Two malic enzymes are present in liver mitochondria from Squalus acanthias (spiny dogfish), a n NAD(P)-dependent and a n NADP-dependent malic enzyme. The NAD(P)-dependent enzyme was partially purified and found to have kinetic properties similar to other malic enzymes; the molecular weight is = 130,000. The maximal rate with NADP is 40% of the maximal rate with NAD. The NADP-dependent malic enzyme was purified to homogeneity and also has properties similar to other malic enzymes, including a subunit molecular weight of = 61,000 and a native molecular weight of -250,000; however, the rate as a function of malate concentration is described by a sigmoid curve. Both enzymes require either Mn2+ or M P for activity, are activated by fumarate or succinate, and are inhibited by ATP. Both enzymes a r e inhibited by low concentrations of oxalacetate; the Ki for competitive inhibition of the NAD(P)-dependent malic enzyme by oxalacetate is 4 pM. The K,,, for Mn2+ is considerably lower than the K,,, for Mg2' for both enzymes; the maximal rate with Mg2+ is about the same a s with Mn2+ for the NADPdependent enzyme, but only 20% of that with Mn2+ for the NAD(P)-dependent enzyme. The possible role of these enzymes in contributing to citrate synthesis by isolated spiny dogfish liver mitochondria is discussed. NAD-or NADP-dependent malic enzyme activity could not be detected in the cytosol. 0 1995 Wiley-Liss, Inc.
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