The two catalytic components of the 2-oxoglutarate dehydrogenase complex in rat cerebral synaptic and nonsynaptic mitochondria: comparison of the response to in vitro treatment with ammonia, hyperammonemia, and hepatic encephalopathy.

The effects of in vitro treatment with ammonium chloride, hepatic encephalopathy (HE) due to thioacetamide (TAA) induced liver failure and chronic hyperammonemia produced by i.p. administration of ammonium acetate on the two components of the multienzyme 2-oxoglutarate dehydrogenase complex (OGDH): 2-oxoglutarate decarboxylase (E1) and lipoamide dehydrogenase (E3), were examined in synaptic and nonsynaptic mitochondria from rat brain. With regard to E1 the response to ammonium ions in vitro (3 mM NH4Cl) was observed in nonsynaptic mitochondria only and was manifested by a 21% decrease of Vmax and a 35% decrease of Km for 2-oxoglutarate (2-OG). By contrast, both in vivo conditions primarily affected the synaptic mitochondrial E1: TAA-induced HE produced an 84% increase of Vmax and a 38% increase of Km for 2-OG. Hyperammonemia elevated Vmax of E1 by 110% and Km for 2-OG by 30%. HE produced no effect at all in nonsynaptic mitochondria while hyperammonemia produced a 35% increase of Vmax and a 30% increase of Km for 2-OG of E1. Both in vivo conditions produced a 20% increase of E3 activity in synaptic mitochondria, but no effect at all in nonsynaptic mitochondria. The preferential sensitivity of E1 to ammonium chloride in vitro in nonsynaptic mitochondria and hyperammonemic conditions in vivo in synaptic mitochondria may play a crucial role in the compartmentation of OGDH responses under analogous conditions. These results confirm the intrinsic differences between the OGDH properties in the synaptic and nonsynaptic brain compartments.