Long-chain saturated fatty acids can be alpha-oxidised by a purified enzyme (M(r) 240,000) in cucumber (Cucumis sativus)

Biotechnological Production of Odor-Active Methyl-Branched Aldehydes by a Novel α-Dioxygenase from Crocosphaera subtropica

As a result of their pleasant odor qualities and low odor thresholds, iso- and anteiso-fatty aldehydes represent promising candidates for applications in flavoring preparations. A novel cyanobacterial α-dioxygenase from Crocosphaera subtropica was heterologously expressed in Escherichia coli and applied for the biotechnological production of C₁₂-C₁₅ branched-chain fatty aldehydes. The enzyme has a sequence identity of less than 40% to well-investigated α-dioxygenase from rice. Contrary to the latter, it efficiently transformed short-chained fatty acids. The kinetic parameters of α-dioxygenase toward unbranched and iso-branched-chain substrates were studied by means of an oxygen-depletion assay. The transformation products (C₁₂-C₁₅ iso- and anteiso-aldehydes) were extensively characterized, including their sensory properties. The aldehydes exhibited green-soapy, sweety odors with partial citrus-like, metallic, peppery, and savory-tallowy nuances. Moreover, the two C₁₄ isomers showed particularly low odor threshold values of 0.2 and 0.3 ng/L in air as determined by means of gas chromatography-olfactometry.

Activation of the fatty acid alpha-dioxygenase pathway during bacterial infection of tobacco leaves. Formation of oxylipins protecting against cell death

A pathogen-induced oxygenase showing homology to prostaglandin endoperoxide synthases-1 and -2 was recently characterized by in vitro experiments as a fatty acid alpha-dioxygenase catalyzing formation of unstable 2(R)-hydroperoxy fatty acids. To study the activity of this enzyme under in vivo conditions and to elucidate the fate of enzymatically produced 2-hydroperoxides, leaves of tobacco were analyzed for the presence of alpha-dioxygenase-generated compounds as well as for lipoxygenase (LOX) products and free fatty acids. Low basal levels of 2-hydroxylinolenic acid (0.4 nmol/g leaves fresh weight) and 8,11,14-heptadecatrienoic acid (0.1 nmol/g) could be demonstrated. These levels increased strongly upon infection with the bacterium Pseudomonas syringae pv syringae (548 and 47 nmol/g, respectively). Transgenic tobacco plants overexpressing alpha-dioxygenase were developed, and incompatible infection of such plants led to a dramatic elevation of 2-hydroxylinolenic acid (1778 nmol/g) and 8,11,14-heptadecatrienoic acid (86 nmol/g), whereas the levels of LOX products were strongly decreased. Further analysis of oxylipins in infected leaves revealed the presence of a number of 2-hydroxy fatty acids differing with respect to chain length and degree of unsaturation as well as two new doubly oxygenated oxylipins identified as 2(R),9(S)-dihydroxy-10(E),12(Z),15(Z)-octadecatrienoic acid and 2(R),9(S)-dihydroxy-10(E),12(Z)-octadecadienoic acid. alpha-Dioxygenase-generated 2-hydroxylinolenic acid, and to a lesser extent lipoxygenase-generated 9-hydroxyoctadecatrienoic acid, exerted a tissue-protective effect in bacterially infected tobacco leaves.

Fatty acid alpha-dioxygenases

This paper will briefly review the biochemistry, molecular biology and functionality of alpha-dioxygenases from tobacco, Arabidopsis, cucumber, pea, rice and algae. Alpha-dioxygenase treated in this review has emerged as a third group of fatty acid dioxygenases along with lipoxygenases and endoperoxide synthases.

alpha-oxidation of fatty acids in higher plants. Identification of a pathogen-inducible oxygenase (piox) as an alpha-dioxygenase and biosynthesis of 2-hydroperoxylinolenic acid

A pathogen-inducible oxygenase in tobacco leaves and a homologous enzyme from Arabidopsis were recently characterized (Sanz, A., Moreno, J. I., and Castresana, C. (1998) Plant Cell 10, 1523-1537). Linolenic acid incubated at 23 degrees C with preparations containing the recombinant enzymes underwent alpha-oxidation with the formation of a chain-shortened aldehyde, i.e., 8(Z),11(Z), 14(Z)-heptadecatrienal (83%), an alpha-hydroxy acid, 2(R)-hydroxy-9(Z),12(Z),15(Z)-octadecatrienoic acid (15%), and a chain-shortened fatty acid, 8(Z),11(Z),14(Z)-heptadecatrienoic acid (2%). When incubations were performed at 0 degrees C, 2(R)-hydroperoxy-9(Z),12(Z),15(Z)-octadecatrienoic acid was obtained as the main product. An intermediary role of 2(R)-hydroperoxy-9(Z), 12(Z),15(Z)-octadecatrienoic acid in alpha-oxidation was demonstrated by re-incubation experiments, in which the hydroperoxide was converted into the same alpha-oxidation products as those formed from linolenic acid. 2(R)-Hydroperoxy-9(Z),12(Z), 15(Z)-octadecatrienoic acid was chemically unstable and had a half-life time in buffer of about 30 min at 23 degrees C. Extracts of cells expressing the recombinant oxygenases accelerated breakdown of the hydroperoxide (half-life time, about 3 min at 23 degrees C), however, this was not attributable to the recombinant enzymes since the same rate of hydroperoxide degradation was observed in the presence of control cells not expressing the enzymes. No significant discrimination between enantiomers was observed in the degradation of 2(R,S)-hydroperoxy-9(Z)-octadecenoic acid in the presence of recombinant oxygenases. A previously studied system for alpha-oxidation in cucumber was re-examined using the newly developed techniques and was found to catalyze the same conversions as those observed with the recombinant enzymes, i.e. enzymatic alpha-dioxygenation of fatty acids into 2(R)-hydroperoxides and a first order, non-stereoselective degradation of hydroperoxides into alpha-oxidation products. It was concluded that the recombinant enzymes from tobacco and Arabidopsis were both alpha-dioxygenases, and that members of this new class of enzymes catalyze the first step of alpha-oxidation in plant tissue.

Intermediates and products formed during fatty acid alpha-oxidation in cucumber (Cucumis sativus)

Fatty acid alpha-oxidation is an essential metabolic pathway both in plants and in mammals which is still not completely understood. We previously described and purified an alpha-oxidation enzyme in cucumber which has been used in the present investigation of the alpha-oxidation reaction mechanism. Free fatty acids, and not the CoA thioesters, were found to undergo alpha-oxidation in cucumber. 2-Hydroxy- and 2-oxopalmitic acids were identified as palmitic acid alpha-oxidation intermediates by high-performance liquid chromatography and gas chromatography-mass spectrometry analysis in cucumber subcellular 150,000 x g(max) pellets obtained by differential centrifugation. Incubation of purified alpha-oxidation enzyme with [1-14C]palmitic acid resulted in the formation of both the above-described intermediates and the Cn-1 product, pentadecanal, and 14CO2. Besides 14CO2, 14C-formate was identified as an alpha-oxidation product from [1-14C]palmitic acid in cucumber subcellular fractions. Fe2+ stimulated the 14CO2 and 14C-formate production, and the addition of ascorbate and 2-oxoglutarate together with Fe2+ resulted in optimal alpha-oxidation activities, suggesting a dioxygenase reaction mechanism, as previously shown in mammals. NADPH and, to a lesser extent, NADH stimulated the total 14C-formate plus 14CO2 production but had only slight or no effects on 14CO2 production. H2O2 showed concentration-dependent inhibitory effects, while FAD had neither effect on 14CO2 nor 14CO2 plus 14C-formate production. The results in the present study demonstrate that an alpha-oxidation enzyme in cucumber is capable of oxidizing palmitic acid via 2-hydroxy- and 2-oxo-palmitic acid to produce pentadecanal and CO2. In contrast to the subcellular 150,000 x g(max) fraction, the purified alpha-oxidation enzyme could neither produce formate nor convert 14C-formate into 14C02, indicating two possible alpha-oxidation routes in cucumber.

Fatty acid alpha-oxidation of tetradecylthioacetic acid and tetradecylthiopropionic acid in cucumber (Cucumis sativus)

Fatty acid alpha-oxidation in cucumber (Cucumis sativus) involves enzymatic conversion of long-chain Cn-fatty acids to the C(n-1)-aldehyde and CO2. However, the mechanism of this process is not well understood. In this study, the alpha-oxidation of the fatty acid analogues tetradecylthioacetic acid (TTA) and tetradecylthiopropionic acid (TTP) with a sulphur atom substituting the methylene group in positions 3 and 4, respectively, was investigated and compared to palmitic acid. Both [1-14C]TTA and [1-14C]TTP could be alpha-oxidised in the cucumber subcellular 150000xgmax fraction. [1-14C]TTP was an even better substrate compared to the natural palmitic acid, while [1-14C]TTA was alpha-oxidised to a lower extent. [2-14C]TTA revealed no 14CO2, indicating that only one cycle of alpha-oxidation occurred. TTA was an inhibitor of the palmitic acid alpha-oxidation, and the inhibitory effects were examined.

Molecular cloning and expression of fatty acid alpha-hydroxylase from Sphingomonas paucimobilis

Fatty acid alpha-hydroxylase (FAAH) catalyzes the initial reaction in alpha-oxidation of fatty acid to produce 2-hydroxy fatty acid. FAAH activity has been detected in a wide range of organisms from prokaryotes to eukaryotes. Here, we describe cloning of the FAAH gene from Sphingomonas paucimobilis, a sphingolipid- and 2-hydroxymyristic acid-rich bacterium. The isolated gene encoded 415 amino acids. A homology search revealed that amino acid sequences highly conserved in cytochrome P450 (P450) were present in FAAH. Although the heme-binding cysteine was recognizable at position 361, the consensus in the heme-binding region was modified by an insertion. Overall, FAAH has no significant identity to the known P450s. CO difference spectrum of recombinant FAAH showed the characteristic one of P450, except this peak was at 445 nm. These results suggest bacterial FAAH is a novel member of the P450 superfamily.