Malic Dehydrogenase from Tamarix Roots

Abstract

Soluble and mitochondrial malic dehydrogenases (MDH) were isolated from root tips of the halophyte Tamarix tetragyna L. grown in the presence and absence of NaCI. The activity of the enzymes isolated from root tips grown in the presence of NaCI was lower than that of the enzymes isolated from roots grown in absence of NaCl. The mitochondrial MDH was much more sensitive to salinity than the soluble MDH. The soluble enzyme from roots grown in NaCl had a higher Km for malate and lower Km for NAD than enzyme from the control roots.Addition of NaCl in vitro at 72 mM significantly stimulated the reductive activity of soluble MDH, while higher NaCl concentrations (240 mM and above) depressed enzyme activity. Another group of organisms showing a very high tolerance to substrate salinity is the group of the halophilic bacteria. The enzymes of these organisms are apparently adapted to function in the presence of high ionic concentrations and are activated by them (1, 2, 6, 13-16, 21, 22). Holmes and Halvorson (13) studied the effect of NaCl on malic dehydrogenase from obligatory halophilic bacteria, facultative halophilic bacteria, and animal tissue. The enyzme from the animal tissue showed maximal activity in the absence of NaCl and the activity decreased with increasing NaCl concentration. The enzyme isolated from obligatory halophilic bacteria, on the other hand, showed the highest activity at the high NaCl concentrations (up to 4 M) and its activity decreased with decreasing NaCl concentrations. Hubbard and Miller (14,15) showed similar to halophilic bacteria, or whether the enzyme was simiIt was of interest, therefore, to examine whether an enzyme of a halophytic plant showed salt requirement for its activity, similar to halophilic bacteria, or whether the enzyme was similar to that of the glycophytes. Malic dehydrogenase (L-malate-NAD oxidoreductase EC 1 .1 .1 . 37) isolated from Tamarix roots was examined. MATERIALS AND METHODSCuttings of Tamarix tetragyna L. were collected from a tree from Mediterranean coastal salt marshes in the Naaman area and were rooted and grown in vermiculite moistened either with half strength Hoagland's solution (12) or with Hoagland's solution containing 0.12 M NaCl (osmotic potential -v = -5 atm). When the cuttings began to root, salinity was stepped up to final salinities of 0.12 M, 0.24 M, 0.36 M, and sometimes 0.48 M NaCl (corresponding to . = -5, -10, -15, and -20 atm, respectively) and higher. Loss of water was compensated daily.Isolation of Mitochondria and Preparation of Enzyme. Root tips, 2 cm long, were collected and blotted with filter paper. Two g of root were ground with sand and 0.5 g of Polyclar AT in 0.1 M tris-HCI buffer (pH 7.4) containing 0.5 M sucrose, 5 mM EDTA, and 1 mm dithioerythritol (buffer I in Fig. 1) and treated as outlined in Figure 1.For kinetic studies, the soluble and the solubilized enzymes were partially purified by precipitation with solid (NH4)2SO,.

Responses of amino acid metabolizing enzymes from plants differing in salt tolerance to NaCl

“…Substrate inhibition is removable by NaC1 and, on the other hand, inhibition by NaC1 at low substrate concentrations can be removed by increasing substrate concentrations. Similar results have been obtained by Sims (1974), von Willert (1974), and Kalir and Poljakoff-Mayber (1975) for malate dehydrogenase from halophytes. NaC1 pretreatment of the plants significantly reduces the effectiveness of such a control of GDH in the less salt-tolerant A triplex species, but has a stimulating effect on GDH of the halophyte A. halimus.…”

“…Such conformational transitions induced by changes in the environmental conditions have been observed with GDH from plants (Ashby et al, 1974;Neumann et al, 1976;Pahlich et al, 1978). In agreement with Wilson and Evans (1968), Greenway and Sims (1974), Kalir and Poljakoff-Mayber (1975), and Greenway and Setter (1977), enzymes from halophytes seem to be adapted to their environmental conditions by structural flexibility rather than by an enhanced de novo synthesis of enzyme proteins and/or changes in the aggregation of subunits. The present results may help to explain not only the wide salt tolerance of A. halimus, but also the requirement of Atriplex species for sodium (Brownell, 1965), especially of those operating with the C4-pathway (Brownell and Crossland, 1972).…”

Effect of Salinity on Respiratory Pathways in Root Tips of Tamarix tetragyna

“…240 360 4k The effect of NaCI salinity on the activity of malic dehydron growth Medium genase and glucose-6-P dehydrogenase were studied. Although the appearance of C 6 from glucose in CO2 was not affected by the substrate salinity, the specific radioactivity of the mitochondrial malic dehydrogenase decreased with increasing salinity in the substrate (4). The activity of glucose-6-P dehydrogenase and that of pyruvate kinase (Fig.…”

“…There are no data available yet about the reversibility of these changes, nor are there any data about the nature of the adaptations enabling the roots to grow in -10 atm NaCI. The study on the properties of malic dehydrogenase (4) suggests that the adaptation is not on the molecular level but at a higher level of organization. Also, there is no information available about the relationships between this ability to endure salinity and the ability of Tamarix to transport salt through the system and secrete it from the salt glands in the leaf.…”

Influence of Arsenate and Phosphate on the Regulation of Growth and TCA Cycle in the Rice (<i>Oryza sativa</i> L.) Cultivars IR64 and Nayanmani

“…The inhibitory effect of arsenic stress on the enzyme is through conformational changes [44] but increased concentration of the substrate i.e. malate counteracted the damaging effects caused by arsenic and thus may have a protective role [45] under arsenate treatment. Malate is known to regulate the pH and limit enzymatic browning by chelating and hindering the phenol enzyme activity.…”