Peptidyl lysine, oxidation

For purposes of this manuscript, we wish to concentrate only on the steps leading to the formation of desmosines, amino acids found predominantly in elastin. With respect to their formation, the following suggests their spontaneous formation from peptidyl lysine and the oxidation product, peptidyl allysine. Narayanan et al. (28,29) have shown that when purified lysyl oxidase and non-crosslined elastin, specifically tropoelastin, are incubated together, the desmosines are formed. Desmosine formation, however, only occurs at temperatures that favor fibrillar arrangements of tropoelastin. Subsequently, it is felt that the maturation of non-crosslinked elastin into cross-linked elastin appears to involve only two major steps, namely insolublization through the formation of fibrils and fixation of the fibrils by crosslinking. 

To form the desmosines, three peptidyl allysine molecules and a molecule of peptidyl lysine must condense. The steps in condensation probably involve the formation of 1,2-dihydropyridines and 1,4-dihydropyridines as shown in Figure 4 (19-24,46,48). Several kinds of chemical evidence (46,48) suggest that the hydropyridines are easily oxidized under normal oxygen tension to corresponding pyridinium ions, such as the desmosines (isodesmosine or desmosine). The exact pathway by which the desmosines are formed, however, is still not clear. 

Lysine tyrosylquinone (LTQ) (Figure 3) is the protein-derived cofactor of mammalian lysyl oxidase, an important enzyme in the metabolism of connective tissue. Lysyl oxidase catalyzes the posttranslational modification of elastin and collagen. It oxidizes selected peptidyl lysine residues to peptidyl a-aminoadipic -semialdehyde residues. This initiates formation of the covalent cross-linkages that insolubilize these extracellular proteins. This enzyme also contains copper as a second prosthetic group. 

B. L, (1970) Oxidation of peptidyl lysine by an amine oxidase from bovine aorta. Biochem. Biophys. Res. Commun, 40 1391. 

Figure 3. Lysyl oxidase. The enzyme, lysyl oxidase, appears to seek out lysyl residues in alanyl- and lysyl-rich regions in the pro fibrillar forms of elastin. The presence of an aromatic amino acid residue adjacent to lysine appears to block its oxidation. The product of oxidation is peptidyl a-aminoadipic-S-semialdehyde. Assays for the enzyme against elastin involve first the preparation of an elastin-rich pellet containing 3H-lysyl residues labeled in the 6 or 4,5 position. This is usually accomplished by incubating embryonic chick aortas in medium containing 3H-lysine plus f3-aminopropionitrile (BAPN) to inhibit endogenous lysyl oxidase activity. BAPN is then removed leaving behind an elastin-rich residue in which the profibrillar forms of elastin labelled with 3H-lysine are only partially crosslinked. When lysyl oxidase preparations are added to this residue the release of tritium represents the assay for activity. It has also been demonstrated that tropoelastin, when incubated with lysyl oxidase, forms a-aminoadipic-S-semialdehyde and eventually crosslinks as shown in Figure 4.

---