Malic enzyme in human liver

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Abstract
In human liver, almost 90% of malic enzyme activity is located within the extramitochondrial compartment, and only approximately 10% in the mitochondrial fraction.Extramitochondrial malic enzyme has been isolated from the post‐mitochondrial supernatant of human liver by (NH4)2SO4 fractionation, chromatography of DEAE‐cellulose, ADP–Sepharose‐4B and Sephacryl S‐300 to apparent homogenity, as judged from polyacrylamide gel electrophoresis. The specific activity of the purified enzyme was 56 μmol · min−1· mg protein−1, which corresponds to about 10000‐fold purification.The molecular mass of the native enzyme determined by gel filtration is 251 kDa. SDS/polyacrylamide gel electrophoresis showed one polypeptide band of molecular mass 63 kDa. Thus, it appears that the native protein is tetramer composed of identical‐molecular‐mass subunits.The isoelectric point of the isolated enzyme was 5.65.The enzyme was shown to carboxylate pyruvate with at least the same rate as the forward reaction.The optimum pH for the carboxylation reaction was at pH 7.25 and that for the NADP‐linked decarboxylation reaction varied with malate concentration.TheKm values determined at pH 7.2 for malate and NADP were 120 μM and 9.2 μM, respectively. TheKm values for pyruvate, NADPH and bicarbonate were 5.9 mM, 5.3 μM and 27.9 mM, respectively.The enzyme converted malate to pyruvate (at optimum pH 6.4) in the presence of 10 mM NAD at approximately 40% of the maximum rate with NADP. TheKm values for malate and NAD were 0.96 mM and 4.6 mM, respectively. NAD‐dependent decarboxylation reaction was not reversible.The purified human liver malic enzyme catalyzed decarboxylation of oxaloacetete and NADPH‐linked reduction of pyruvate at about 1.3% and 5.4% of the maximum rate of NADP‐linked oxidative decarboxylation of malate, respectively.The results indicate that malic enzyme from human liver exhibits similar properties to the enzyme from animal liver.