Biosynthesis of alkanes by particulate and solubilized enzyme preparations from pea leaves (Pisum sativum)

Occurrence of n-Alkanes in Vegetable Oils and Their Analytical Determination

Vegetable oils contain endogenous linear hydrocarbons, namely n-alkanes, ranging from n-C21 to n-C35, with odd chain lengths prevalent. Different vegetable oils, as well as oils of the same type, but of different variety and provenience, show typical n-alkane patterns, which could be used as a fingerprint to characterize them. In the first part of this review, data on the occurrence of n-alkanes in different vegetable oils (total and predominant n-alkanes) are given, with a focus on obtaining information regarding variety and geographical origin. The second part aims to provide the state of the art on available analytical methods for their determination. In particular, a detailed description of the sample preparation protocols and analytical determination is reported, pointing out the main drawbacks of traditional sample preparation and possible solutions to implement the analysis with the aim to shift toward rapid and solvent-sparing methods.

Fatty Acyl Synthetases and Thioesterases in Plant Lipid Metabolism: Diverse Functions and Biotechnological Applications

Plants use fatty acids to synthesize acyl lipids for many different cellular, physiological, and defensive roles. These roles include the synthesis of essential membrane, storage, or surface lipids, as well as the production of various fatty acid-derived metabolites used for signaling or defense. Fatty acids are activated for metabolic processing via a thioester linkage to either coenzyme A or acyl carrier protein. Acyl synthetases metabolically activate fatty acids to their thioester forms, and acyl thioesterases deactivate fatty acyl thioesters to free fatty acids by hydrolysis. These two enzyme classes therefore play critical roles in lipid metabolism. This review highlights the surprisingly complex and varying roles of fatty acyl synthetases in plant lipid metabolism, including roles in the intracellular trafficking of fatty acids. This review also surveys the many specialized fatty acyl thioesterases characterized to date in plants, which produce a great diversity of fatty acid products in a tissue-specific manner. While some acyl thioesterases produce fatty acids that clearly play roles in plant-insect or plant-microbial interactions, most plant acyl thioesterases have yet to be fully characterized both in terms of their substrate specificities and their functions. The biotechnological applications of plant acyl thioesterases and synthetases are also discussed, as there is significant interest in these enzymes as catalysts for the sustainable production of fatty acids and their derivatives for industrial uses.

Mountain pine beetle (Dendroctonus ponderosae) CYP4Gs convert long and short chain alcohols and aldehydes to hydrocarbons

Hydrocarbon biosynthesis in insects involves the elongation of fatty acyl-CoAs to very-long chain fatty acyl-CoAs that are then reduced and converted to hydrocarbon, with the last step involving the oxidative decarbonylation of an aldehyde to hydrocarbon and carbon dioxide. Cytochromes P450 in the 4G family decarbonylate aldehydes to hydrocarbon. All insect acyl-CoA reductases studied to date reduce fatty acyl-CoAs to alcohols. The results of the work reported herein demonstrate that CYP4G55 and CYP4G56 from the mountain pine beetle, Dendroctonus ponderosae, expressed as fusion proteins with house fly cytochrome P450 reductase (CPR), convert both long chain aldehydes and long chain alcohols to hydrocarbons. CYP4G55 and CYP4G56 appear to prefer primary alcohols to aldehydes as substrates. These data strongly suggest that hydrocarbon biosynthesis in insects occurs by the two-step reduction of very long chain fatty acyl-CoAs to alcohols, which are then oxidized to aldehydes and then oxidatively decarbonylated to hydrocarbon by CYP4G enzymes. In addition, both CYP4G55 and CYP4G56 fusion proteins convert C10 alcohols and aldehydes to hydrocarbons, including the conversion of (Z)-7-decenal, a putative intermediate in the exo-brevicomin pheromone biosynthetic pathway, to (Z)-3-nonene. These data demonstrate that the highly conserved CYP4G enzymes accept a broad range of carbon chain lengths, including C10 and C18, and have evolved to function in cuticular hydrocarbon biosynthesis and pheromone production.

Allocation of freshly assimilated carbon into primary and secondary metabolites after in situ ¹³C pulse labelling of Norway spruce (Picea abies)

Plants allocate carbon (C) to sink tissues depending on phenological, physiological or environmental factors. We still have little knowledge on C partitioning into various cellular compounds and metabolic pathways at various ecophysiological stages. We used compound-specific stable isotope analysis to investigate C partitioning of freshly assimilated C into tree compartments (needles, branches and stem) as well as into needle water-soluble organic C (WSOC), non-hydrolysable structural organic C (stOC) and individual chemical compound classes (amino acids, hemicellulose sugars, fatty acids and alkanes) of Norway spruce (Picea abies) following in situ (13)C pulse labelling 15 days after bud break. The (13)C allocation within the above-ground tree biomass demonstrated needles as a major C sink, accounting for 86% of the freshly assimilated C 6 h after labelling. In needles, the highest allocation occurred not only into the WSOC pool (44.1% of recovered needle (13)C) but also into stOC (33.9%). Needle growth, however, also caused high (13)C allocation into pathways not involved in the formation of structural compounds: (i) pathways in secondary metabolism, (ii) C-1 metabolism and (iii) amino acid synthesis from photorespiration. These pathways could be identified by a high (13)C enrichment of their key amino acids. In addition, (13)C was strongly allocated into the n-alkyl lipid fraction (0.3% of recovered (13)C), whereby (13)C allocation into cellular and cuticular exceeded that of epicuticular fatty acids. (13)C allocation decreased along the lipid transformation and translocation pathways: the allocation was highest for precursor fatty acids, lower for elongated fatty acids and lowest for the decarbonylated n-alkanes. The combination of (13)C pulse labelling with compound-specific (13)C analysis of key metabolites enabled tracing relevant C allocation pathways under field conditions. Besides the primary metabolism synthesizing structural cell compounds, a complex network of pathways consumed the assimilated (13)C and kept most of the assimilated C in the growing needles.

Arabidopsis long-chain acyl-CoA synthetase 1 (LACS1), LACS2, and LACS3 facilitate fatty acid uptake in yeast

The plant cuticle is a lipid-based barrier on the aerial surfaces of plants that plays a variety of protective roles. The cuticle is comprised largely of long-chain and very-long-chain fatty acids and their derivatives. In Arabidopsis, LONG-CHAIN ACYL-COA SYNTHETASE1 (LACS1), LACS2, and LACS3 are known or suspected cuticle biosynthetic genes. Very-long-chain acyl-coenzyme A (CoA) synthetase activity has been demonstrated for LACS1 and LACS2, although the role for such an activity in cuticle biosynthesis is currently unclear. In yeast and mammalian systems, some very-long-chain acyl-CoA synthetases are also called fatty acid transport proteins (FATPs) due to a second function of mediating transmembrane movement of fatty acids. We sought to determine if LACS1-3 also have this dual functionality. A yeast fat1Δ mutant is deficient in both very-long-chain acyl-CoA synthetase activity and exogenous fatty acid uptake. We demonstrate that heterologous expression of LACS1, 2, or 3 is able to complement both of these deficiencies. Furthermore, expression of each LACS enzyme in yeast resulted in uptake of the long-chain fatty acid analogue, C(1)-BODIPY-C(12). Only expression of LACS1 resulted in uptake of the very-long-chain fatty acid analogue, BODIPY-C(16). These results demonstrate that LACS1, LACS2, and LACS3 have the dual functionality of yeast and mammalian FATP enzymes. These findings have implications in the transmembrane transport and intracellular trafficking of plant lipids destined for export to the cuticle.

Characterization and quantification of the aliphatic hydrocarbon fraction during linseed development ( Linum usitatissimum L.)

Changes in hydrocarbon composition were investigated during maturation of three varieties of linseed (H52, O116, and P129) cultivated in Tunisia. The hydrocarbon fraction of the three linseed oil samples was found to contain mainly n-alkanes and squalene. The greatest decrease of these components occurred between 7 and 21 days after flowering (DAF); thereafter, the changes were slight. At 7 DAF, P129 had a significantly higher level of squalene (27.24 mg/100 g of oil) than H52 (3.36 mg/100 g of oil), but from this date until 21 DAF squalene decreased much more actively in P129, resulting in similar levels in H52 (0.57 mg/100 g of oil) and P129 (0.52 mg/100 g of oil) at full maturity. In three varieties of linseed, 13 n-alkanes were detected ranging from C(22) to C(34) carbon atoms. The n-alkane composition of linseed oil was influenced by the ripening stage of seeds. At 7 DAF, C29 was the most predominant hydrocarbon (19.84 mg/100 g of total oil), followed by C(27) (11.82 mg/100 g) and C(25) (11.28 mg/100 g). C(29) exhibited the most significant decrease from 7 to 21 DAF, as a result C(27) being the most significant n-alkane component for the remainder of the period. At full maturity, the content of total n-alkanes in three varieties of linseed ranged from 4.0 to 4.26 mg/100 g of oil.

Plant surface lipid biosynthetic pathways and their utility for metabolic engineering of waxes and hydrocarbon biofuels

Due to their unique physical properties, waxes are high-value materials that are used in a variety of industrial applications. They are generated by chemical synthesis, extracted from fossil sources, or harvested from a small number of plant and animal species. As a result, the diversity of chemical structures in commercial waxes is low and so are their yields. These limitations can be overcome by engineering of wax biosynthetic pathways in the seeds of high-yielding oil crops to produce designer waxes for specific industrial end uses. In this review, we first summarize the current knowledge regarding the genes and enzymes generating the chemical diversity of cuticular waxes that accumulate at the surfaces of primary plant organs. We then consider the potential of cuticle biosynthetic genes for biotechnological wax production, focusing on selected examples of wax ester chain lengths and isomers. Finally, we discuss the genes/enzymes of cuticular alkane biosynthesis and their potential in future metabolic engineering of plants for the production of renewable hydrocarbon fuels.

Sealing plant surfaces: cuticular wax formation by epidermal cells

The vital importance of plant surface wax in protecting tissue from environmental stresses is reflected in the huge commitment of epidermal cells to cuticle formation. During cuticle deposition, a massive flux of lipids occurs from the sites of lipid synthesis in the plastid and the endoplasmic reticulum to the plant surface. Recent genetic studies in Arabidopsis have improved our understanding of fatty acid elongation and of the subsequent modification of the elongated products into primary alcohols, wax esters, secondary alcohols, and ketones, shedding light on the enzymes involved in these pathways. In contrast, the biosynthesis of alkanes is still poorly understood, as are the mechanisms of wax transport from the site of biosynthesis to the cuticle. Currently, nothing is known about wax trafficking from the endoplasmic reticulum to the plasma membrane, or about translocation through the cell wall to the cuticle. However, a first breakthrough toward an understanding of wax export recently came with the discovery of ATP binding cassette (ABC) transporters that are involved in releasing wax from the plasma membrane into the apoplast. An overview of our present knowledge of wax biosynthesis and transport and the regulation of these processes during cuticle assembly is presented, including the evidence for coordination of cutin polyester and wax production.

Ontogenic variations in n-alkanes during somatic embryogenesis of flax (Linum usitatissimum L.)

Hypocotyl segments (HS) of flax seedlings germinated in vitro, were used to induce indirect somatic embryogenesis on solid medium. The composition and distribution of n-alkanes in flax tissues collected at different developmental stages were studied by capillary gas chromatography (GC) and capillary gas chromatography-mass spectrometry (GC-MS). During induction and development of callus from hypocotyl tissues a decrease in the percentage of total lipids was observed. In all types of tissue sampled - HS used as primary explants, HS with differentiating calli at the cut ends (HSC), embryogenic (EC) and non-embryogenic calli (NEC) and somatic embryos (SE) - a skewed-normal distribution of n-alkanes with a low mass range (C(13)-C(21)) were found. The highest content of n-alkanes occurred in the primary hypocotyl explants and in the early stages of callus development. Longer carbon chain n-alkanes were observed only in the mature or differentiated tissues of hypocotyls and SE. Although the n-alkane contents decreased with time, in SE and calli, a significantly lower n-alkane content was observed in EC when compared to NEC independent of the time in culture. These results suggest the utilisation of n-alkanes for heterotrophic cellular growth as well as its mobilisation from EC to developing SE.

Solubilization, partial purification, and characterization of a fatty aldehyde decarbonylase from a higher plant, Pisum sativum

Enzymatic decarbonylation of fatty aldehydes generates hydrocarbons. The particulate enzyme that catalyzes the decarbonylation has not been solubilized and purified from any organism but a green alga. Here we report the solubilization, purification, and partial characterization of the decarbonylase from a higher plant. Decarbonylase from a particulate preparation from pea (Pisum sativum) leaves, enriched in decarbonylase, was solubilized with beta-octyl glucoside and partially purified. SDS-PAGE showed a major protein band at 67 kDa. Rabbit antibodies raised against this protein specifically cross-reacted with the 67-kDa protein in solubilized microsomal preparations; anti-ribulose bisphosphate carboxylase cross-reacted only with the 49-kDa large subunit of the carboxylase, but not with any protein near 67 kDa, showing the absence of any contamination from cross-linked small-large subunit of the carboxylase found in the green algal enzyme preparation. Anti-67-kDa protein antibodies inhibited decarbonylation catalyzed by the enzyme preparations, showing that this protein represents the decarbonylase. Decarbonylase activity of the purified enzyme required phospholipids for activity; phosphatidylcholine was the preferred lipid although phosphatidylserine and phosphatidylethanolamine could substitute less effectively. Half-maximal activity was observed at 40 microM octadecanal. The purified enzyme produced alkane and CO and was inhibited by O2, NADPH, and DTE. Metal ion chelators severely inhibited the enzyme and Cu2+ fully restored the enzyme activity. Purified enzyme preparations consistently showed the presence of Cu, and copper protoporphyrin IX catalyzed decarbonylation. These results suggest that this higher plant enzyme probably is a Cu enzyme unlike the green algal enzyme that was found to have Co.

Resolution and purification of an aldehyde-generating and an alcohol-generating fatty acyl-CoA reductase from pea leaves (Pisum sativum L.)

Higher plant tissues produce both wax esters generated from fatty alcohols and hydrocarbons generated from fatty aldehydes. If two different reductases are responsible for the synthesis of aldehydes and alcohols, both types of reductases may be present in such tissues. To test for this possibility, pea leaves, known to produce both types of wax components, were examined. Subcellular fractionation showed that acyl-CoA reductase activities were localized mainly in the microsomal fraction. Fatty aldehyde formation was rectilinear for 30 min and subsequently decreased, whereas fatty alcohol formation remained linear for 2 h. The two activities in the microsomes were differently affected by pH; alcohol formation was optimal between pH 5 and pH 6, whereas aldehyde formation was optimal at around pH 7.5. With solubilized microsomes, protein concentration dependence of alcohol formation showed a sigmoidal pattern, possibly suggesting inhibition by hexadecanoyl-CoA at low protein concentrations. Bovine serum albumin (BSA) enhanced alcohol formation. In contrast, the aldehyde generation showed a typical protein concentration dependence, and BSA severely inhibited aldehyde generation. Phosphatidylcholine showed over twofold stimulation for alcohol formation, whereas aldehyde formation was only slightly stimulated. All of this biochemical evidence suggested the presence of two different reductases. Confirming this hypothesis, an aldehyde-generating and an alcohol-generating reductase were resolved from the solubilized microsomal proteins using Blue A agarose, gel filtration, and hexadecanoyl-CoA affinity chromatography. SDS-PAGE of the purified proteins showed that the alcohol-generating enzyme was a 58-kDa protein and the aldehyde-forming one was a 28-kDa protein. It is proposed that two different elongating systems are functionally coupled to the alcohol-generating and aldehyde-generating reductases, which in turn are coupled to the transacylase to produce wax esters and to the decarbonylase to produce hydrocarbons, respectively.

Hydrocarbon biosynthesis in Triatoma infestans eggs

Triatoma infestans eggs are shown to synthesize hydrocarbons. Radio-gas chromatography was used to demonstrate metabolism of [1-14C]propionate into precursor methyl-branched fatty acids and into methyl-branched hydrocarbons in T. infestans eggs. These reactions have not been demonstrated previously in insect eggs. An in vivo study showed that hydrocarbons are also transported to eggs by the hemolymph. Inhibition of hydrocarbon synthesis by sodium trichloroacetate (NaTCA) was correlated with reduced oviposition, reduced hatchability, and reduced insect survival. Scanning electron microscopy showed impoverishment of the eggs' epicuticular waxes following NaTCA treatment.

Regulation of sex pheromone biosynthesis in the housefly, Musca domestica: relative contribution of the elongation and reductive steps

The regulation of production of the sex pheromone (Z)-9-tricosene (Z9-23:Hy) in the housefly, Musca domestica, was studied by examining the chain length specificity of the fatty acyl-CoA elongation reactions and the reductive conversion of fatty acyl-CoAs to alkenes in 1- and 4-day-old male and female houseflies. Microsomal preparations from 4-day-old female insects produced as the predominant alkene Z9-23:Hy when incubated with malonyl-CoA, NADPH, and [9,10-3H2]oleoyl-CoA (18:1-CoA), whereas microsomal preparations from 4-day-old male insects produced predominantly (Z)-9-heptacosene (Z9-27:Hy). These are the major alkenes produced in vivo by Day 4 females and males, respectively. Microsomes prepared from both Day 1 males and Day 1 females produced Z9-27:Hy as the major alkene from labeled 18:1-CoA. This is the major alkene produced in vivo by both sexes at Day 1. An examination of the chain length specificity of the elongation reactions showed that microsomes prepared from Day 4 male insects readily elongated both 18:1-CoA and 15-[15,16-3H2]tetracosenoyl-CoA (24:1-CoA) to 28-carbon moieties, whereas microsomes from Day 4 female insects did not efficiently elongate either substrate beyond 24 carbons. With high substrate concentrations, microsomes prepared from male insects converted 24:1-CoA to Z9-23:Hy more efficiently than did those from females, whereas under lower and presumably more physiological substrate concentrations, microsomes from females had slightly higher activity than did those from males. Taken together, these data show that the regulation of the chain length of the alkenes, and thus sex pheromone production, in the housefly resides predominantly in the elongation reactions and not in the step which converts the fatty acyl-CoA to hydrocarbon.

A cobalt-porphyrin enzyme converts a fatty aldehyde to a hydrocarbon and CO

The final step in hydrocarbon biosynthesis involves loss of CO from a fatty aldehyde. This decarbonylation is catalyzed by microsomes from Botyrococcus braunii. Among the several detergents tested for solubilizing the decarbonylase, octyl beta-glucoside (0.1%) was found to be the most effective and released 65% of the enzyme activity in soluble form. FPLC of the solubilized enzyme preparation with Superose 6 followed by ion-exchange FPLC with Mono Q resulted in 200-fold increase in specific activity with 7% recovery. The purified enzyme released nearly 1 mol of CO for each mol of hydrocarbon. SDS/PAGE of the enzyme preparation showed two protein bands of equal intensity at 66 and 55 kDa. The absorption spectrum of the enzyme with bands at 410 nm, 425 nm, 580 nm, and 620 nm suggests the presence of a porphyrin. Electron microprobe analysis revealed that the enzyme contained Co. Purification of the decarbonylase from B. braunii grown in 57CoCl2 showed that 57Co coeluted with the decarbonylase. These results suggest that the enzyme contains Co that might be part of a Co-porphyrin, although a corrin structure cannot be ruled out. Co-protoporphyrin IX itself caused decarbonylation of octadecanal at 60 degrees C, whereas the metal ion or protoporphyrin alone, or several other metal porphyrins, did not cause decarbonylation. These results strongly suggest that biosynthesis of hydrocarbons is effected by a microsomal Co-porphyrin-containing enzyme that catalyzes decarbonylation of aldehydes and, thus, reveal a biological function for Co in plants.

A microsomal fatty acid synthetase from the integument of Blattella germanica synthesizes methyl-branched fatty acids, precursors to hydrocarbon and contact sex pheromone

Methyl-branched fatty acids present in the integument of the German cockroach, Blattella germanica, were identified by gas chromatography-mass spectrometry of their methyl esters and reduction products (alkanes) as n-3-, n-4-, n-5-, n-7-, n-8-, and n-9-monomethyl fatty acids and as n-5,9-, n-3,9-, and n-3,11-dimethyl fatty acids with 16 to 20 total carbons. These fatty acids have the same branching patterns as do the major hydrocarbons of this insect, including 3,11-dimethylnonacosane, the precursor to the major contact sex pheromone, and are presumed to be intermediates in hydrocarbon formation. A novel microsomal fatty acid synthetase (FAS) located in the integument of this insect incorporated [methyl-14C]methylmalonyl-CoA into methyl-branched fatty acids as demonstrated by radio-high-performance liquid chromatography. A cytosolic FAS is also present in the integument. Both the microsomal and the soluble FAS incorporated [methyl-14C]methylmalonyl-CoA into fatty acids, but only the microsomal FAS was able to efficiently use methylmalonyl-CoA as the sole elongating agent. This is the first report of the characterization of methyl-branched fatty acids from the integument of an insect and of an integumental microsomal FAS that incorporates methylmalonyl-CoA into branched fatty acids.

Alkane biosynthesis by decarbonylation of aldehyde catalyzed by a microsomal preparation from Botryococcus braunii

The final step in the synthesis of n-hydrocarbons in an animal and a higher plant involves enzymatic decarbonylation of aldehydes to the corresponding alkanes by loss of the carbonyl carbon. Whether such a novel reaction is involved in hydrocarbon synthesis in the colonial microalga, Botryococcus braunii, which is known to produce unusually high levels (up to 32% of dry weight) of n-C27, C29, and C31 alka-dienes and -trienes, was investigated. Dithioerythritol severely inhibited the incorporation of [1-14C]acetate into these hydrocarbons with accumulation of the label in the aldehyde fraction in the B. braunii cells. Microsomal preparations of the alga synthesized alkane from fatty acid and aldehyde in the absence of O2. Conversion of fatty acid to alkane required CoA, ATP, and NADH, whereas conversion of aldehyde to alkane did not require the addition of cofactors. That the alkane synthesis involves a decarbonylation was shown by the production of CO and heptadecane from octadecanal. CO was identified by adsorption to RhCl[(C6H6)3P]3. The decarbonylase had a pH optimum at 7.0, an apparent Km of 65 microM, a Vmax of 1.36 nmol/min/mg and was inhibited by the metal chelators EDTA, O-phenanthroline and 8-hydroxyquinoline. It was stimulated nearly threefold by 2 mM ascorbate and inhibited by the presence of O2. A partial (28%) retention of the aldehydic hydrogen of [1-3H]octadecanal in the heptadecane was observed; the remaining 3H was lost to H2O. The microsomal preparation also catalyzed the oxidation of 14CO to 14CO2, with a pH optimum of 7.0. This accounts for the nonstoichiometry of CO to heptadecane observed. In vivo studies with 14CO showed that the label was incorporated into metabolic products. This metabolic conversion of CO, not found in the previously examined hydrocarbon synthesizing systems, may be necessary for organisms that produce large amounts of hydrocarbons such as the present alga. The mechanism of the decarbonylation and the nature of the decarbonylase remain to be elucidated.

Identification and characterization of nonsedimentable lipid-protein microvesicles

Previously uncharacterized lipid-protein microvesicles have been isolated from young and senescing bean cotyledon tissue. The microvesicles are nonsedimentable and enriched in phospholipid degradation products (free fatty acids, long-chain aldehydes, and long-chain hydrocarbons). They range from 70 to 170 nm (radius) with a mean radius of 132 nm, and it is clear from freeze-fracture electron micrographs that they are bilayered in nature. Nonsedimentable lipid-protein microvesicles containing the same products of phospholipid degradation but smaller were also formed in vitro when smooth microsomal membranes from young cotyledon tissue were treated with Ca2+ to stimulate enzymatic degradation of phospholipids. The data suggest that these microvesicles comprise an intermediate stage of membrane lipid deterioration. They appear to serve as a vehicle for moving phospholipid degradation products out of membranes into the cytosol during senescence and perhaps also during normal membrane lipid turnover.

Tissue and chain length specificity of the fatty acyl-CoA elongation system in the American cockroach

The elongation of fatty acyl-CoAs, reactions involved in hydrocarbon biosynthesis, was examined in the cockroach, Periplaneta americana. Products were analyzed by radio-HPLC and radio-GLC. The majority of the elongation activity was observed in microsomes prepared from abdominal epidermal tissue. Linoleoyl-CoA (18:2-CoA) was elongated most efficiently followed by stearoyl-CoA (18:0-CoA), linolenoyl-CoA (18:3-CoA; n-3) and oleoyl-CoA (18:1-CoA). The products of 18:2-CoA elongation included all even numbered acyl groups up to 28 carbons, and the products of 18:0-CoA included all even numbered acyl groups to 26 carbons. The 18:3-CoA was elongated only to 20 and 22 carbons. Radioactivity from both 18:2-CoA (5.4%) and 18:0-CoA (1.2%) was recovered in the hydrocarbon fraction. Analysis of this hydrocarbon fraction showed that the radio-activity from 18:2-CoA was present in (Z,Z)-6,9-heptacosadiene and that the radioactivity from 18:0-CoA was present in n-pentacosane. These data demonstrate for the first time in an in vitro insect system that the fatty acid elongation reactions are coupled with the conversion of the elongated product to hydrocarbon. Thus, each of the expected intermediates in the conversion of 18:0 and 18:2 to 25 and 27 carbon hydrocarbons, respectively, was observed, and the results demonstrate high tissue, substrate, and product specificity.