Glutamic dehydrogenase activity in rat heart: demonstration of two forms of enzyme activity

Glutamic dehydrogenase (GDH) activity in rat heart was found to be 2.1 U/g of heart (wet wt). The mitochondrial glutamic dehydrogenase activity accounted for only 18% of the total. This percentage of the total activity in heart mitochondria was not altered by nagarse treatment, acetone extraction, sonication in Triton X-100, and extraction with buffer containing a protease inhibitor. The remainder of the activity was present in the cytosol. Cytosolic GDH activity differed from mitochondrial GDH activity by its pH curve, stability to heat, Arrhenius plot, and the effect of different nucleotides. Acetone extraction of the mitochondria resulted in GDH that was stable to heat and had a shallow temperature activation curve resembling cytosolic GDH. Acetone extraction of cytosolic GDH inactivated it. The cytosolic activity was purified 288-fold and the mitochondrial activity 100-fold. Purified cytosolic and mitochondrial GDH enzymes had different monomeric molecular weights on sucrose density gradient centrifugation. Gel filtration of cytosolic and mitochondrial GDH also showed different monomeric molecular weights. We conclude that rat heart GDH exists in two forms with different physical and kinetic characteristics. The majority of GDH activity in rat heart is cytosolic. The mitochondrial enzyme has a lipid-soluble component that can be removed with acetone without destroying its activity.