Long chain fatty acid CoA-activation by microsomes from Allium porrum epidermal cells

Evidence that oleoyl-CoA and ATP-dependent elongations coexist in rapeseed (Brassica napus L.)

The elongation of different substrates was studied using several subcellular fractions from Brassica napus rapeseed. In the presence of malonyl-CoA, NADH and NADPH, very-long-chain fatty acid (VLCFA) synthesis was observed from either oleoyl-CoA (acyl-CoA elongation) or endogenous primers (ATP-dependent elongation). No activity was detected using oleic acid as precursor. Acyl-CoA and ATP-dependent elongation activities were mainly associated with the 15 000 g/25 min membrane fraction. Reverse-phase TLC analysis showed that the proportions of fatty acids synthesized by these activities were different. Acyl-CoA elongation increased up to 60 microM oleoyl-CoA, and ATP-dependent elongation was maximum at 1 mM ATP. Both activities increased with malonyl-CoA concentration (up to 200 microM). Under all conditions tested, acyl-CoA elongation was higher than ATP-dependent elongation, and, in the presence of both ATP and oleoyl-CoA, the elongation activity was always lower. ATP strongly inhibited acyl-CoA elongation, whereas ATP-dependent elongation was slightly stimulated by low oleoyl-CoA concentrations (up to 15 microM) and decreased in the presence of higher concentrations. CoA (up to 150 microM) had no effect on the ATP-dependent elongation, whereas it inhibited the acyl-CoA elongation. These marked differences strongly support the presence in maturing rapeseed of two different elongating activities differently modulated by ATP and oleoyl-CoA.

Characterization of fatty acid elongase enzymes from germinating pea seeds

In vitro biosynthesis of radioactive arachidate and behenate was observed when microsomal fractions of germinating pea seeds were incubated with exogenous stearoyl-CoA (18:0-CoA) or arachidoyl-CA (20:0-CoA) in the presence of NADPH, [2-14C]malonyl-CoA and ATP. Characterization of parameters required for optimal stearoyl- and arachidoyl-CoA elongation revealed that, at least, two chain-length-specific elongases are necessary for very-long-chain fatty acid synthesis. Both enzymes were found to be sensitive to the group-selective reagents, p-CMB, NEM, iodoacetate, arsenite and phenylglyoxal. Subcellular fractionation studies indicated that both of these elongases were localized mainly in the endoplasmic reticulum.

BIOCHEMISTRY AND MOLECULAR BIOLOGY OF WAX PRODUCTION IN PLANTS

The aerial surfaces of plants are covered with a wax layer that is primarily a waterproof barrier but that also provides protection against environmental stresses. The ubiquitous presence of cuticular wax is testimony to its essential function. Genetic and environmental factors influence wax quantity and composition, which suggests that it is an actively regulated process. The basic biochemistry of wax production has been elucidated over the past three decades; however, we still know very little about its regulation. This review presents a discussion along with new perspectives on the regulatory aspects of wax biosynthesis. Among the topics discussed are the partitioning of fatty acid precursors into wax biosynthesis and the elongation of fatty acids with particular emphasis on the nature of the acyl primer, and the role of ATP in fatty acid elongation. The recent cloning of wax biosynthetic genes and the transport of wax to plant surfaces are also discussed.

The membrane of leaf peroxisomes contains a porin-like channel

Spinach leaf peroxisomes were purified by Percoll density gradient centrifugation. After several freeze-thaw cycles, the peroxisomal membranes were separated from the matrix enzymes by sucrose density gradient centrifugation. The purity of the peroxisomal membranes was checked by measuring the activities of marker enzymes and by using antibodies. Lipid bilayer membrane experiments with the purified peroxisomal membranes, solubilized with a detergent, demonstrated that the membranes contain a channel-forming component, which may represent the major permeability pathway of these membranes. Control experiments with membranes of other cell organelles showed that the peroxisomal channel was not caused by the contamination of the peroxisomes with mitochondria or chloroplasts. The peroxisomal channel had a comparatively small single channel conductance of 350 pS in 1 M KCl as compared with channels from other cell organelles. The channel is slightly anion selective, which is in accordance with its physiological function. The single channel conductance was found to be only moderately dependent on the salt concentration in the aqueous phase. This may be explained by the presence of positive point net charges in or near the channel or by the presence of a saturable binding site inside the channel. The possible role of the channel in peroxisomal metabolism is discussed.

Biosynthesis of very long chain fatty acids in microsomes from epidermal cells of Allium porrum L

The elongation system present in leek epidermal cells functions to synthesize very long chain fatty acids which, in turn, are the precursors to alkanes. The elongation system is microsomal, employs only saturated acyl components of the endogenous lipid pool as acceptors, utilizes malonyl-CoA as the C2 donor, has an absolute requirement for ATP, and is markedly inhibited by acetyl-ACP. Only saturated acyl-CoAs are readily elongated to very long chain fatty acids by malonyl-CoA in the absence of ATP. ACP is not required by the microsomal system.