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1. The effects produced by a rapid freezing and thawing on the activities of some enzymes of nuclear (nicotinamide adenine dinucleotide (NAD)-pyrophosphorylase) and mitochondrial (succinoxidase, succinic dehydrogenase, and glutamic dehydrogenase) origin are presented. These effects are compared with those produced by sonic treatment of fresh tissue.    2. The activity of NAD-pyrophosphorylase was unaffected by the freezing procedures employed, namely dry ice at –79°C, isopentane at –155°C, and propane at –175°C, or by sonic treatment of homogenates of fresh tissue.    3. Succinoxidase activity was decreased to some extent by all the procedures employed. Sonic treatment produced a 47% loss in the enzyme activity, while freezing on dry ice, in isopentane, or in propane produced losses of 26, 29, and 37%, respectively.    4. The activity of succinic dehydrogenase was least affected by sonic treatment or by freezing on dry ice. Quenching in isopentane or in propane, however, produced 11% and 23% loss respectively in the dehydrogenase activity.    5. Free glutamic dehydrogenase activity, which is low in fresh-tissue preparations, was increased to the same extent by all methods employed. The unsedimentable activity depended upon the procedure used, however. Sonic treatment and freezing with propane, isopentane, or dry ice gave 78, 48, 33, and 20%, respectively, of the total enzyme activity.    6. The fact that the increase in unsedimentable glutamic dehydrogenase activity roughly parallels a decrease in succinoxidase activity suggests that a rapid freezing followed by thawing produces alterations in the mitochondrial membranes.

EFFECT OF FREEZING AND THAWING ON CERTAIN NUCLEAR AND MITOCHONDRIAL ENZYMES OF MOUSE LIVER