Human malic enzymes in heart and muscle: evidence of a selective distribution

Methylglyoxal-induced modification of arginine residues decreases the activity of NADPH-generating enzymes

Inadequate control of plasma and cellular glucose and ketone levels in diabetes is associated with increased generation of reactive aldehydes, including methylglyoxal (MGO). These aldehydes react with protein side chains to form advanced glycation end-products (AGEs). Arg residues are particularly susceptible to MGO glycation and are essential for binding NADP(+) in several enzymes that generate NADPH, a coenzyme for many critical metabolic and antioxidant enzymes. In most animal cells, NADPH is produced predominantly by glucose-6-phosphate dehydrogenase (G6PD) in the oxidative phase of the pentose phosphate pathway and, to a lesser extent, by isocitrate dehydrogenase (IDH) and malic enzyme (ME). In this study, the activities of isolated G6PD, IDH, and ME were inhibited by MGO (0-2.5mM, 2-3h, 37°C), in a dose- and time-dependent manner, with G6PD and IDH more sensitive to modification than ME. Significant inhibition of these two enzymes occurred with MGO levels ≥500μM. Incubation with radiolabeled MGO (0-500µM, 0-3h, 37°C) demonstrated dose- and time-dependent adduction to G6PD and IDH. HPLC analysis provided evidence for AGE formation and particularly the hydroimidazolones MG-H1 and MG-H2 from Arg residues, with corresponding loss of parent Arg residues. Peptide mass mapping studies confirmed hydroimidazolone formation on multiple peptides in G6PD and IDH, including those critical for NADP(+) binding, and substrate binding, in the case of IDH. These results suggest that modification of NADPH-producing enzymes by reactive aldehydes may result in alterations to the cellular redox environment, potentially predisposing cells to further damage by oxidants and reactive aldehydes.

Molecular cloning and characterization of the sheep malic enzyme cDNA

Malic enzyme catalyzes decarboxylation of malate to pyruvate and CO(2), providing de novo biosynthesis of fatty acids with NADPH. Since lipogenesis in ruminants, contrarily to some monogastric species like human and rodents, occurs predominantly in adipose tissue, the activity of many lipogenic enzymes is higher in adipose tissue compared to liver. Expression of malic enzyme is regulated by nutrition; refeeding after a period of starvation results to an induction of the enzyme. Here we present the nucleotide sequence of two transcripts of the ovine cytosolic malic enzyme gene that differ at the length of the 3' UTR. These are the first published cDNA sequences for ruminant species and share high similarity with the corresponding sequences of other species. Malic enzyme mRNA was present in every ovine tissue that was examined. In agreement with the fact that adipose tissue is the major lipogenic site for ruminants, mRNA levels in adipose tissue were higher than in liver. Refeeding after two weeks of caloric restriction resulted in a two-fold increase of the mRNA level of malic enzyme in adipose tissue.

Characterization of cytosolic malic enzyme in human tumor cells

Cytosolic NADP(+)-dependent malic enzyme (ME) from human tumor cells was characterized in detail and compared to ME from normal human tissues produced in recombinant E. coli. Kinetic properties, size as seen in SDS gels, and HPLC elution profiles of tryptic digests of human 'normal cell' ME and NADP(+)-ME from tumor cells were identical. Thus, NADP(+)-ME found in tumor cells does not constitute a tumor-specific isoform as suggested by other studies but is identical to the 'housekeeping protein' predominantly expressed in human liver and white adipose tissue.