**1. Introduction**

It is accepted in modern genomics that only a minor portion (1-5%) of the genome contains information about the primary structure of protein molecules. The higher is the organism on the evolutionary ladder, the lower the density of genetic information, in the modern sense of the term, per kilobase (kb). For instance (Table 1), this parameter is 1-1.7 kb per gene in bacteria and more than 30 kb per gene in human.


**Table 1.** Comparison of the genome size and gene number (Kiselev, L.L., 2000)

The genome can be conventionally divided into three groups of nucleotide sequences: unique sequences, moderate repeats (DNA segments repeated frequently), and fast repeats (satellite DNA). This division is exclusively conventional; the assignment to a particular group is determined by the capability of fragmented and denatured DNA to find complementary regions upon reassociation in solution. Yet such a division makes it possible to study the functional role of individual genome regions. It was shown, for instance, that unique DNA sequences are responsible mostly for encoding the primary structure of protein molecules. Such sequences occur in a few copies in the genome. Part of information of repetitive DNA systems (histone, rRNA, and tRNA genes) is encoded by so-called slow DNA repeats

The role of fast repeats and satellite DNA is still enigmatic to a great extent. Such DNA is often called selfish or waste, suggesting a lack of information valuable for the organism.

© 2012 Zimnitskii, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Surprisingly, it is this DNA that increases in amount in organisms with the highest level of organization (for instance, repetitive sequences account for more than 95% of the human genome). Data accumulating in recent decades for inverted repeats, regulatory gene regions, centromeric and telomeric satellite DNAs, introns, and mobile genetic systems still fail to improve the understanding of the functional role of tandem repeats, which quantitatively constitute the basis of the genome. The hypothesis that repeats are just dilutors of genetic information is below the level of current knowledge, testifying to a poor understanding of the phenomenon rather than helping to elucidate their role (Singer M., Berg P. 1998).

Some authors believe that tandem repeats play an important, though still unknown, part in evolutionary improvement of organisms. Thus repeats show species specificity (Mednikov B.M., et al., 2001) and occur in hundred of thousands or even millions of copies in the genome (Table 2). Simple tandem repeats, such as 5-CA, 5-GA, 5-AT, and 5-GC, are present in virtually all eukaryotes and have huge copy numbers.


**Table 2.** Some tandem repeats dispersed through the genomes of various eukaryotes (Singer, M., Berg, P., 1998)

In terms of base groups, the sequences shown in Table 2 can be presented as multiple repeats of (Pyr-Pyr), (Pur-Pur), and (Pur-Pyr), where Pyr is a pyrimidine (Т or C) and Pur is a purine (А or G). Thus, the genome contains extended monopurine and/or monopyrimidine arrays as well as tandem repeats whose averaged unit contains Pur and Pyr bases in nearly equal proportions. It is also beyond doubt that excess information is read from DNA during transcription yielding gnRNA, part of which is excised during RNA maturation. Information is transferred from DNA to mRNA almost directly, without processing, in prokaryotes, whereas the situation observed in higher organisms is similar to that with DNA repeats. The higher the position of an organism on the evolutionary ladder, the greater the amount of information that is transmitted to RNA and is senseless in terms of the polypeptide sequence (Singer M., Berg P., 1998).

While DNA determines the ontogenetic and phylogenetic development of organisms and is structurally described as a polymer of dimeric units, a natural question is whether the cell possesses another structurally similar biopolymer (apart from RNA). As is well known, proteins are unfit for this role, because the triplet code of polypeptides requires at least three different monomers for their implementation. There is only one group of tandem biopolymers playing a key role in the functioning of higher organisms as multicellular entities. This group is polysaccharides.

Polysaccharides are biopolymers that consist of monosaccharides linked to each other. It was believed until the 1960s (Colman Y., Rem C.-G. 2000), that carbohydrates serve only as an energy source (monosaccharides and storage polysaccharides such as starch and glycogen) and structural material (e.g., cellulose in plants and chitin in insects). Interest in carbohydrates was moderated by their extreme structural complexity. While there is only one way of linking together monomers of nucleic acids (nucleotides) or proteins (amino acids), monosaccharide units of oligosaccharides and polysaccharides can be linked in several ways at several different positions. Thus only 256 tetranucleotides can be obtained with four different nucleotides, whereas four different monosaccharides can form 35,560 unique tetrasaccharides (Colman Y., Rem C.-G. 2000). Consequently, biological polymorphism of polysaccharides is immeasurably higher than that of proteins or nucleic acids (NA).

By composition, polysaccharides are usually divided into homopolysaccharides (homoglycans) and heteropolysaccharides (heteroglycans). In almost all cases, the chemical structure can be reduced to a dimeric unit repeated many times (Fig. 1, Table 3). Glycosaminoglycans (GAG) are the most common natural heteroglycans.

hyaluronic acid

**Figure 1.** Structures of common polysaccharides.

258 The Complex World of Polysaccharides

5-CAAA GTTTT GTTTGA

P., 1998)

Surprisingly, it is this DNA that increases in amount in organisms with the highest level of organization (for instance, repetitive sequences account for more than 95% of the human genome). Data accumulating in recent decades for inverted repeats, regulatory gene regions, centromeric and telomeric satellite DNAs, introns, and mobile genetic systems still fail to improve the understanding of the functional role of tandem repeats, which quantitatively constitute the basis of the genome. The hypothesis that repeats are just dilutors of genetic information is below the level of current knowledge, testifying to a poor understanding of

the phenomenon rather than helping to elucidate their role (Singer M., Berg P. 1998).

5-GGAAG birds 2:0 5-CA many eukaryotes 1:1 5-GA many eukaryotes 2:0 5-GT many eukaryotes 1:1 5-AT many eukaryotes 1:1

5-TCTCC birds 0:2

**Table 2.** Some tandem repeats dispersed through the genomes of various eukaryotes (Singer, M., Berg,

In terms of base groups, the sequences shown in Table 2 can be presented as multiple repeats of (Pyr-Pyr), (Pur-Pur), and (Pur-Pyr), where Pyr is a pyrimidine (Т or C) and Pur is a purine (А or G). Thus, the genome contains extended monopurine and/or monopyrimidine arrays as well as tandem repeats whose averaged unit contains Pur and Pyr bases in nearly equal proportions. It is also beyond doubt that excess information is read from DNA during transcription yielding gnRNA, part of which is excised during RNA maturation. Information is transferred from DNA to mRNA almost directly, without processing, in prokaryotes, whereas the situation observed in higher organisms is similar to that with DNA repeats. The higher the position of an organism on the evolutionary ladder, the greater the amount of information that is transmitted to RNA and is senseless in terms of

While DNA determines the ontogenetic and phylogenetic development of organisms and is structurally described as a polymer of dimeric units, a natural question is whether the cell possesses another structurally similar biopolymer (apart from RNA). As is well known,

in virtually all eukaryotes and have huge copy numbers.

the polypeptide sequence (Singer M., Berg P., 1998).

Some authors believe that tandem repeats play an important, though still unknown, part in evolutionary improvement of organisms. Thus repeats show species specificity (Mednikov B.M., et al., 2001) and occur in hundred of thousands or even millions of copies in the genome (Table 2). Simple tandem repeats, such as 5-CA, 5-GA, 5-AT, and 5-GC, are present

Repeat Organisms Purine:pyrimidine ratio

*Xenopus laevis* 1:0.9

GAG is a polymer that consists of hexuronic acid and hexosamine residues linked by Oglycoside bonds. The three major GAG classes are nonsulfated hyaluronic acid (HA), moderately sulfated chondroitin sulfates (CS), and highly sulfated heparan sulfates (HS). GAG of the last two classes belong to proteoglycans. GAG contained in proteoglycans play various vital parts in the intercellular matrix and within the cell (Zimnitskii A.N., Bashkatov S.A. 2004).


**Table 3.** Uronic acid/hexose ratio in storage and structural polysaccharides (PS)

HA biosynthesis was studied in group A hemolytic streptococci (Dorfman A 1958, 1964,1965). Similar synthetic processes occur in higher animals (Glaser L., Brown D.H. 1955). Roseman (Roseman S., Ludowieg J., et. al. 1953, Roseman S., Moses F.E., et. al. 1953, Roseman S., Moses F.E, et. al. 1954 ) have shown that D-glucose is a precursor of glycosamine and glucuronic acid, which are HA monomers. The immediate precursors of HA are uridine diphosphate (UDP) derivatives of glucuronic acid and N-acetylglucosamine, which act as donors of carbohydrate residues during HA synthesis (Cifonelli J.A., et al. 1957, Markovic O., et al. 1964, Markovitz A., et al. 1958, Lorenzel I. 1959). A cell-free system allowing HA synthesis in the presence of UDP-glucuronic acid, UDP-glucosamine, and Mg2+ was obtained by sonicating a microbial culture producing HA (Markovic O., Huttl S. 1964). The enzyme responsible for HA synthesis sedimented at 100.000 g. A similar enzyme was found in rat embryo skin by Schiler (Schiller S. 1964, Schiler S. 1965) and in various chicken, rodent, and human tissues by Altschuler (Altschuler C.H., et al. 1963). As in microorganisms, nucleotide derivatives result from the reactions catalyzed by pyrophosphorylases in higher animal tissues producing HA (Strominger J.L. 1964). Acetylation of UDP-glucose involves acetyl coenzyme A. Glucose is transformed into glucosamine before being linked to UDP.

260 The Complex World of Polysaccharides

Polysaccharides of microorganisms

Polyuronides of

Chitin of insects and crustaceans

Hyaluronic acid of

Dermatan sulfate of

Chondroitin sulfate (CS) of animals

Heparan sulfate (HS) of animals

Amylose and cellulose of plants, glycogen of animals

plants

animals

animals

**PS, source PS formula Uronic acid:hexose ratio**

Heparan of animals GlcUA - GlcN 1:1

CS A of animals GlcUA – GalNAc4SО4 1:1 CS C of animals GlcUA – GalNAc6SО4 1:1 CS of animals GlcUA3S – GalNAc4SО4 1:1 CS of animals GlcUA2S – GalNAc6S 1:1 CS of animals GlcUA - GalNAc(4,6-diSО4) 1:1 CS of animals IdoUA – GalNAc4SО4 1:1 CS of animals IdoUA2(3)SО4 - GalNAc 1:1 CS of animals IdoUA2(3)SО4 - GalNAc4SО4 1:1 CS of animals IdoUA2SО4 – GalNAc6SО4 1:1 CS of animals IdoUA – GalNAc(4,6-diSО4) 1:1

HS of animals IdoUA – GlcNSО46SО4 1:1 HS of animals IdoUA2(3)SО4 - GlcNSО4 1:1 HS of animals IdoUA2(3) – bGlcNSО46SО4 1:1 HS of animals GlcUA - GlcNSО4 1:1 HS of animals GlcUA – GlcNAc6SО4 1:1 HS of animals GlcUA – GlcNS6SО4 1:1 HS of animals GlcUA2SО4 – GlcNSО46SО4 1:1 HS of animals GlcUA2SО4 - GlcNSО4 1:1 HS of animals GlcUA2SО4 -GlcNSО4SО4 1:1

**Table 3.** Uronic acid/hexose ratio in storage and structural polysaccharides (PS)

CS of animals IdoUA2(3)SО4 – GalNAc(4,6 diSО4)

GlcUA - Glc 1:1

GlcUA - GlcUA 2:0

Glc-Glc 0:2

GlcNAc – GlcNAc 0:2

GlcUA - GlcNAc 1:1

IdoUA - GalNAc 1:1

GlcUA - GalNAc 1:1

IdoUA - GlcNSО4 1:1

1:1

As Slutskii (Slutskii L.I. 1969). showed in the late 1970s, when biosynthesis of heteroglycans of the intracellular matrix were studied most intensely in higher animals, and modern data demonstrate, the enzyme involved in synthesis of polysaccharide fragments of GAG is associated with membrane structures in the intact cell. One is forced to accept the fact that little is still understood about the mechanism of final steps of GAG biosynthesis: how it is possible that a strict alternation of acetylglucosamine and glucuronic acid residues in the macromolecular glycan chain is achieved in the absence of a template.

Studies showed that synthesis of sulfated GAG is essentially similar to synthesis of nonsulfated HA and that D-glucose is a precursor of monosaccharide residues as in the case of HA. However, a necessary step is epimerization of glucosamine into galactosamine in the case of CS and glucuronic acid into iduronic acid in the case of dermatan sulfate. Several works demonstrated epimerization of UDP-monosaccharides in the presence of NADPH+ and specific epimerases (Maley F., et al. 1959, Roden L., et al. 1958, Rondle C.J.M., et al. 1955). White (White B.N., et al. 1965) observed the reversible character of epimerization with the example of transformation of galactosamine into glucosamine during GAG synthesis. During synthesis of HS and, in particular, heparin, nucleotide derivatives of hexuronic acids and hexosamines form not only β-glycosidic bonds, characteristic of HA and CS, but also α-glycosidic bonds, characteristic of HS (Silbert J.J. 1963).

According to current views, the first step of proteoglycan synthesis is translation of the protein core mRNA on ribosomes of the rough endoplasmic reticulum. Then glycosylation of the protein core is initiated: a site is generated at which the prospective GAG is to be linked to the polypeptide chain. This process has not been localized so far; it is known only that the protein core already has a polysaccharide fragment when delivered into the endoplasmic reticulum (Zimina N.P., et al. 1992, Zimina N.P., et al. 1987, White J, et al. 1978, Silbert J.E. et al, 1995). It is thought that synthesis of GAG chains is template-independent:

glycosyltransferases just consecutively add monosaccharides of donor UDP-saccharides to the growing carbohydrate chain. The substrate specificity of glycosyltransferases is determined by the monosaccharide sequence. The growth of GAG chains is initiated by xylosyltransferase. However, only some serine residues of the protein core are subject to xylosylation. The site of attachment of a GAG chain is presumably selected depending on the amino acid sequence of the region to be glycosylated (Zimina N.P., et al. 1992, Zimina N.P., et al. 1987, Jeffrey D Esko, et al. 1996). After a xylose residue is linked to the protein, a carbohydrate chain is formed via adding consecutively two galactose residues and one glucuronic acid residue by galactosyltransferases I and II and glucuronyltransferase I. The newly synthesized tetrasaccharide serves as an acceptor in the first reaction of GAG synthesis. Polymeric GAG chains result from multiple reactions performed alternately by two enzymes associated with the membrane Golgi complex. These enzymes are Nacetylgalactosaminyltransferase and glucuronyltransferase II in the case of CS and dermatan sulfates. In the case of HS and heparin, polymerization is catalyzed by Nacetylglucosaminyltransferase and glucuronyltransferase (Zimina N.P., et al. 1992, Zimina N.P., et al. 1987, Sugahara K., et al. 2000, Silbert J.E. 1982). The structure of the protein core is critical for the structure of GAG chains (Jeffrey D Esko, et al. 1996).

The immediate substrates of GAG synthesis are nucleotide (UDP) derivatives of monosaccharides; these derivatives are generated in the reactions catalyzed by pyrophosphorylases. Glucuronic acid is produced from UDP-glucose via two-step oxidation of C6, with transformation of the hydroxyl group into a carboxyl group. In dermatan sulfate synthesis, UDP-glucuronic acid is epimerized into UDP-iduronic acid by specific epimerase.

Thus, during GAG biosynthesis, one enzyme complex elongates the heteroglycan chain by adding consecutively monosaccharide units. Treatment with ribonucleases or deoxyribonucleases did not prevent chain elongation in a model HA-producing system. This finding was interpreted as demonstrating the template-independent character of GAG synthesis. We think, however, that template independence of GAG synthesis is not evident from this finding, because the glycans produced in the model system were not compared with natural glycans. It is beyond doubt that, in the absence of a template, the enzyme complex is capable of producing a heteroglycan chain at random, as is the case with NA synthesis in *in vitro* systems.

To eliminate the contradiction between the clearly ordered GAG structure (Zimina N.P., et al. 1992, Zimina N.P., et al. 1987 ) and the concept of non-template GAG synthesis, attempts were made over the past decade to associate the ordered character of the GAG structure with template synthesis of the protein core of proteoglycans; i.e., the amino acid sequence of the protein core was considered as a kind of a template for GAG synthesis. Although some achievements were made in the field, they mostly revealed statistical, rather than cause-andeffect, relationships. It seems that this situation has been accepted as satisfactory, and the concept of non-template GAG synthesis associated with the protein core is now described in textbooks. Yet the concept is shattered completely by the fact that that HA, reaching a molecular weight of 107 Da, is an individual GAG lacking any protein core, in contrast to CS and HS.

Current views of proteoglycan biosynthesis are related to studies of the role of intracellular membranes. It is commonly known that cell proteoglycans are almost always associated with membrane structures. The processes of proteoglycan biosynthesis, intracellular transport to organelles, and transfer onto the cell surface in the intercellular matrix are coupled with the functioning of membranes. GAG accumulation in lysosomes and subsequent cleavage to monosaccharides are also connected with the functioning of membrane structures. Free proteoglycans and GAG that are not associated with membranes are detectable only at the last degradation stages in lysosomes and some other cell structures such as the nucleus and mast cell granules.

262 The Complex World of Polysaccharides

synthesis in *in vitro* systems.

and HS.

glycosyltransferases just consecutively add monosaccharides of donor UDP-saccharides to the growing carbohydrate chain. The substrate specificity of glycosyltransferases is determined by the monosaccharide sequence. The growth of GAG chains is initiated by xylosyltransferase. However, only some serine residues of the protein core are subject to xylosylation. The site of attachment of a GAG chain is presumably selected depending on the amino acid sequence of the region to be glycosylated (Zimina N.P., et al. 1992, Zimina N.P., et al. 1987, Jeffrey D Esko, et al. 1996). After a xylose residue is linked to the protein, a carbohydrate chain is formed via adding consecutively two galactose residues and one glucuronic acid residue by galactosyltransferases I and II and glucuronyltransferase I. The newly synthesized tetrasaccharide serves as an acceptor in the first reaction of GAG synthesis. Polymeric GAG chains result from multiple reactions performed alternately by two enzymes associated with the membrane Golgi complex. These enzymes are Nacetylgalactosaminyltransferase and glucuronyltransferase II in the case of CS and dermatan sulfates. In the case of HS and heparin, polymerization is catalyzed by Nacetylglucosaminyltransferase and glucuronyltransferase (Zimina N.P., et al. 1992, Zimina N.P., et al. 1987, Sugahara K., et al. 2000, Silbert J.E. 1982). The structure of the protein core

The immediate substrates of GAG synthesis are nucleotide (UDP) derivatives of monosaccharides; these derivatives are generated in the reactions catalyzed by pyrophosphorylases. Glucuronic acid is produced from UDP-glucose via two-step oxidation of C6, with transformation of the hydroxyl group into a carboxyl group. In dermatan sulfate synthesis, UDP-glucuronic acid is epimerized into UDP-iduronic acid by specific epimerase. Thus, during GAG biosynthesis, one enzyme complex elongates the heteroglycan chain by adding consecutively monosaccharide units. Treatment with ribonucleases or deoxyribonucleases did not prevent chain elongation in a model HA-producing system. This finding was interpreted as demonstrating the template-independent character of GAG synthesis. We think, however, that template independence of GAG synthesis is not evident from this finding, because the glycans produced in the model system were not compared with natural glycans. It is beyond doubt that, in the absence of a template, the enzyme complex is capable of producing a heteroglycan chain at random, as is the case with NA

To eliminate the contradiction between the clearly ordered GAG structure (Zimina N.P., et al. 1992, Zimina N.P., et al. 1987 ) and the concept of non-template GAG synthesis, attempts were made over the past decade to associate the ordered character of the GAG structure with template synthesis of the protein core of proteoglycans; i.e., the amino acid sequence of the protein core was considered as a kind of a template for GAG synthesis. Although some achievements were made in the field, they mostly revealed statistical, rather than cause-andeffect, relationships. It seems that this situation has been accepted as satisfactory, and the concept of non-template GAG synthesis associated with the protein core is now described in textbooks. Yet the concept is shattered completely by the fact that that HA, reaching a molecular weight of 107 Da, is an individual GAG lacking any protein core, in contrast to CS

is critical for the structure of GAG chains (Jeffrey D Esko, et al. 1996).

Scarce, if any, information is available concerning the early steps of GAG synthesis and GAG transport to the sites where polysaccharide chains are generated (Silbert J.E. et al. 1995). As the views of the structure of the protein core are generally discrepant, it is still unclear whether the protein core determines the formation of proteoglycans. The primary structure of a core protein with a potential serine xylosylation site suggests that the protein molecule plays no crucial role in recognition of the xylosylation site. Hence it is probable that the conformation of membrane structures is a factor determining the recognition of xylosylation sites in protein molecules of proteoglycans. This assumption can be advanced because the main determinant of xylosylation is probably associated with membrane structures of the endoplasmic reticulum and Golgi complex, where glycosyltransferase activity is high (Silbert J.E. et al. 1995).

Synthesis of the linker tetrasaccharide (an uronic acid residue is added to the trisaccharide) initiates addition of a particular N-acetylhexosamine and thereby determines which GAG chain is to be generated. As a result, the nascent proteoglycan is transferred into the corresponding site, containing either galactose N-acetyltransferases or glucose Nacetyltransferases; this determines the chain to be synthesized. It is the amino acid sequence of the peptide moiety that determines the position of the nascent proteoglycan on membrane structures and, consequently, the direction of synthesis and modification of the polysaccharide chain by the corresponding enzyme systems.

The primary structure of GAG in proteoglycan molecules is an important factor determining their function. For instance, the GAG chain size, the number and location of sulfo groups, and epimerization affect the properties and biological role of proteoglycans. The extent of modification of the polysaccharide chain and its size probably depend on the structure of membrane-associated complexes responsible for proteoglycan synthesis. Membrane enzyme complexes play a key role in epimerization and/or sulfation of GAG polysaccharide chains (Silbert J.E. et al. 1995).

In 1987, Lindahl (Lindahl U., et al. 1987) showed convincingly that the results of GAG biosynthesis *in vitro* cannot be extrapolated to the *in vivo* situation. Studying heparin biosynthesis, these researchers observed that polysaccharides obtained with intact microsomes contain extended sequences of both N-sulfated and N-acetylated disaccharides, suggesting a nonrandom character of their synthesis. In contrast, products obtained with a solubilized microsomal system displayed a random distribution of such groups with a

greater portion of N-acetylated and N-sulfated disaccharides (Lindahl U., et al. 1987). In fact, this finding demonstrates that synthesis of the glycoside moiety of proteoglycans is genetically determined *in vivo*, because their primary structure is tissue- and speciesspecific.

A basic unit monomers of polysaccharides is glucose, which occurs in solution both in the α and in the β form owing to the mutarotation reaction. In addition, glucose can be epimerized to other hexoses and be oxidized to yield glucuronic and other hexuronic acids. Owing to such lability of glucose, biosynthetic systems always contain sufficient amounts of the α and β forms of hexoses and hexuronic acids. Modified with UDP at C1, these monosaccharides provide the main components for synthesis of polysaccharide chains.

As Fig. 1 and Table 3 demonstrate, the majority of known structural and storage polysaccharides each consist of two monomers, a hexose and hexuronic acid, which occur in the α or β form, are linked through (1-3), (1-4) O-glycoside bonds, and are modified to a various extent at various carbon atoms as a result of acetylation, amination, sulfation, etc. All these biopolymers can be combined in one group with a universal structure of multiply repeated (A-B), (A-A), or (B-B) units, where A is a hexose and B is a hexuronic acid.

We think that the periodicity of the primary structure is similar between polysaccharides and DNA tandem repeats. In view of this structural similarity, it was justified and important to study a possible complementarity between monosaccharides of glycans and bases of NA.

To check the hypothesis of complementarity of NA bases to hexoses and hexuronic acids, quantum chemical methods were used in our lab for a particular case of glucose and glucuronic acid contained in the heteropolysaccharide HA. It is clear that such an approach is computational and that the relevant conclusions need experimental verification. In view of this, it was necessary to obtain empirical information supporting or contradicting the results of quantum chemical computations. For this purpose, we employed UV spectrophotometry and dot hybridization, which allow detection of specific complexes of biopolymers.

Since the mechanism initiating GAG biosynthesis is unclear, it was expedient to study glycan synthesis in the rat liver upon administration of elevated doses of glucose. The use of 35SO42- as a radioactive label is inadequate for studying GAG synthesis, and we decided to label a precursor of the glycan polysaccharide chain. We used glucose as such a precursor: glucose is transformed into UDP-glucose and then into UDP-glucuronic acid, which is utilized in GAG synthesis. It should be noted that ribose 5-phosphate, which is formed from glucose 6-phosphate, is incorporated in NA. Glucose is converted into ribose in the pentose phosphate cycle by eliminating C1, which is released as carbon dioxide. Hence, we used [14С]glucose labeled at C1 to prevent generation of radiolabeled NA in our experiments. We studied the composition of rat liver polysaccharides in cell nuclei, microsomes, and in a total liver homogenate. In addition, the nuclear and microsomal fractions were used to monitor the accumulation of radiolabeled saccharides, which are polysaccharide precursors.
