Abstract
1. 1. Intracellular localization studies show that in turtle ( Pseudemys scripta elegans) liver and heart, 99 and 85 per cent of the pyruvate kinase (PK) and malate dehydrogenase (MDH) activities, respectively, are extramitochondrial (24,500 g supernate), while 71 per cent of the liver and 93 per cent of the heart phosphoenolpyruvate carboxykinase (PEPCK) activities are extramitochondrial. 2. 2. Of the three enzymes assayed, PK shows the highest activity in both liver and heart; PEPCK shows the lowest activity in both organs, While MDH is intermediate in activity. 3. 3. PK and MDH are only one-fourth as active in liver as in heart, while PEPCK is over twice as active in liver as in either heart or skeletal muscle. 4. 4. The apparent K m( PEP) values for liver (81 μM) and heart (48 μM) PEPCK are smaller than K m( PEP) values for liver (2141 μM) and heart (172 μM) PK. 5. 5. In the presence of 100 μM fructose-1,6-diphosphate the K m( PEP) of liver PK decreases to 214 μM.
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