Characterization of the kinetics of cardiac cytosolic malate dehydrogenase and comparative analysis of cytosolic and mitochondrial isoforms.

Because the mitochondrial inner membrane is impermeable to pyridine nucleotides, transport of reducing equivalents between the mitochondrial matrix and the cytoplasm relies on shuttle mechanisms, including the malate-aspartate shuttle and the glycerol-3-phosphate shuttle. These shuttles are needed for reducing equivalents generated by metabolic reactions in the cytosol to be oxidized via aerobic metabolism. Two isoenzymes of malate dehydrogenase (MDH) operate as components of the malate-aspartate shuttle, in which a reducing equivalent is transported via malate, which when oxidized to oxaloacetate, transfers an electron pair to reduce NAD to NADH.

Determination of the catalytic mechanism for mitochondrial malate dehydrogenase.

The kinetics of malate dehydrogenase (MDH) catalyzed oxidation/reduction of L-malate/oxaloacetate is pH-dependent due to the proton generated/taken up during the reaction. Previous kinetic studies on the mitochondrial MDH did not yield a consensus kinetic model that explains both substrate and pH dependency of the initial velocity. In this study, we propose, to our knowledge, a new kinetic mechanism to explain kinetic data acquired over a range of pH and substrate concentrations.

An evaluation of muscle maintenance costs during fiber hypertrophy in the lobster Homarus americanus: are larger muscle fibers cheaper to maintain?

Large muscle fiber size imposes constraints on muscle function while imparting no obvious advantages, making it difficult to explain why muscle fibers are among the largest cell type. Johnston and colleagues proposed the 'optimal fiber size' hypothesis, which states that some fish have large fibers that balance the need for short diffusion distances against metabolic cost savings associated with large fibers. We tested this hypothesis in hypertrophically growing fibers in the lobster Homarus americanus.