Cytoplasmic and mytochondrial malicdehydrogenases of chicken liver

Abstract

Cytoplasmic and Mytochondrial Malicdehydro- genases of Chicken Liver. Rev. esp. Fisiol., 30, 241-248. 1974.

Chicken liver cytoplasmic (MDH-s) and mytocondrial (MDH-m) malicdehydro- genases appear to be in the forms of the four and of the three possible isoenzymes, respectively. The optimum pH for MDH-s lies between 6.9-7.5 and that of MDH-m between 7.3-7.7; the activity of the first mentioned enzyme considerably diminishes with alkaline pH, whereas the activity of the second one decreases in the acid zone.

The optimum temperature of both MDHs is 50° C; at higher temperatures, MDH-m becomes almost totally inactive whilst MDH-s partially retains its activity.

MDH-s shows two Km (O.A.A.) valúes ([NADH] = 1.4 X 10-4 M) that are of the order of 10“fi M and 10-4 M respectively, and in both cases, the general mechanism of the catalysis is sequential.

MDH-m does not show this particular feature, and its action seems to be adjusted to a ping-pong mechanism, thus diferring from mytochondrial enzymes from other sources. The real parameters of the enzyme systems calcúlales by using the Vestling-Flo- rini and Dalziel methods are as follows: MDH-s: Km (O.A.A.) = 2.45 X 10“’ M;

Km (NADH) = 1.3 X 10“’ M; Koaa-nadh = 0.93 X 10“’ M! when the O.A.A. are smaller than 7 X 10-s M, whereas when the O.A.A. are higher than 7 X 10“’ M, the valúes the obtained are: Km (O.A.A.) = 3.15 X 10“4 M; K,”ž (NADH) = 1.18 X 10“’ M and Koaa-nadh = 1-21 X 10“8 M2.

The real parameters of the MDH-m are: Km (O.A.A.) = 5.4 X 10“’ M; Km (NADH) = 6.04 X 10“’ M and Koaa-nadh = 0. Oxalacetate inhibits MDH-m non-competitively by excess of substrate (when [O.A.A.] is greater than 1.2 X 10-4 M), and shows to be an acompetitive inhibitor of the MDH-s (when [O.A.A.] higher the 1.2X10“’ M), determined, in both cases, with respect to NADH.