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Particulate Preparation from Pisum Sativum (Plant Wax/CO Production/Hydrocarbon) T Proc. Natl. Acad. Sci. USA Vol. 81, pp. 6613-6617, November 1984 Biochemistry Alkane biosynthesis by decarbonylation of aldehydes catalyzed by a particulate preparation from Pisum sativum (plant wax/CO production/hydrocarbon) T. M. CHEESBROUGH AND P. E. KOLATTUKUDY* Institute of Biological Chemistry and Biochemistry/Biophysics Program, Washington State University. Pullman, WA 99164-6340 Communicated by P. K. Stumpf, June 29, 1984 ABSTRACT Mechanism of enzymatic conversion of a fat- MATERIALS AND METHODS ty acid to the corresponding alkane by the loss of the carboxyl Materials. Omnifluor, [1-_4C]stearic acid, [U-3H]tetraco- carbon was investigated with particulate preparations from Pi- sanoic acid, [1-'4Cjsteroyl-CoA, and LiAl3H4 were from sum sativum. A heavy particulate preparation (sp. gr., 1.30 New England Nuclear. Sodium [1-14C]cyanide was from g/cm3) isolated by two density-gradient centrifugation steps ICN (Chemical and Radioisotopes Division). Pyridinium catalyzed conversion of octadecanal to heptadecane and CO. chlorochromate, meso-tetraphenylporphyrin, and RhCP Experiments with [1-3H,1_14C]octadecanal showed the stoichi- 3H2O were from Aldrich. LiAIH4, di-t-butylchlorophos- ometry of the reaction and retention of the aldehydic hydrogen phine, and Ru(CO)12 were from Alfa Products. The rhodium in the alkane during this enzymatic decarbonylation. This de- chelate was synthesized by the procedure of Monson (8). carbonylase showed an optimal pH of 7.0 and a Km of 35 ,uM Ruthenium dicarbonyl tetraphenylporphyrin [Ru(CO)2- for the aldehyde. This enzyme was severely inhibited by metal (TTP)] was synthesized (9) and activated by substitution of ion chelators and showed no requirement for any cofactors. one CO with di-t-butyl-phosphine (10). The bright red crys- Microsomal preparations and the particulate fractions from tals of RuCO(TTP)[(C4H9)2P1 gave a spectrum identical to the first density-gradient step catalyzed acyl-CoA reduction to that reported (11). The oxidizing contaminants in Triton X- the corresponding aldehyde. Electron microscopic examina- 100 were removed as described (12). tion showed the presence of fragments of cell wall/cuticle but [1-14C]Octadecanal was synthesized from [1-'4C]octade- no vesicles in the decarbonylase preparation. It is concluded canol generated by LiAlH4 reduction of the corresponding that the aldehydes produced by the acyl-CoA reductase located acid using pyridinium chlorochromate, and the resulting al- in the endomembranes of the epidermal cells are converted to dehyde was purified as described (13). [2-14C]Octadecanal alkanes by the decarbonylase located in the cell wall/cuticle was synthesized by two cycles of nitrile elongation using a region. microscale adaptation of the method of Vederas et al. (14), followed by LiAlH4 reduction of the resulting acid and reoxi- Alkanes are widely distributed in the plant and animal king- dation of the alcohol to the aldehyde. [1-3H]Octadecanal was doms (1). Biological hydrocarbons usually have an odd num- synthesized by reduction of methyl octadecanoate with ber of carbon atoms, suggesting that they are derived by the LiAl3H4, followed by oxidation of the resulting alcohol with loss of one carbon atom from fatty acids with even numbers pyridinium chlorochromate as indicated above. [2-3H]Octa- of carbon atoms. Experiments with higher plant tissue slices decanoic acid was synthesized from 2-bromooctadecanoic strongly suggested that hydrocarbons are formed by chain acid generated by bromination of octadecanoic acid by the elongation of fatty acids followed by loss of the carboxyl Hell-Vollard-Zelensky reaction as described (15). The carbon, presumably by decarboxylation (2, 3). Subsequent methyl ester of the bromo acid was reduced with LiAl3H4 in work with insects (4) and mammals (5) supported this mech- tetrahydrofuran, and the resulting [1,2-3H]octadecanol was anism for alkane biosynthesis. Experiments with cell-free oxidized with CrO3 to [2-3H]octadecanoic acid. [U-3H]Te- preparations from pea leaves showed that oxygen and ascor- tracosanoyl-CoA was synthesized from the acid as described bate were required for the conversion of C32 fatty acid to (16). alkane and that metal ion chelators strongly inhibited alkane Enzyme Preparations. Pisum sativum (var. dark green per- synthesis (6). Subsequent studies showed that a major part fection) was grown in a growth room with a 220C day, 15'C of this alkane-generating activity was located in a crude mi- night, and a 15.5-hr photoperiod of 16,000 Ix. The apical crosomal fraction and that C18 to C32 fatty acids could serve meristem and its enclosing unopened leaflets were harvested as substrates for alkane formation (7). All of these substrates from 28- to 36-day-old plants. About 15 g of tissue was ho- gave rise to mainly alkanes containing two carbon atoms less mogenized three times for 10 sec each in an Omnimix with than the parent acid. Evidence was presented suggesting that 0.1 M potassium phosphate, pH 7.0/0.3 M sucrose. The ho- in vitro the aldehyde generated from the parent acid by the mogenate was filtered through two layers of cheesecloth and classical a-oxidation was the immediate precursor of the al- centrifuged for 20 min at 10,800 x g. The resulting superna- kane, whereas in vivo aldehydes generated by acyl-CoA re- tant was centrifuged at 165,000 x g, for 1 hr. The microsom- ductase might be the immediate precursor of alkanes. It was al pellet (P-2), resuspended in 24 ml of 0.1 M potassium suggested that the aldehyde might be decarborlylated to al- phosphate, pH 7.0/18% (wt/vol) sucrose, was layered on a kane. In the present paper, we describe the isolation of a discontinuous density gradient. Each tube, containing 6 ml particulate fraction devoid of a-oxidation activity, and we of sample layered on 7 ml of 25%, 6 ml of 30%, 8 ml of 40%, present direct experimental evidence for enzymatic decar- and 6 ml of 60% sucrose solutions in 0.1 M potassium phos- bonylation of an aldehyde to alkane. phate (pH 7.0), was centrifuged at 64,000 x g for 3 hr. The first 4 ml of 60% sucrose (G-1) collected from the bottom The publication costs of this article were defrayed in part by page charge was diluted 1:2 with 0.1 M potassium phosphate and centri- payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. *To whom reprint requests should be addressed. 6613 Downloaded by guest on September 28, 2021 6614 Biochemistry: Cheesbrough and Kolattukudy Proc. Natl. Acad Sci. USA 81 (1984) fuged for 3.5 hr at 64,000 x g on a second discontinuous sucrose density gradient, which consisted of 8 ml of sample, 60 40 30 25 18 4 ml of 40% sucrose, and 3 ml of 60% sucrose. The first 2.5 ml of 60% sucrose collected from the bottom was diluted 1:2 1.2 Aldehyde fl with 0.1 M potassium phosphate and centrifuged at 165,000 n- x g for 1.5 hr. The pellet (G-2) was resuspended in 1 ml of 0.1 M potassium phosphate buffer (pH 7.0) and used as the 0.6 source of the decarbonylase enzyme. Electron Microscopic and Chemical Examination. The par- ticulate fraction, recovered by a 1:2 dilution and centrifuga- tion at x was 165,000 g for 30 min, fixed with 2% gluteralde- E 1.2 hyde and 2% OS04 and stained with lead citrate/uranyl ace- tate. The particulate fraction recovered from the gradients r- was depolymerized with LiAlH4, and the products were ex- 0.6 amined by combined gas-liquid chromatography and mass spectrometry as described (17). H= Decarbonylase Assays. The reactions were run in 16 x 125 mm test tubes with serum caps through which polypropy- lene cups were fitted. Each cup contained 1 mg of Rh- 0.03p- Cl[(C6H6)3P]3 on a strip of wetted filter paper. Each reaction mixture contained 0.1 M potassium phosphate (pH 7.0), 36 A.M octadecanal, and enzyme in 2.0 ml. The substrate solu- tion was prepared by sonicating 72 nmol of octadecanal in 0.c 0.5 ml of 0.1 M potassium phosphate (pH 7.0) with 0.1% )1, (vol/vol) purified Triton X-100. After incubating the mix- tures at 30'C for 45 min, 200 A.l of 2 M HCl was added, and 0 5 10 15 any bound CO was released by photolysis by placing a 22 W Fraction fluorescent light 4 cm away from the reaction mixture for 2 min. The filter paper trap containing the CO adduct was FIG. 1. Sucrose density-gradient fractionation of pea leaf micro- placed directly in scintillation fluid and assayed for 14C. The somes. The microsomal suspension was centrifuged on a discontinu- lipids were recovered from the reaction mixtures with ous sucrose density gradient; the interfaces of the 18%, 25%, 30c, CHCl3/CH30H (2:1; vol/vol), and the alkanes were isolated 40%, and 60%o sucrose layers are marked by arrows. Each fraction by thin-layer chromatography and assayed for radioactivity was assayed for a-oxidation activity with [1-14C]palmitic acid and as described (2). The alkane fraction was analyzed by radio for alkane synthesis with [U-3H]tetracosanoic acid. Aldehyde pro- duction in the latter assay was measured by thin-layer chromatogra- gas-liquid chromatography. phy. Trypsin Digestion of G-2. An aliquot (100 ,ul) of the enzyme was with 25 strokes in a 2-ml Ten preparation homogenized decanoic acid, the major cutin monomer (3), only in the frac- Broeck homogenizer with 1.1 ml of 0.1 M potassium phos- tions that contained decarbonylase activity including G-2, phate buffer, pH 7.0/20 ,ug of L-1-tosylamido-2-phenylethyl the final particulate preparation.
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