| Abstract |
Cobalamin must be metabolized intracellularly in order to bind two
enzymes: methionine synthase in cytoplasm and methylmalonyl-CoA mutase in
mitochondria. Defects in this process cause different inborn errors of
cobalamin metabolism (cblA-cblG and mut). A previous study described a
cobalamin-binding protein, in addition to methylmalonyl-CoA mutase, in
crude mitochondrial fractions. The amount of [57Co]cobalamin bound to this
protein was increased in cblB, mut and cblD variant cell lines, compared
to control cell lines. In the present study, this protein was identified
as transcobalamin (TC). Mitochondrial fractions from a cblB cell line were
incubated with anti-TC antibodies, which precipitated the cobalamin-bound
protein. Analysis of mitochondrial and cytoplasmic fractions isolated from
a chloroquine-incubated cblF cell line showed that isolated mitochondrial
fractions contain lysosomal material, suggesting that the identified TC is
lysosomal. Quantification of cobalamin-bound TC levels in whole cell
extracts showed significant increases in cblB and mut groups compared to
control cell lines.
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