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Abstract:
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In eukaryotes , folate -dependent one -carbon (1 -C ) metabolism is composed of two parallel pathways compartmentalized to either the cytoplasm or mitochondria . In each , 1 -C units , carried on tetrahydrofolate (THF ) , are interconverted by four catalytic activities . Serine hydroxymethyltransferase transfers the 3 -carbon of serine to THF forming 5 ,10 -methylene -THF which is oxidized in 3 successive steps to formate via the intermediates , 5 ,10 -methenyl -THF and 10 -formyl -THF . Because of the redox potential in each compartment , 1 -C flux is thought by most authors to be from formate to serine in the cytosol and in the opposite direction in mitochondria . Transport of serine , glycine and formate across the mitochondrial membranes creates a 1 -C cycle . All eukaryotes characterized to date contain a cytoplasmic trifunctional C1 -THF synthase possessing 5 ,10 -methylene -THF dehydrogenase , 5 ,10 -methenyl -THF cyclohydrolase and 10 -formyl -THF synthetase activities which interconvert the catalytic intermediates between 5 ,10 -methylene -THF and formate . However , despite the observation that adult rat liver mitochondria oxidize serine to formate , no known enzymatic activities correlate with those of cytoplasmic C1 -THF synthase . In embryos , a bifunctional protein , containing 5 ,10 -methylene -THF dehydrogenase and 5 ,10 -methenyl -THF cyclohydrolase , accounts for two of these activities . But the 10 -formyl -THF synthetase activity has no associated enzyme in mitochondria . Reported here is the discovery of a monofunctional homolog of C1 -THF synthase in mammalian mitochondria . Characterization of the protein confirms mitochondrial localization and 10 -formyl -THF synthetase activity . Likewise , the adult human transcript is present and differs in size and tissue distribution from cytosolic C1 -THF synthase . In mouse embryos , the temporal expression of the mRNA starts out relatively low and increases as the embryos age . The spatial distribution of the transcript is ubiquitous but with areas of elevated expression corresponding to proliferative regions within the embryo . The temporal expression pattern of the protein and transcript correspond well . However , mitochondrial flux studies and immunoblotting data suggest that mitochondrial C1 -THF synthase is not the rate -limiting enzyme in mitochondria , at least during the mid to later stages of embryogenesis . Additionally , studies modulating the expression of mitochondria 1 -C proteins demonstrate the likelihood that most cytoplasmic 1 -C units are mitochondrially derived . |