Oleyl-coenzyme a metabolization by sub-cellular fractions from growing pea leaves

Exogenous nitric oxide alleviates manganese toxicity in bean plants by modulating photosynthesis in relation to leaf lipid composition

Nitric oxide (NO) is a signaling molecule controlling several steps of plant development and defense process under stress conditions. NO-induced alleviation of manganese (Mn) toxicity was investigated on bean plants submitted for 28 days to 500 µM MnCl₂. Manganese excess decreased plant dry weight and elongation and increased levels of reactive oxygen species and lipid peroxidation leading to up-regulation of superoxide dismutase, catalase, and ascorbate peroxidase activities. The inhibitory effects of Mn on plant growth were associated to reduction of light-saturated carbon assimilation (A_max), stomatal conductance (g_s), and transpiration (E). By contrast, Mn induced significant increase in the apparent quantum yield (ɸ) and light compensation point (LCP). Interestingly, intracellular CO₂ (Ci) remains stable under Mn stress. Concomitantly, leaf membrane lipids have drastically reduced under high Mn concentration. After Mn exposition, leaf fatty acids exhibited a significant loss of linolenic acid, accompanied by an accumulation of palmitoleic, stearic, and linoleic acids leading to alteration of lipid desaturation. NO supply reversed Mn toxicity as evidenced by enhancement of growth biomass and recovery of A_max, E, ɸ, and LCP. Similarly, NO addition has positive effects on leaf lipid content and composition leading to restoration of lipid unsaturation. The modulation of fatty acid composition can be a way to reduce leaf membrane damages and maintain optimal photosynthesis and plant growth. Despite the absence of enough evidences in how NO is involved in lipid and photosynthesis recovery under Mn stress conditions, it is assumed that NO beneficial effects are attributable to NO/Mn cross-talk.

Solubilization, purification and kinetic properties of three membrane-bound long-chain acyl-coenzyme-A thioesterases from microsomes of photosynthetic tissue

Microsomal membranes of developing pea (Pisum sativum) leaves contained almost one third of the total long-chain acyl-CoA thioesterase activity found in the leaf cell. Three distinct forms of long-chain acyl-CoA thioesterase were purified by a combination of cholate-solubilization, dialysis, ion-exchange, and gel-filtration chromatography. Purification factors of 4600, 100 and 280 were achieved for the thioesterase forms I, II and III, respectively. Apparent molecular masses were: form I, 28 kDa; form II, 140 kDa; form III, 139 kDa. All the three thioesterases showed overlapping specificities towards palmitoyl-CoA, stearoyl-CoA, and oleoyl-CoA but were inactive towards short-chain acyl-CoAs, such as acetyl-CoA and malonyl-CoA. Each thioesterase exhibited complex kinetic behavior, which was consistent with differential affinities of the enzymes for monomeric and micellar forms of their substrates. The significance of the kinetic behavior and possible regulatory role of these enzymes are discussed.

Functional association of a monoacylglycerophosphocholine acyltransferase and the oleoylglycerophosphocholine desaturase in microsomes from developing leaves

The biosynthesis of linoleic acid has been investigated, using oleoyl-CoA as a substrate, in microsomal preparations from young leaves of Pisum sativum. Oleoyl moieties from oleoyl-CoA were preferentially acylated to lysophosphatidylcholine by an acyltransferase to produce an oleoylglycerophosphocholine. Kinetic data are presented which argue for a direct desaturation of the oleoyl moieties of this oleoyl glycerophosphocholine to linoleoyl moieties. There was no evidence of a subsequent acyltransfer of linoleoyl moieties either to form thioesters or oxygen esters in other complex lipids. The kinetics were also consistent with a functional coupling of the lysophosphatidylcholine acyltransferase with the oleate desaturase. There was little exchange of the oleoyl glycerophosphocholine from the bulk membrane lipid with that newly synthesised by the lysophosphatidylcholine acyltransferase. Rather, the newly synthesised oleoylglycerophosphocholine seemed to be directly channelled to the vicinity of the desaturase. The results are discussed in the context of 'metabolite channelling'. The consequences for desaturase activity and its regulation are also examined.

Acetyl coenzyme A biosynthesis in the chloroplast : What is the physiological precursor?

Acetyl coenzyme A (CoA) biosynthesis in spinach chloroplasts has been investigated by following the incorporation of bicarbonate and acetate into fatty acids under a variety of conditions. Both substrates were readily incorporated into fatty acids in a light-dependent manner by intact photosynthesising chloroplasts, but when the concentrations of these substrates were adjusted to those found in vivo, i.e. 200 μM acetate, 10 μM bicarbonate, then acetate was found to supply carbon atoms for fatty acids biosynthesis via acetyl CoA at forty times the rate of bicarbonate. It is proposed that extra-chloroplastic free acetate is the pricipal substrate for chloroplasts acetyl CoA biosynthesis in spinach.