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Tetrahydrobiopterin and inherited hyperphenylalaninemias.
[classical phenylketonuria]
Tetrahydrobiopterin
deficiency
,
a
variant
of
hyperphenylalaninemia
,
may
be
caused
by
deficiency
of
one
of
the
following
enzymes
:
guanosine
triphosphate
cyclohydrolase
1
,
6
-
pyruvoyltetrahydropterin
synthase
,
dihydropteridin
reductase
and
pterin-
4
a-carbinolamine
dehydratase
.
The
first
two
enzymes
are
involved
in
the
biosynthesis
of
tetrahydrobiopterin
,
the
last
two
in
its
regeneration
.
Although
these
diseases
are
rare
,
early
detection
by
selective
screening
is
essential
for
the
treatment
and
outcome
.
Tetrahydrobiopterin
deficiencies
are
very
heterogenous
ranging
from
mild
forms
requiring
only
marginal
if
any
treatment
to
severe
forms
which
are
in
some
cases
very
difficult
to
treat
.
All
variants
of
tetrahydrobiopterin
deficiency
can
be
differentiated
from
the
classical
phenylketonuria
(
PKU
)
by
measurement
of
pterin
metabolites
in
patients
'
urine
,
tetrahydrobiopterin
loading
test
,
and
by
dihydropteridine
reductase
activity
in
erythrocytes
from
the
Guthrie
card
.