PURIFICATION AND PROPERTIES OF 2 MALIC ENZYME-ACTIVITIES IN LIVER-MITOCHONDRIA OF SQUALUS-ACANTHIAS (SPINY DOGFISH)

Two malic enzymes are present in liver mitochondria from Squalus acant hias (spiny dogfish), an NAD(P)-dependent and an NADP-dependent malic enzyme. The NAD(P)-dependent enzyme was partially purified and found t o have kinetic properties similar to other malic enzymes; the molecula r weight is approximate to 130,000. The maximal rate with NADP is 40% of the maximal rate with NAD. The NADP-dependent malic enzyme was puri fied to homogeneity and also has properties similar to other malic enz ymes, including a subunit molecular weight of approximate to 61,000 an d a native molecular weight of approximate to 250,000; however, the ra te as a function of malate concentration is described by a sigmoid cur ve. Both enzymes require either Mn2+ or Mg2+ for activity, are activat ed by fumarate or succinate, and are inhibited by ATP. Both enzymes ar e inhibited by low concentrations of oxalacetate; the K-i for competit ive inhibition of the NAD(P)-dependent malic enzyme by oxalacetate is 4 mu M. The K-m for Mn2+ is considerably lower than the K-m for Mg2+ f or both enzymes; the maximal rate with Mg2+ is about the same as with Mn2+ for the NADP-dependent enzyme, but only 20% of that with Mn2+ for the NAD(P)-dependent enzyme. The possible role of these enzymes in co ntributing to citrate synthesis by isolated spiny dogfish liver mitoch ondria is discussed. NAD- or NADP-dependent malic enzyme activity coul d not be detected in the cytosol. (C) 1995 Wiley-Liss, Inc.