NADP-specific malic enzyme

Hrdy I, Mertens E, Van Schaftingen E (1993) Identification, purification and separation of different isozymes of NADP-specific malic enzyme from Tritrichomonas foetus. Mol... 

Aside from PEPCase, a number of other CAM-related genes have been partially characterised (Table 1). These include cDNA clones for pyruvate, orthophosphate dikinase (PPDK), a specific NADP malate dehydrogenase (MDH), glyceraldehyde phosphate dehydrogenase (GaPDH) and NADP-dependent malic enzyme (MOD). Previous studies indicated that the enzymatic activities of these gene products increased upon salt stress in the ice plant (Holtum Winter, 1982). As in the case... 

Some plants, such as corn and sugar cane, have evolved an auxiliary C4-dicarboxylic acid cycle< > that cooperates with the reductive pentose cycle in the photosynthetic assimilation of CO2. In plants with this cycle (sometimes referred to as the Hatch and Slack cycle), chloroplasts in the mesophyll cells near the surface on the leaf contain three C4-pathway specific enzymes pyruvate, phosphate-dikinase that directly converts pyruvate into phosphoenolpyruvate (PEP) with ATP, PEP carboxylase that catalyzes the carboxyla-tion of PEP to oxaloacetate, and malate dehydrogenase that finally reduces oxaloacetate to malate with NADPH. The purpose of these steps is apparently to incorporate CO2 and NADPH into malate in order to translocate them to the vascular bundle sheath cells, where they are again released by the action of a NADP-dependent malic enzyme. The malic enzyme is located in the bundle sheath chloroplasts together with the en mes of the Calvin cycle. CO2 is then reduced to carbohydrates while pyruvate is presumably transported back to the mesophyll cells. Besides the malate-type C4-plants, there is a second and larger group of species (aspartate type) that contains little malic enzyme and utilizes aspartate as the COj carrier. 

It is not surprising that the pyruvic acid intermediate seemed plausible because in a paper earlier in that same year (23), the authors described a malic enzyme from pigeon liver. This enzyme was shown to form appreciable amounts of pyruvic acid from malic acid, but it was NADP instead of NAD specific. The end product was shown to be pyruvic acid by spectrophotometric assay involving lactate dehydrogenase. 

Ochoa reported that malic enzyme from L. plantarum was NAD and not NADP specific. The malic enzyme of cauliflower bud mitochondria (31) is NAD and NADP specific, with NAD being the preferred cofactor. Both the malo-lactic activity and NADH producing activity of the Leuconostoc oenos system (6,7, 8) was strictly NAD specific. Nicotinamide-adenine dinucleotide phosphate, flavin adenine dinucleotide, and flavin mononucleotide could not substitute in either of these activities. 

It has been purified (445) and shares some properties in common with malic enzymes from mammals and birds in being NADP-dependent, heat-stable and able to decarboxylate oxaloacetate. The malic enzyme of H. microstoma also has a marked specificity for NADP (216), contrasting with that of Spirometra mansonoides, which appears to be both NAD- and NADP-linked (220). Malic enzyme has been demonstrated in a range of other cestodes including Mesocestoides corti (399), Schistocephalus solidus (406), Moniezia expansa (60), Echinococcus spp. (500) and L. intestinalis (502). 

Periodate-oxidized NADP has been used as an affinity label for other NADP-specific enzymes including pigeon liver malic enzyme (72, 73), spinach ferre-doxin-NADP reductase (74, 75), and Leuconostoc mesenteroides glucose-6-phosphate dehydrogenase (76). Recently, a new fluorescent dialdehyde derivative has been reported as an affinity label of NADP sites, namely, periodate-oxidized 3-aminopyridine adenine dinucleotide phosphate (77). 

FIGURE 2. Reversible light-activation of several of the mesophyll cell-specific steps of the C., cycle (i ) in an NADP-malic enzyme-type C4 plant such as maize (see text for discussion). BS, bundle sheath OA, oxaloacetate. 

Carbon for FAS could also be produced via ATP-citrate lyase. This activity, which will convert citrate to oxaloacetate plus acetyl-CoA, has been demonstrated in soybean extracts. But there is no current evidence for citrate transport into the plastid nor of localisation of this activity in the plctstid to support citrate cleaving enzyme as a source of carbon for FAS, at least for avocado mesoccirp plastids. Extracts of developing soybean also contain a NADP+-dependent medic enzyme.20 NAD+-dependent malic enzyme, which produces pyruvate ind Cctrbon dioxide from malate, is an enzyme specific to the mitochondrial matrix in higher pleints.21 The localisation of NADP+-malic enzyme in immature soybeans, eind the possibility of pyruvate production other than by pyruvate kinase, and the utilisation of this pyruvate in FAS, remain to be determined. 

Malic enzymels), L-malatt-NADP oxidortdm-tase, decarboxylating (EC 1.1.1.40) an important enzyme found in most organisms, which catalyses the decarboxylation of L-malate to pyruvate and CO2, with concomitant reduction of NADP to NADPH (or the synthesis of malate by the reverse reaction) HOOC-CHj-CHOH-COOH + NADF CHj-CO-COOH + CO2 + NADPH + H. M. e. has various metabolic roles 1. Synthesis of malate by the action of M.e. may serve as an Anaplerotic reaction (see) of the TCA-cycle 2. An important route for the total combustion of any TCA-[Pg.380]

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