The physiological role of the isoenzymes of lactate dehydrogenase in potatoes

Involvement of osmoregulation, glyoxalase, and non-glyoxalase systems in signaling molecule glutamic acid-boosted thermotolerance in maize seedlings

Glutamic acid (Glu) is not only an important protein building block, but also a signaling molecule in plants. However, the Glu-boosted thermotolerance and its underlying mechanisms in plants still remain unclear. In this study, the maize seedlings were irrigated with Glu solution prior to exposure to heat stress (HS), the seedlings' thermotolerance as well as osmoregulation, glyoxalase, and non-glyoxalase systems were evaluated. The results manifested that the seedling survival and tissue vitality after HS were boosted by Glu, while membrane damage was reduced in comparison with the control seedlings without Glu treatment, indicating Glu boosted the thermotolerance of maize seedlings. Additionally, root-irrigation with Glu increased its endogenous level, reinforced osmoregulation system (i.e., an increase in the levels of proline, glycine betaine, trehalose, and total soluble sugar, as well as the activities of pyrroline-5-carboxylate synthase, betaine dehydrogenase, and trehalose-5-phosphate phosphatase) in maize seedlings under non-HS and HS conditions compared with the control. Also, Glu treatment heightened endogenous methylglyoxal level and the activities of glyoxalase system (glyoxalase I, glyoxalase II, and glyoxalase III) and non-glyoxalase system (methylglyoxal reductase, lactate dehydrogenase, aldo-ketoreductase, and alkenal/alkenone reductase) in maize seedlings under non-HS and HS conditions as compared to the control. These data hint that osmoregulation, glyoxalase, and non-glyoxalase systems are involved in signaling molecule Glu-boosted thermotolerance of maize seedlings.

Evaluation of the role of lactate dehydrogenase in oxalate synthesis

The kinetic properties of lactate dehydrogenase (LDH) (EC 1.1.1.27) from spinach leaves were studied in order to evaluate the possible roles of LDH in the production of oxalate. LDH was purified by affinity chromatography on affigel blue and oxamate agarose columns. The pH optimum for the reduction of pyruvate was 7.25, while that for the reduction of glyoxylate was 7. The rate of reduction of pyruvate and glyoxylate at the optimum pH indicated substrate inhibition at concentrations above 8 and 40 mM, respectively. The maximum activity of LDH with pyruvate was about three times greater than with glyoxylate. The pH optimum for the oxidation of lactate was 9, with very low activity below pH 8. Substrate inhibition was apparent at lactate concentrations above 10 mM. LDH was inactive with glyoxylate in the oxidative reaction, which would lead to the biosynthesis of oxalate.

Soybean L-(+)-lactate dehydrogenases: purification, characterization, and resolution of subunit structure

Lactate dehydrogenase (LDH) (EC 1.1.1.27) from soybean (Glycine max) was purified 2360-fold to homogeneity using ion-exchange, hydroxyapatite, affinity, and hydrophobic chromatographies. The molecular weight of the holoenzyme is 150,000 +/- 5000. Two-dimensional (isoelectrofocusing-sodium dodecyl sulfate) gel electrophoresis reveals two polypeptides subunits of 5.9 and 6.5 pI and of 36,000 +/- 1000 and 37,000 +/- 1000 Mr, respectively. Nondissociating electrophoresis and isoelectric focusing of LDH resolved five tetrameric isoenzymes with pI's between 6.0 and 6.5. The data suggest that these LDH isoenzymes are derived from random association of the products of two different, but most probably related, genes. Kinetic measurements revealed substrate inhibition at high concentrations of lactic acid and biphasic kinetics with NAD.