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9.3 Alternative sources of NADPH

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Not all cells that require large amounts of NADPH have a high activity of the hexose monophosphate shunt. Some alternative mechanisms that can provide NADPH are:

1. The combination of pyruvate carboxylase, malate dehydrogenase, and malic enzyme, which generates NADPH from NADH at the expense of ATP (see section 12.1). I have not run across evidence that this mechanism is indeed quantitatively important.
2. NADP⁺-dependendent isocitrate dehydrogenase, which occurs alongside the NAD⁺-dependent enzyme in mitochondria but also in the cytosol (see section 5.2.1). This mechanism seems to be particularly important for fatty acid synthesis in the liver and the fat tissue (see below).

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