In vitro evidence for the involvement of mitochondrial folate metabolism in the supply of cytoplasmic one-carbon units.

We present in vitro evidence for a novel intercompartmental pathway in which folate-mediated reactions in mitochondria generate one-carbon units for utilization in cytoplasmic processes. Rat liver mitochondria are shown to contain the enzymatic activities for catabolism of serine or sarcosine to produce formate. Intact mitochondria rapidly convert the 3-carbon of serine or the N-methyl group of sarcosine to formate, which exits the mitochondria. Labeled formate is incorporated into purine by a cytoplasmic purine synthesizing system only after activation to 10-formyl-THF via the ATP-dependent 10-formyl-THF synthetase reaction. In a coupled system where one-carbon donors are catabolized by mitochondria before addition to the cytoplasmic purine synthesizing system, incorporation into purine shows a marked dependence on ATP. These observations demonstrate that mitochondria can metabolize one-carbon donors via THF-dependent reactions to the level of formate which then exits mitochondria for utilization in the cytoplasm. The proposed pathway is discussed in relation to genetic evidence for its operation in vivo as well as compartmentation of folate coenzymes and their one-carbon units.