Unreactive epsilon-amino groups in collagens

Unravelling the mechanism of the interactions of oxazolidine A and E with collagens in ovine skin

Cross-linking agents play an important part in the physical properties of collagen based biomaterials. Oxazolidines are novel aldehydic tanning agents that are widely used to stabilise collagens in the leather industry. The exact mechanism through which they cross-link collagens is, however, not well understood. When they are combined with vegetable tannins, it is thought that oxazolidines form carbocationic intermediates through ring opening, which are then able to interact with the amino acid side chains of collagens and flavonoid ring systems of vegetable tannins. In this study, the interactions of oxazolidines, with collagens, have been investigated using a number of analytical techniques. High pressure liquid chromatography (HPLC) analysis of oxazolidine tanned collagen samples showed that there is an irreversible reaction with tyrosine side chains. Mass spectrometry (MS) revealed the formation of a Schiff's base adduct with lysine residues, which was reversible in nature. MS analysis of reaction of oxazolidines with a model peptide Suc-Ala-Phe-Lys-AMC in presence of NaCNBH(3), indicated the formation of a product with an increase in molecular weight of 28 kD characteristic of the addition of two methyl groups to lysine. Differential scanning calorimetry showed a synergistic effect for combination tannage, with best results being obtained when vegetable tan was added prior to the aldehydic tanning agents. Circular dichroism (CD) studies of collagen in presence of the more reactive oxazolidine A showed that there was a loss in ellipticity simply because of aggregation of collagen molecules rather than a change in the secondary structure. Based on the results obtained, a scheme has been proposed to explain the possible mechanism of action of oxazolidines with the collagen amino acid side chains.

Guanidination of the lysine residues present in fibrous elastin and in soluble elastin peptides

We have found that only 20 to 30% of the lysine residues measured after acid hydrolysis of insoluble elastin and alpha-elastin were blocked by prior guanidination of elastin with o-methylisourea. In contrast free lysine was guanidinated almost 100% and a purified, highly crosslinked peptide fraction of low molecular weight, 85%. The non-reactivity of lysine does not seem to be due to steric hindrance in the fibrous protein or in the large peptides derived therefrom but rather to the presence of epsilon [delta-adipic acid]-lysine, the oxidized form of lysinonorleucine described by Bailey et al. Since photolysis of desmosine and isodesmosine leads also to the formation of new lysine residues, methods will have to be devised to distinguish lysine-containing peptides derived from photolyzed elastin.

Glycosaminoglycans: structure and interaction

In the last few years, there has been considerable progress in the studies on glycosaminoglycans, a group of acidic polysaccharides present in the intercellular matrix of connective tissue. X-ray diffraction studies have indicated that these polymers can exist in the condensed phase in some helical form. Chiroptical and hydrodynamic measurements have provided significant information regarding the molecular conformation in solution and other physicochemical properties of the polymers. Studies related to the interaction properties of glycosaminoglycans with polypeptides, metal ions, and other molecules are numerous. This review covers mainly the results and their interpretations of both published and as yet unpublished material of the 1970s, but certain previous data are also included. A present-day concept regarding the structure and interaction properties of these molecules on the basis of various physicochemical measurements is presented. The biosynthesis and metabolism of glycosaminoglycans, and the structure of proteoglycans and glycoproteins, are not discussed.