Can Oxaloacetate cross the outer mitochondrial membrane?

Question

I am aware of the Malate–Aspartate Shuttle, but something is not clear to me and different sources seem to contradict each other. Some show oxaloacetate (OAA) being reduced to malate in the mitochondrial inter-membrane space (IMS), whereas others show the reduction happening in the cytosol. Where does the OAA → malate reduction happen? i.e., can OAA cross the outer mitochondrial membrane (from the cytosol into the IMS) so that it can be reduced in the IMS, or must it be reduced in the cytosol before crossing either of the two mitochondrial membranes?

Answer 1

The outer mitochondrial membrane is permeable to all metabolites including oxaloacetate and NADH but is impermeable to most proteins, they must be specifically imported into the IMS. The cytosolic malate dehydrogenase is found in the cytosol, not the IMS. Hence reduction of oxaloacetate to malate occurs in the cytosol because that is where the enzyme is found, not because oxaloacetate cannot cross the outer mitochondrial membrane.