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Genetic defects of folate and cobalamin metabolism.
[homocystinuria without methylmalonic aciduria]
Deficient
activity
of
an
enzyme
can
result
from
a
defect
in
the
conversion
of
the
vitamin
to
a
co
-enzyme
as
well
from
an
abnormal
apo-enzyme
or
disturbed
binding
of
coenzyme
to
enzyme
.
Conversion
of
dietary
vitamin
to
intracellular
active
co
-enzyme
can
be
complex
and
require
many
physiological
and
biochemical
processes
including
stomach
release
of
bound
vitamin
,
intestinal
uptake
,
carriers
/
transport
,
blood
transport
,
cellular
uptake
,
intracellular
release
and
intracellular
compartmentalisation
.
Disorders
of
malabsorption
(
food
cobalamin
malabsorption
,
intrinsic
factor
deficiency
and
abnormal
enterocyte
cobalamin
processing
)
and
transport
proteins
(
transcobalamin
II
deficiency
,
R-
binder
deficiency
)
mostly
lead
to
disturbed
function
of
the
two
cobalamin
requiring
enzymes
,
methylmalonyl
CoA
mutase
and
methionine
synthase
.
Defects
of
early
steps
of
intracellular
cobalamin
(
cblF
,
cbl
C
/
D
)
result
in
marked
deficiencies
of
both
cobalamin
co
-enzymes
and
homocystinuria
combined
with
methylmalonic
aciduria
.
Defective
synthesis
of
adenosyl
cobalamin
in
the
cbl
A
/
B
defects
leads
to
methylmalonyl
CoA
mutase
.
Isolated
methionine
synthase
deficiency
is
also
classified
as
a
cobalamin
disorder
due
to
its
associated
deficient
formation
of
methylcobalamin
.
Folate
disorders
include
methylene-tetrahydrofolate
reductase
deficiency
and
glutamate
formimino-transferase
deficiency
.
In
addition
a
hereditary
disorder
of
intestinal
folate
transport
has
been
described
.
Less
well
established
are
disorders
of
dihydrofolate
reductase
,
methenyl-tetrahydrofolate
cyclohydrolase
,
and
defects
of
cellular
folate
uptake
.
Diseases
Validation
Diseases presenting
"marked deficiencies of both cobalamin co-enzymes"
symptom
homocystinuria without methylmalonic aciduria
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