Affinity chromatography on 2',5'-ADP-Sepharose 4B for purification of malic enzyme from crustacean muscle

Role for malic enzyme, pyruvate carboxylation, and mitochondrial malate import in glucose-stimulated insulin secretion

Pyruvate cycling has been implicated in glucose-stimulated insulin secretion (GSIS) from pancreatic beta-cells. The operation of some pyruvate cycling pathways is proposed to necessitate malate export from the mitochondria and NADP(+)-dependent decarboxylation of malate to pyruvate by cytosolic malic enzyme (ME1). Evidence in favor of and against a role of ME1 in GSIS has been presented by others using small interfering RNA-mediated suppression of ME1. ME1 was also proposed to account for methyl succinate-stimulated insulin secretion (MSSIS), which has been hypothesized to occur via succinate entry into the mitochondria in exchange for malate and subsequent malate conversion to pyruvate. In contrast to rat, mouse beta-cells lack ME1 activity, which was suggested to explain their lack of MSSIS. However, this hypothesis was not tested. In this report, we demonstrate that although adenoviral-mediated overexpression of ME1 greatly augments GSIS in rat insulinoma INS-1 832/13 cells, it does not restore MSSIS, nor does it significantly affect GSIS in mouse islets. The increase in GSIS following ME1 overexpression in INS-1 832/13 cells did not alter the ATP-to-ADP ratio but was accompanied by increases in malate and citrate levels. Increased malate and citrate levels were also observed after INS-1 832/13 cells were treated with the malate-permeable analog dimethyl malate. These data suggest that although ME1 overexpression augments anaplerosis and GSIS in INS-1 832/13 cells, it is not likely involved in MSSIS and GSIS in pancreatic islets.

Purification of tumor mitochondrial malic enzyme by specific ligand affinity chromatography

A two-step chromatographic procedure, based on a specific ligand-binding approach, for the purification of tumor NAD(P)(+)-dependent malic enzyme is described. The enzyme was purified to near homogeneity by extraction from mitochondria, negative cellulose phosphate chromatography, ammonium sulfate precipitation, and application of specific elution from a malate-agarose column. The rationale for the use of the affinity column is also described.

Mitochondrial malic enzyme from crustacean and fish muscle

1. In contrast to mammalian skeletal muscle mitochondria, the only substrate that crustacean and fish mitochondria oxidize at a high rate is malate. 2. The mitochondria isolated from muscles of fish and crayfish exhibit a high activity of malic enzyme. 3. Assuming that malic enzyme is responsible for the conversion of malate to pyruvate in animal muscle, it could be expected that the mitochondria which possess high activity of this enzyme should oxidize malate very rapidly when oxygen is available. 4. Some properties of different molecular forms of malic enzyme are reviewed.