Characterization of the folate-dependent mitochondrial oxidation of carbon 3 of serine.

The folate-dependent catabolism of serine was studied in intact rat liver mitochondria and soluble extracts from sonicated mitochondria. Formate and CO2 are both known to be products of the mitochondrial oxidation of carbon 3 of serine. The present work tests the proposal [Barlowe, C. K., & Appling, D. R. (1988) Biofactors 1, 171-176] that carbon 3 of serine is first oxidized to 10-formyltetrahydrofolate, which can be either oxidized to CO2 or converted to formate. Oxidation of carbon 3 of serine to formate and CO2 was shown to be dependent on the respiratory state of the mitochondria. Formate production was greatest in state-3 (actively respiring) mitochondria and lowest in uncoupled mitochondria. In contrast, CO2 production was greatest in uncoupled mitochondria and lowest in respiratory-inhibited mitochondria. Formate production appeared to be favored when high concentrations of NADP+ and ADP were present, but there was no clear correlation between the NADP+:NADPH redox state and CO2 production. In soluble mitochondrial extracts, CO2 production depended on NADP+ and tetrahydrofolate, whereas formate production required ADP in addition to NADP+ and the reduced folate cofactor. Unlike CO2 production, however, formate production showed a complete dependence on a polyglutamylated form of the folate cofactor. These experiments support the proposed folate-mediated serine oxidation as a major pathway for the flux of one-carbon units through mitochondria.