The most active NADH shuttle, which functions in liver, kidney, and heart mitochondria, is the malate-aspartate shuttle. The cytosolic reducing equivalents of NADH are first transferred to the cytosolic oxaloacetate to yield malate, and this is catalysed by cytosolic malate dehydrogenase. The malate thus formed passes through the inner membrane via the malate–ketoglutarate transporter. Within the matrix, the reducing equivalents are passed to NAD by the action of matrix malate dehydrogenase, forming NADH; this NADH can pass electrons directly to the respiratory chain. Cytosolic oxaloacetate must be regenerated by transamination reactions and the activity of membrane transporters to start another cycle of the shuttle.