The Stereochemistry of α‐Oxidation of Fatty Acids in Plants

Abstract

ration of acetone-dried powder has been described previously [1,2]. This was stirred in about forty times its weight of cold 0.2M phosphate buffer (pH 7.0) for 1 h and briefly centrifuged to remove most of the insoluble material. Homogenates were made by coarsely chopping the leaves into about five times their weight of cold 0.2 M phosphate buffer (pH7.0) and homogenising with an Ultra Turrax tissue grinder (Hudes Merchandising, Leeke St., London) for about 45 sec a t a temperature below 10". Homogenates and acetone-dried powder solutions were used immediately after preparation.The Radiochemical Centre (Amersham) provided [1-14C]palmitic acid (specific activity, 44 mC/mmole). Preparation of ~-[ 2 -~H ] 1 6 :0 and ~-[ 2 -~H ] 1 6 : 0The preparation of D-and L-2-hydroxypalmitic acid, essentially by the method of Horn and Pretorius [5], has been described previously [3]. Each enantiomer was converted into its tosyl derivative with retention of configuration by treatment with p-toluenesulphonyl chloride in pyridine. Each tosylate was refluxed in anhydrous tetrahydrofuran for 20 h with lithium boron [3H]hydride, when hydrogenolysis with inversion of configuration occurs [6,7]. The hexadecanol formed was oxidised t o the acid with chromium trioxide in acetic acid; D-16h:O thus gave ~-[2-~H]16 : 0, which was purified by thin-layer and gas-liquid chromatography. Similarly L-16 h : 0 yielded ~-[ 2 -~H ] 1 6 : 0 (Fig.2). METHODSSubstrates were prepared by mixing [1-l4C]16: 0 and the [2-3H]16:0 in benzene solution, aliquots of which were assayed for 14C and 3H simultaneously in a Packard Tri Carb spectrometer. The 3H :14C ratio

Glyceryl-ether monooxygenase [EC 1.14.16.5]. A microsomal enzyme of ether lipid metabolism

“…140,232 The formation of the carbon-oxygen bond can occur with retention or inversion of configuration at C1Ј, but this cannot be deduced from these experiments and should await the isolation of the intermediate hemiacetal (see Scheme 1). However, by analogy with the known stereospecificities of hydroxylations by mixed function monooxygenases, all of which occurred with retention of configuration, [233][234][235][236][237] the hydroxylation by glyceryl-ether monooxygenase can tentatively be said to occur with retention of configuration at C1Ј of the glyceryl ethers.…”

Mechanisms and stereochemistry in fatty acid metabolism

“…When the D-2-hydroxypalmitic acids produced by pea leaf preparations froin these D-and L-[1-_4C,2-3H]palmitic acid substrates were isolated it was found that the tritium in the D-configuration in the substrate had been lost on hydroxylation whereas the L-tritium had been retained (Morris & Hitchcock, 1968). Therefore the formation of longchain ac-hydroxy fatty acids in these higher-plant leaf systems proceeds by direct hydroxylation at the 2-position with retention ofconfiguration at that position.…”

The stereochemistry of α-oxidation of fatty acids in plants: the configuration of biosynthetic long-chain 2-hydroxy acids

“…All natural long-chain 2-hydroxy acids that have been isolated and tested by polarimetry are optically active and have the D-configuration at C-2; in particular, lipids of pea (Pi8um sativum) leaves contain a small proportion of D-2-hydroxypalmitate (Hitchcock, Morris & James, 1968b). This evidently accumulates after biosynthesis by a-hydroxylation of palmitate (Hitchcock & James, 1966;Morris & Hitchcock, 1968), though ac-oxidation of palmitate apparently involves preferential formation and oxidation of L-2-hydroxypalmitate (Hitchcock, Morris & James, 1968a). When [1-14C]palmitic acid is incubated with pea leaves or solutions of their acetone-dried powders, 2-hydroxy[1-14C]palmitate accumulates; the configuration of this metabolite cannot be determined directly, but appears to be the D-hydroxy acid by crystallization of the isolated hydroxy esters with authentic enantiomers (Hitchcock et al 1968b).…”