Changes in NADP+-linked isocitrate dehydrogenase during tomato fruit ripening

Abstract

The activaty of NADP+-specific isocitrate dehydrogenase (NADP +-IDH, EC 1.1.1.42) was investigated during the ripening of tomato (Lycopersicon esculenturn Mill.) fruit. In the breaker stage, NADP +-IDH activity declined but a substantial recovery was observed in the late ripening stages when most lycopene synthesis occurs. These changes resulted in higher NADP+-IDH activity and specific polypeptide abundance in ripe than in green fruit pericarp. Most of the enzyme corresponded to the predominant cytosolic isoform which was purified from both green and ripe fruits. Fruit NADP+-IDH seems to be a dimeric enzyme having a subunit size of 48 kDa. The K m values of the enzymes from green and ripe pericarp for NADP +, isocitrate and Mg 2+ were not significantly different. The similar molecular and kinetic properties and chromatographic behaviour of the enzymes from the two kinds of tissue strongly suggest that the ripening process is not accompanied by a change in isoenzyme complement. The increase in NADP+-IDH in the late stage of ripening also suggests that this enzyme is involved in the metabolism of C6 organic acids and in glutamate accumulation in ripe tissues.

Characterization of a Stress-induced NADP-isocitrate Dehydrogenase Gene in Maize Confers Salt Tolerance in Arabidopsis

“…Nicotinamide adenine dinucleotide phosphate-dependent isocitrate dehydrogenase (ICDH; EC 1.1.1.42) catalyzes the reversible conversion of isocitrate to 2-oxoglutarate (2-OG) and links C and N metabolism (Hurley et al 1991;Gallardo et al 1995). ICDH activity is mainly localized in the cytosol, although minor activity has been found in mitochondria, peroxisomes, and chloroplasts in higher plants (Henson et al 1986).…”

“…Since ICDH is a source of 2-OG for amino acid synthesis (Chen and Gadal 1990), NADP-ICDH has a role in the supply of NADPH to the cytosol (Gallardo et al 1995;Fieuw et al 1995). Because ICDH requires either NAD + -or NADP + -producing NADH and NADPH, several authors have suggested that ICDH is one of the most important NADPH generating mechanisms.…”

Metabolic analysis of guava (Psidium guajava L.) fruits at different ripening stages using different data-processing approaches

“…In the TCA cycle, citrate is sequentially metabolized to isocitrate, 2-oxoglutarate and glutamate. Glutamate is catabolized through GABA [33]. Therefore, as would be expected, citrate in the current study was decreased while GABA was increased in guava fruit during ripening.…”

Plant NADP-dependent isocitrate dehydrogenases are predominantly localized in the cytosol

“…There has been increasing evidence to support the operation of the pathway in vivo. First, the oxidation of malate or oxaloacetate in the mitochondria of illuminated leaves produces mainly citrate but not a-KG (Hanning and Heldt 1993); second, cytosolic aconitase has been found in several plant species (Courtois-Verniquet and Douce 1993;De Bellis et al 1995); third, an increase in the cytosolic NADP-IDH activity and its gene expression has been observed wherever there is a high demand for a- KG (Fiew et al 1995;Gallardo et al 1995;Scheible et al 1997). The ubiquitous presence of cytosolic NADP-IDH in higher plants determined by this study strongly supports the hypothesis that the cytosol is a major site for production of a-KG used in nitrogen assimilation.…”