Solute Effects on Mitochondrial Respiration: the Kinetics of Proline Oxidation by Mitochondria from the Ventricle of the Marine Clam Mercenaria Mercenaria

-1. The effects of osmolarity on the maximal velocity (Vmax) and apparent Michaelis constant (K~) for the oxidation of proline by both intact and sonicated mitochondria from the ventricle of the marine clam Mercenaria mercenaria have been determined. 2. Compared to the isoosmotic state (1040mOsm/I), the rate of oxidation at any concentration of proline by intact mitochondria is greater in the hypoosmotic state and lower in the hyperosmotic state. 3. Maximal rates of proline oxidation at low osmolarities equal the maximal rates of electron transport over the range 500 to 700 mOsm/l. 4. The response of the apparent Michaelis constant for proline to the osmolarity of the medium may be due to changes in the inner mitochondrial membrane. Such changes were monitored by measuring the rate of NADH oxidation by intact mitochondria. 5. It is suggested that volume changes in mitochondria during the early stages of osmotic stress may be responsible for adjustments in intracellular concentration of certain amino acids observed during volume regulation in marine bivalves.