The C-4 pathway in Pennisetum purpureum : II. Malate dehydrogenase and malic enzyme.

The enzymes NAD-specific malate dehydrogenase (E.C.1.1.1.37) and malic enzyme (E.C. 1.1.1.40) have been partially purified from leaves of P. purpureum and their kinetic properties examined. With malate dehydrogenase, assayed in the direction of malate synthesis, Lineweaver-Burk plots with either oxaloacetate or NADH as variable substrate were parallel to one another, indicating an ordered reaction mechanism of the ping-pong type. Studies of end-product inhibition suggested that the mechanism was iso-ping-pong-bi-bi. Michaelis constants were about 5×10(-5) M for NADH and 3×10(-4) M for oxaloacetate. With the malic enzyme Michaelis constants were about 2×10(-6) M for NADP(+), 4×10(-4) M for malate and 8×10(-6) M for Mn(2+). Results are discussed in terms of the roles of these enzymes in the metabolism of carbon through the C-4 pathway of photosynthesis.