Hyaluronan

A. Arrangement of Sugar Residues

Hyaluronic acid containing glucuronic and N-acetylglucosamine was extracted and purified from bovine vitreous in 1934 by Meyer and Palmer (1). This macromolecule was precipitated under acidic conditions and it was thought, therefore, to be an acid. Because it contained uronic acid, Meyer named this compound as hyaluronic acid as it was isolated from hyalos ( = glassy, vitreous) and contained uronic acid (2). It took almost 20 years before

Repeating Disaccharide

Repeating Disaccharide

N-acetyl- Glucuronic glucosamine acid

Figure 1 Structure of disaccharide units of hyaluronan.

N-acetyl- Glucuronic glucosamine acid

Figure 1 Structure of disaccharide units of hyaluronan.

Meyer could link the two sugar residues together in the disaccharide repeat unit correctly: glucuronic acid-b-1,3-N-acetylglucosamine-b-1,4 that forms glycosaminoglycan (Fig. 1) (3).

Repeating disaccharide units of different glycosaminoglycans are summarized in Table 1. The resolution is based on the combination of their migration properties in a barium acetate buffer and their differential sensitivity to ethanol precipitation. The migration of the glycosaminoglycan chains on cellulose acetate plate electrophoresis stained with Alcian Blue occurs (4) in the following order HP < DS < HS < HA < CS (Fig. 2). At physiological pH the COOH groups of uronic acid in HA are dissociated. Therefore, Balazs et al. (5) changed its name from hyaluronic acid to hyaluronan.

B. Role in Different Connective Tissues

Hyaluronan can retain water almost 100 times its weight. It is believed that hyaluronan plays a structural role, i.e., able to regulate water homeostasis and

Table 1 Arrangement of Sugars in Different Glycosaminoglycans

Glycosaminoglycan

Saccharide backbone

Nac

SO3"

Hyaluronan

! 4)-b-D-GlcA(1 ! 3)-b-D-GlcN(1 !

1

0

Chondroitin sulfate

! 4)-b-D-GlcA(1 ! 3)-b-D-GalN(1 !

1

1

Dermatan sulfate

! 4)-a-L-IdoA(1 ! 3)-b-D-GalN(1 !

1

1

[ ! 4)-b-D-GlcA)(1 ! ]

Heparan sulfate

! 4)-a-L-IdoA(1 ! 4)-a-D-GlcN(1 !

< 1

0-2

[ ! 4)-b-D-GlcA)(1 ! ]

Heparin

! 4)-a-L-IdoA(1 ! 4)-a-D-GlcN(1 !

P 1

2

[ ! 4)-b-D-GlcA)(1 ! ]

GlcA, glucuronic acid, residue shown in brackets is minor component; IdoA, iduronic acid; GlcN, glucosamine; GalN, galactosamine. (Source: Ref. 6).

GlcA, glucuronic acid, residue shown in brackets is minor component; IdoA, iduronic acid; GlcN, glucosamine; GalN, galactosamine. (Source: Ref. 6).

HA HS

Origin

Figure 2 Migration of glycosaminoglycan chains on cellulose acetate plate. CS, chondroitin sulfate; HA, Hyaluronan; HA, Heparan sulfate; DS, dermatan sulfate; HP, heparin.

transport in the interstitium (7,8). Its ability to sequester water, to self-aggregate and to bind to many proteins make hyaluronan an ideal molecule to organize a net work in the aqueous subphase, e.g., in vitreous humor, lung interstitium, cartilage etc. In other cases, it is elastic, i.e., it absorbs energy and bounces back to its original shape, e.g., helping knees in absorbing the impact of jumping. In addition to these functions, hyaluronan works as Silly Putty. The various roles of hyaluronan in connective tissues are summarized in Table 2.

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