EPENDYMOMA
relatively dry climate. Most respiratory
complaints, in contrast, are more com-
mon during the winter.
ALTITUDE
Although
mountainous
regions
have
much less atmospheric pollution, they
are not necessarily beneficial to health
because the air becomes thinner as alti-
tude increases. People with a chest con-
dition who ascend in a few days from
sea level to 1,500 m may find that their
breathing
difficulty
worsens.
Above
about 3,000 m, breathing becomes dif-
ficult even for healthy people. Rapid
ascent from sea level to 3,600 m or
higher carries a risk of altitude sickness
(see
mountain sickness
) ,
w hich can cause
sleeplessness, nausea,
coma
(a state of
unconsciousness and unresponsiveness
to stimuli), or death.
Sustained life seems to be impossible
above
6,000
m because at that level the
blood cells increase in number to com-
pensate for the lack of oxygen. This
increase puts strain on the heart and
causes a predisposition to
thrombosis
(abnormal blood clotting).
SUNLIGHT
Fair-skinned people who live in sunny
climates
may
suffer
ill
effects from
repeated
exposure
to
sunlight
(see
sunlight, adverse effects of
) ,
including
premature w rinkling of the skin and an
increased risk of
cataract
(loss of trans-
parency of the lens of the eye). There is
also an increased risk of developing
skin
cancers
,
such as malignant melanoma
(see
melanoma, malignant
) and
basal cell
carcinoma
,
as well as the precancerous
condition solar
keratosis
.
These
risks
have been increased by damage caused
by environmental pollutants to the pro-
tective layers of ozone in the upper
atmosphere. Health risks from sunlight
may be reduced by protection of the
skin, such as by using
sunscreens
.
MINERALS
Variations in the distribution of certain
minerals
in the environment are known
to have an effect on health. For example,
there is a higher-than-average incidence
of cancer in areas where the radioactive
gas
radon
is emitted from granitic rocks.
In contrast, there is a lower-than-average
incidence of tooth decay
(see
caries,
dental
) in populations in regions where
the water has a high
fluoride
content.
enzyme
A
protein
that regulates the rate of a
chemical reaction in the body. There are
thousands of enzymes, each with a dif-
ferent
chemical
structure.
It
is
this
structure that determines the specific
reaction regulated by the enzyme.
Every
cell
in
the
body
produces
various types o f enzymes; different sets
o f enzymes occur in different tissues,
reflecting their specialized functions.
For example, the
pancreas
produces the
digestive enzymes lipase, protease, and
amylase,
and
among
the
numerous
enzymes produced by the
liver
are some
that metabolize drugs.
In order to function properly, many
enzymes need an additional compon-
ent, known as a coenzyme, w hich is
often derived from a
vitamin
or
mineral
.
INDUCTION
AND
INHIBITION
Enzyme activity is influenced by many
factors. One o f these factors is the
action o f drugs. Liver enzyme activity
is increased by certain drugs, such as
barbiturate drugs
,
w hich affect the rate
at w hich other drugs are metabolized
by the liver cells. This effect, w hich is
called enzyme induction, is responsi-
ble for a variety o f important drug
interactions (see
drug
).
Conversely, many drugs inhibit or
block enzyme action. Some
antibiotic
drugs
destroy bacteria by blocking bac-
terial enzymes while leaving human
enzymes
unaffected.
Similarly,
some
anticancer drugs
act by blocking enzyme
activity in tumour cells, affecting nor-
mal body cells to a lesser degree.
ENZYMES AND
DISEASE
Measuring enzyme levels in the blood
can be useful in
diagnosing
certain
disorders. For
example, the level o f
heart muscle enzymes is raised follow-
ing a
myocardial infarction
(heart attack)
because
the
damaged
heart
muscle
releases enzymes into the bloodstream,
and muscle enzyme levels are raised in
muscular dystrophy
.
Many inherited metabolic disorders,
including
phenylketonuria
,
galactosaemia
,
and
G6PD deficiency
,
are caused by defects
in, or deficiencies of, enzymes. Abnor-
mal enzymes or levels can be detected
in tests on blood or other body fluids.
ENZYMES AND
TREATMENT
Enzymes can play a valuable role in
treating
certain
conditions.
Pancreatic
enzymes may be given to aid digestion
in people who have
malabsorption
related
to pancreatic disease.
Enzymes
such as
streptokinase
and
alteplase (see
tissue-plasminogen activator
)
are used to treat acute
thrombosis
and
embolism
(conditions that cause block-
age o f blood vessels) by dissolving clots.
THE ACTION OF ENZYMES
An enzyme is a protein that acts as
a catalyst for a chemical change in
the body (that is, it greatly speeds
up the rate at which the change
occurs). The change may be a small
modification to the structure of a
substrate (the particular chemical
on which the enzyme acts) in a body
tissue, the splitting of a substrate,
or the joining of two substrates.
a
I
The shape ofan enzyme determines
its activity. Each enzyme molecule will
combine only with a particular substrate
that has molecules of a complementary
shape.
C o m b i n e d
s u b s t r a t e
a n d e n z y m e
2
When the enzyme and substrate
combine, their interaction causes a
chemical change within the substrate. In the
example shown here, the chemical is split
into two products.
E n z y m e
3
After the chemical reaction has ended,
the enzyme molecule remains unchanged
and can move on to combine with another
substrate molecule and repeat the process.
eosinophil
A type of leukocyte
(
white blood cell)
that
plays a role in allergic responses (see
allergy
)
and fighting parasitic infections.
ependymoma
A rare
brain tumour
of the
glioma
type
(arising from supporting glial cells in
the
nervous system
)
that occurs most
commonly in children.
E
275
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