Plasmalemma-associated malate dehydrogenase activity in onion root cells

Plasma membrane vesicles isolated from onion roots showed oxaloacetate redu ctase activity as well as other oxidoreductase activities. Purification and further sequencing showed that the protein responsible for the activity is a 40 kDa protein which corresponds to the cytosolic soluble malate dehydro genase. However, the activity remained bound to the membrane after repeated freezing and thawing cycles and further washing, excluding a cytosolic con tamination as the source of the activity. Furthermore, a second 28 kDa prot ein has been copurified together with the 40 kDa protein. The plasmalemma o xaloacetate reductase activity shows both donor and acceptor sites located towards the cytroplasmic side of the plasma membrane. This enzyme catalyzed the oxidation of NADH by oxaloacetate and the reduction of NAD(+) by malat e in the presence of an oxaloacetate-withdrawing system. We conclude that a significant amount of the cytosolic malate dehydrogenase can be specifical ly attached to the cyrosolic face of the plasmalemma. A possible role in a putative malate shuttle associated to the plasma membrane is discussed.