The Utilization of Glycolytic Intermediates as Precursors for Fatty Acid Biosynthesis by Pea Root Plastids

Abstract

Radiolabeled pyruvate, glucose, glucose-6-phosphate, acetate, and malate are all variously utilized for fatty acid and glycerolipid biosynthesis by isolated pea (Pisum safivum 1.) root plastids. At the highest concentrations tested (3-5 mM), the rates of incorporation of these precursors into fatty acids were 183, 154, 125, 99, and 57 nmol h-' mg-' protein, respectively. In all cases, cold pyruvate consistently caused the greatest reduction, whereas cold acetate consistently caused the least reduction, i n the amounts of each of the other radioactive precursors utilized for fatty acid biosynthesis.Acetate incorporation into fatty acids was approximately 55% dependent on exogenously supplied reduced nucleotides (NADH and NADPH), whereas the utilization of the remaining precursors was only approximately 1 O and 20% dependent on added NAD(P)H. In contrast, the utilization of all precursors was greatly dependent (85-95O/0) on exogenously supplied ATP. Palmitate, stearate, and oleate were the only fatty acids synthesized from radioactive precursors. Higher concentrations of each precursor caused increased proportions of oleate and decreased proportions of palmitate synthesized. Radioactive fatty acids from all precursors were incorporated into glycerolipids. The data presented indicate that the entire pathway from glucose, including glycolysis, to fatty acids and glycerolipids is operating i n pea root plastids. This pathway can supply both carbon and reduced nucleotides required for fatty acid biosynthesis but only a small portion of the ATP required.Plastids, including chloroplasts and chromoplasts, oilseed plastids, and root plastids, are the site of de novo fatty acid biosynthesis in plants. This subject has been thoroughly reviewed in a number of works (Stumpf, 1984;Harwood, 1988;Dennis, 1989; Sparace and KleppingerSparace, 1993). Traditionally, most in vitro studies of fatty acid biosynthesis utilize acetate as a radioactive tracer largely because it is efficiently incorporated into fatty acids. This has been justified by the belief that extraplastidic acetate originates through the action of mitochondrial pyruvate dehydrogenase and acetyl-COA hydrolase (Murphy and Stumpf, 1981), as well as by the occurrence of envelope-bound acetyl-COA synthetase (Kuhn et al., 1981). However, more recent studies indicate that most plastids contain their own pyruvate dehydrogenase (Reid et al

Metabolism of glucose-6-phosphate and utilization of multiple metabolites for fatty acid synthesis by plastids from developing oilseed rape embryos

“…This contrasts with observations made using plastids isolated from other tissues where the simultaneous supply of two substrates for fatty acid synthesis has led to a reduction in the rate of incorporation of one substrate relative to that when it was supplied alone (e.g. Smith et al 1992;Fuhrmann et al 1994;Qi et al 1995). For example, Glc6P reduced incorporation from pyruvate into fatty acids by 25%, and pyruvate reduced that from Glc6P by up to 50% when these metabolites were supplied concurrently to plastids isolated from pea roots (Qi et al 1995).…”

Multiple Metabolic Roles for the Nonphotosynthetic Plastid of the Green Alga Prototheca wickerhamii

“…The identification of several clusters encoding homologues of enzymes related to the glycolytic pathway (phosphoglucomutase, phosphoglycerate dehydrogenase, and pyruvate kinase) including two members of the pyruvate dehydrogenase complex, suggests that the imported carbohydrates can be converted to acetyl-coenzyme A in the plastid of P. wickerhamii. A similar process is known to take place in the leucoplasts of photosynthetically competent land plants (37,40). The pyruvate dehydrogenase complex is also present in Helicosporidium sp.…”

Evidence That a Malate/Inorganic Phosphate Exchange Translocator Imports Carbon across the Leucoplast Envelope for Fatty Acid Synthesis in Developing Castor Seed Endosperm

“…Plastids isolated from developing embryos of rapeseed (Buassica napus), roots of pea (Pisum sativum), and leaves of maize (Zea mays) can also utilize malate as a substrate for fatty acid synthesis (Kang and Rawsthorne, 1994;Preiss et al, 1994;Qi et al, 1995). In Z. mays the rate of incorporation of carbon from malate into fatty acids by isolated chloroplasts is greatly stimulated by subjecting the plants to conditions of salt stress (Preiss et al, 1994).…”