Hydrogen shuttles in air versus water breathing fishes

1. The malate-aspartate cycle was demonstrable in subcellular preparations of hearts from Arapaima, Lepidosiren, and Synbranchus (obligate air breathers), Hoplerythriunus (facultative air breather), and Osteoglossum and Hoplias (obligate water breathers). 2. Although no respiratory evidence for significant alpha-glycerophosphate cycle participation could be shown in the air breathers, this cycle was demonstrable in hearts of water breathers. 3. In agreement with the O2 uptake studies, it was possible to reconstruct the malate-aspartate, but not the alpha-glycerophosphate cycle, in isolated mitochondria from air breathers, while both shuttles could be reconstructed with heart mitochondria in the case of water breathing fishes.