Preprint: Under Hypoxia, Mouse Cells Transfer Electrons to Valine and Isoleucine Breakdown Products

Preprint: Under Hypoxia, Mouse Cells Transfer Electrons to Valine and Isoleucine Breakdown Products

When oxygen is scarce, mitochondria have more difficulty removing electrons from NADH. Cells must regenerate NAD+ to maintain metabolism. Valine and isoleucine derivatives accept these electrons, and their levels changed in human datasets following resistance exercise, in severe COVID, and with alcohol consumption.

On August 25, the authors posted a preprint describing how cells respond to oxygen deprivation. NADH carries electrons, which mitochondria remove using oxygen. When oxygen is scarce, NADH accumulates, NAD+ becomes less available, and reactions that extract energy from nutrients slow down.

Cells usually transfer excess electrons to pyruvate, producing lactate. Valine and isoleucine are two amino acids in the BCAA group, and cells convert them into keto acids. Under hypoxia, these intermediates accumulate in the cytoplasm, outside the mitochondria. There, LDHA, the enzyme that usually helps produce lactate, transfers electrons from NADH to these compounds. This reaction produces branched-chain hydroxy acids (BCHA) and regenerates NAD+ for metabolism.

“The electrons still need somewhere to go, and through BCAA fermentation, we found another place they can go,” the authors write in an Arc Institute overview.

In mice exposed to 8% oxygen, plasma levels of all three BCHA increased at every timepoint from three hours to three weeks and in all six organs examined. Cultured mouse fat cells produced more isotope-labeled BCHA at 1.5% oxygen than under normal oxygen conditions, showing that the cells generated these compounds themselves. Deleting the Ldha gene weakened the pathway, while blocking mitochondrial respiration enhanced it even under normal oxygen conditions. These experiments link the pathway to excess NADH in the cytoplasm.

In previously published human datasets, all three BCHA increased after resistance exercise. Among patients with severe COVID, the valine derivative was higher when blood oxygen saturation was lower. In another dataset, BCHA levels were associated with self-reported alcohol consumption. The authors therefore consider BCHA potential markers of reductive stress, a state in which NADH accumulates faster than the cell can oxidize it.

The authors tested a separate function in mouse sperm. In these cells, LDHC, a form of the enzyme expressed in the testes, reduces keto acids derived from valine and isoleucine more efficiently. When respiration was blocked, adding these keto acids lowered the NADH/NAD+ ratio. In an isotope-labeling experiment, BCHA accounted for more than half of the labeled products released by the sperm. The same combination increased hyperactivated flagellar motility, which sperm need to penetrate the egg’s outer layer.

In mouse tissues, this pathway leaves a measurable trace of excess NADH. In experiments with mouse sperm, it also helps maintain flagellar movement.