Protectins: Their biosynthesis, metabolism and structure-functions

Abstract

Several lipoxygenase enzymes and cyclooxygenase-2 stereoselectively convert the polyunsaturated fatty acids arachidonic acid, eicosapentaenoic acid, docosahexaenoic acid, and n-3 docosapentaenoic acid into numerous oxygenated products. Biosynthetic pathway studies have shown, during the resolution phase of acute inflammation, that distinct families of endogenous products are formed. These products were named specialized pro-resolving mediators, given their specialized functions in the inflammation-resolution circuit, enhancing the return of inflamed and injured tissue to homeostasis. The lipoxins, resolvins, protectins and maresins, together with the sulfido-conjugates of the resolvins, protectins and maresins, constitute the four individual families of these local mediators. When administrated in vivo in a wide range of human disease models, the specialized pro-resolving mediators display potent bioactions. The detailed and individual biosynthetic steps constituting the biochemical pathways, the metabolism, recent reports on structure-function studies and pharmacodynamic data of the protectins, are presented herein. Emphasis is on the structure-function results on the recent members of the sulfido conjugated protectins and further metabolism of protectin D1. Moreover, the members of the individual families of specialized pro-resolving mediators and their biosynthetic precursor are presented. Today 43 specialized pro-resolving mediators possessing pro-resolution and anti-inflammatory bioactions are reported and confirmed, constituting a basis for resolution pharmacology. This emerging biomedical field provides a new approach for drug discovery, that is also discussed.

Keywords: Biosynthesis; Cyclooxygenase-2; Lipoxygenases; Protectins; Resolution pharmacology; Specialized pro-resolving mediators.

Figure 1.

An outline of the individual families of SPMs biosynthesized from PUFAs. The protectins, marked in blue, are the focus of this review. RCTRs: resolvin conjugates in tissue regeneration 1–3; PCTRs: protectin conjugates in tissue regeneration 1–3; MCTRs: maresin conjugates in tissue regeneration 1–3.

Figure 2.

Chemical structures of protectin D1 (PD1) and protectin D2 (PD2), biosynthesized from DHA, and the more recent members PD1_{n-3 DPA} and PD2_{n-3 DPA}, biosynthesized from n-3 DPA. PD1_{n-3 DPA} and PD2_{n-3 DPA} are congeners of PD1 and PD2, respectively, due to the lack of the Z-configured C4-C5 double bond in their n-3 DPA precursor.

Figure 3.

The detailed biosynthetic pathways of PD1 and PDX from the common intermediate 16S,17S-epoxyprotectin. The non-enzymatically formed stereoisomers are also shown.

Figure 4.

Biosynthesis of PD2 occurs stereoselectively in a S_N2-type reaction.

Figure 5.

Biosynthesis, via 16S,17S-epoxyprotectin, of the sulfido protectin conjugate PCTR1 and its enzymatic conversion into PCTR2 and PCTR3.

Figure 6.

Outline of the enzymatic conversion of PD1 into the metabolites 22-OH-PD1, dinor-PD1 and tetranor-PD1. The synthetic analogs 22-F-PD1 and 3-oxa-PD1_{n-3 DPA} are also shown.