Protein cross-linking by radical reactions is normally associated with aging-related tissue damage, but on page 879 Edens et al. report that worms specifically use cross-linking of collagen to make a strong cuticle. Without the combined oxidase/peroxidase that does the cross-linking, the cuticle separates into distinct layers, resulting in translucent worms that often suffer from massive blisters and defective movement.
The clue that the cross-linking enzyme (called Duox for Dual Oxidase) might exist came from studies of phagocyte NADPH-oxidase. The latter enzyme generates bursts of superoxide to effect killing of engulfed cells. But other cell types, some of which lack phagocyte NADPH-oxidase, generate lower levels of reactive oxygen species.
Duox may be one of the sources of this oxidative activity. Edens et al. isolated genes for human and worm Duox enzymes based on similarity to phagocyte NADPH-oxidase, then investigated the function of the two worm genes by RNAi. The RNAi animals appeared similar to collagen mutants, and lacked di- and tri-tyrosine cross-links normally present in cuticular collagen. Furthermore, the peroxidase domains of both the human and worm Duox enzymes (which set Duox apart from phagocyte NADPH-oxidase) could cross-link tyrosine ethyl esters in a bacterial lysate.
The combined biochemical activities suggest the following model for Duox action. First the intracellular flavoprotein domain pulls electrons off NADPH. After passing through the membrane, these electrons transform molecular oxygen to superoxide, which spontaneously decays to hydrogen peroxide. The hydrogen peroxide reacts with the heme iron in Duox's peroxidase domain to form a powerful oxidant that can then oxidize the tyrosine residues, creating tyrosyl radicals that react to form a cross-link between two protein chains.
Such an activity is likely to hit any protein that is nearby. Worms minimize the danger of unwanted cross-links by using Duox primarily or perhaps solely in the cuticle. Humans express Duox in a number of tissues, notably lung, where cross-linking of elastin may help create the unique extracellular matrix that makes lungs so resilient to stretching. ▪