only detected after release by alpha-amylase treatment. Based on analysis by polymerase chain reaction, the predominant isoform in liver was glycogenin-2beta. Glycogenin-2 was found in Ewing's sarcoma RD-ES cells where, however, it was not associated with high Mr carbohydrate. Both human liver and human RD-ES cell extracts also contained glycogenin-1. Glycogenin-1 and glycogenin-2 interact with one another, based on in vitro interactions and co-immunoprecipitation from liver and cell extracts. Mutation of Tyr-196 in glycogenin-2 to a Phe residue abolished the ability of glycogenin-2 to self-glucosylate but not to interact with glycogenin-1. Stable overexpression of glycogenin-2alpha in Rat-1 fibroblast cells resulted in a 5-fold increase in the level of glycogen present in the low speed pellet but little change in the low speed supernatant. This result is important since it indicates that the level of glycogenin-2 can determine glycogen accumulation and hence has the potential to control glycogen synthesis.

Purification and characterization of an alpha1,2,-L-fucosyltransferase, which modifies the cytosolic protein FP21,from the cytosol of Dictyostelium

A novel fucosyltransferase (cFTase) activity has been enriched over 10(6)-fold from the cytosolic compartment of Dictyostelium based on transfer of [3H]fucose from GDP-[3H]fucose to Galbeta1,3 GlcNAc beta-paranitrophenyl (paranitrophenyl-lacto-N-bioside or pNP-LNB). The activity behaved as a single component during purification over DEAE-, phenyl-, Reactive Blue-4-, GDP-adipate-, GDP-hexanolamine-, and Superdex gel filtration resins. The purified activity possessed an apparent Mr of 95 X 10(3), was Mg2+-dependent with a neutral pH optimum, and exhibited a Km for GDP-fucose of 0.34 microM, a Km for pNP-LNB of 0.6 mM, and a Vmax for pN-P-LNB of 620 nmol/min/mg protein. SDS-polyacrylamide gel electrophoresis analysis of the Superdex elution profile identified a polypeptide with an apparent Mr of 85 X 10(3), which coeluted with the cFTase activity and could be specifically photolabeled with the donor substrate inhibitor GDP-hexanolaminyl-azido-125I-salicylate. Based on substrate analogue studies, exoglycosidase digestions, and co-chromatography with fucosylated standards, the product of the reaction with pNP-LNB was Fucalpha1, 2Galbeta1,3GIcNAcbeta-pNP. The cFTase preferred substrates with a Galbeta1,3linkage, and thus its acceptor substrate specificity resembles the human Secretor-type alpha1,2- FTase. Afucosyl isoforms of the FP21 glycoprotein, GP21-I and GP21-II, were purified from the cytosol of a Dictyostelium mutant and found to be substrates for the cFTase, which exhibited an apparent K(m) of 0.21 microM and an apparent V(max) of 460 nmol/min/mg protein toward GP21-II. The highly purified cFTase was inhibited by the reaction products Fucalpha1,2Galbeta1,3GlcNAcbeta-pNP and FP21-II. FP21-I and recombinant FP21 were not inhibitory, suggesting that acceptor substrate specificity is based primarily on carbohydrate recognition. A cytosolic location for this step of FP21 glycosylation is implied by the isolation of the cFTase from the cytosolic fraction, its high affinity for its substrates, and its failure to be detected in crude membrane preparations.

Inhibition of glycogenin-catalyzed glucosyl and xylosyl transfer by cytidine 5'-diphosphate and related compounds

The self-glucosylation of beef kidney glycogenin was inhibited by the following pyrimidine nucleotides and nucleotide sugars, listed in order of decreasing effectiveness: CDP-glucose, CDP, UDP-xylose, UDP-N-acetylglucosamine, UDP-galactose, UDP, CTP, CDP-choline, UDP-glucuronic acid, beta-S-UDP-glucose, and CMP. In contrast, the purine nucleotide sugars, ADP-glucose and GDP-glucose, were essentially ineffective, as was the pyrimidine nucleoside, cytidine. UDP-Xylose may be utilized by glycogenin as an alternative sugar donor instead of UDP-glucose (Rodén, L., Ananth, S., Campbell, P., Manzella, S., and Meezan, E. (1994) J. Biol. Chem. 269, 11509-11513) and therefore presumably inhibited the glucosyl transfer reaction by being a competitive substrate. Like glucosyl transfer, xylosyl incorporation into glycogenin was also inhibited effectively by CDP. On the other hand, UDP-xylose:proteoglycan core protein xylosyltransferase (EC 2.4.2.26) was not affected by CDP, nor was it inhibited by UDP-glucose. Addition of CDP or UDP-glucose to reaction mixtures containing both enzymes therefore made it possible to assay xylosyltransferase EC 2.4.2.26 reliably without the extensive product characterization that is otherwise necessary. The CDP effect on glycogenin further allowed the development of an improved procedure for the purification of this enzyme, in which specific elution of an affinity matrix (UDP-glucuronic acid-agarose) was carried out with CDP as the eluant.

Catalytic activities of glycogenin additional to autocatalytic self-glucosylation

Glycogenin is the autocatalytic, self-glucosylating protein that initiates glycogen synthesis in muscle and other tissues. We have sequenced the cDNA for rabbit muscle glycogenin and expressed and purified the protein in high yield as well as two mutant proteins in which Phe or Thr replaces Tyr-194, the site of glucosylation. While the wild-type protein can self-glucosylate, the mutants cannot, but all three utilize alternative acceptors by intermolecular glucose transfer for which the mutants have altered specificity. Tyr-194 is therefore not essential for the catalytic activity of glycogenin. All three proteins also hydrolyze UDP-glucose to glucose at rates comparable with the rate of self-glucosylation. The hydrolysis is competitive with glucose transfer to p-nitrophenyl alpha-maltoside. Self-glucosylation, glucosylation of other acceptors, and hydrolysis all appear to be catalyzed by the same active center. In the absence of peptidase inhibitors, the homogenous recombinant proteins of M(r) 37,000 break down to equally active species having M(r) 32,000. The kinetics of self-glucosylation catalyzed by the wild-type enzyme suggest that the reaction could be intermolecular rather than, as previously reported, intramolecular. The wild-type recombinant enzyme and native muscle glycogenin, which is phosphorylated, are inhibited quite differently by ATP at physiological concentration.

Identification of a novel glycosaminoglycan core-like molecule. I. 500 MHz 1H NMR analysis using a nano-NMR probe indicates the presence of a terminal alpha-GalNAc residue capping 4-methylumbelliferyl-beta-D-xylosides

beta-Xylosides compete with endogenous proteoglycan core proteins and act as alternate acceptors for synthesizing protein-free glycosaminoglycan chains. Their assembly on these alternate acceptors utilizes the same glycosyltransferases that make the protein-bound chains. Most studies using alternate acceptors focus on the production of sulfated glycosaminoglycan chains that are thought to be the major products. However, we previously showed that labeling melanoma cells with [6-3H]galactose in the presence of 4-methylumbelliferyl (MU) or p-nitrophenyl (pNP) beta-xylosides led to the synthesis of mostly di- to tetrasaccharide products including incomplete core structures. We have solved the structure of one of the previously unidentified products as, GalNAc alpha(1,4)GlcA beta(1,3)Gal beta(1,3)Gal beta(1,4)Xyl beta MU, based on compositional analysis by high performance liquid chromatography, fast atom bombardment, electrospray mass spectrometry, and one-dimensional and two-dimensional 1H NMR spectroscopy. The novel aspect of this molecule is the presence of a terminal alpha-Gal-NAc residue at a position that is normally occupied by beta-GalNAc in chondroitin/dermatan sulfate or by alpha-Glc-NAc in heparin or heparan sulfate chains. An alpha-GalNAc residue at this critical location may prevent further chain extension or influence the type of chain subsequently added to the common tetrasaccharide core.

New and specific nucleoside diphosphate glucose substrates for glycogenin

Glycogenin, the autocatalytic, self-glucosylating primer for glycogen synthesis by glycogen synthase, is presumed, in vivo, to use UDP-glucose as the source of the glucose residues it adds to itself. When we tested its ability to utilize other nucleoside diphosphate glucoses, it emerged that purine nucleotides are not utilized but two pyrimidine nucleotides are used, in addition to UDP-glucose. These are CDP-glucose and TDP-glucose. CDP-glucose is utilized at 70% of the rate of UDP-glucose. While there is no evidence that CDP-glucose is a natural substrate for glycogenin, it has the advantage over UDP-glucose in that it can be used specifically to detect and assay glycogenin in the presence of glycogen synthase because CDP-glucose, unlike UDP-glucose, is not a substrate for the synthase.

Properties of carbohydrate-free recombinant glycogenin expressed in an Escherichia coli mutant lacking UDP-glucose pyrophosphorylase activity

Glycogenin, the self-glucosylating primer for glycogen synthesis, is expressed in wild-type E. coli as a recombinant protein in an already partly glucosylated form, owing to the presence of its substrate, UDP-glucose. By using an E. coli mutant strain lacking in UDP-glucose pyrophosphorylase activity, we have succeeded in expressing carbohydrate-free glycogenin (apo-glycogenin) in good yield. When provided with UDPxylose, it autocatalytically adds 1 xylose residue. With UDP-glucose, an average of 8 glucose residues are added. However, release of the self-synthesized maltosaccharide chains with isoamylase reveals them to be a mixture. Chains as long as 11 glucose residues (maltoundecaose) are present. The ability of recombinant apo-glycogenin to self-glucosylate is further proof that a separate enzyme is not needed for the addition of the first glucose residue to Tyr-194 of the protein.