Porphyria Educational Services
PORPHYRIA EDUCATIONAL SERVICES BULLETIN
Vol. 2 No. 22 May 28, 2000
FOCUS: Heme, Cytochrome P-450 and Porphyria
Heme Biosynthesis is the backbone of the porphyrias.
Heme is synthesized in largest amounts by the bone marrow. In the
bone marrow the heme is incorporated into hemoglobin. The
hemoglobin which is an oxygen transport protein takes it from
there.
The heme is then off to the liver. At the liver the heme is
metabolized by the liver, where most of the heme is incorporated
into cytochromes.
The cytochromes are electron transport proteins. The most
abundant cytochromes in liver are the cytochrome P-450 enzymes
that metabolize drugs and many other foreign and endogenous
chemicals. It is for this reason that all porphyrics must be very
careful in avoiding all drugs that metabolize in the liver. They
are often referred to as P-450 drugs or toxins.
Heme biosynthesis is controlled differently in liver and bone
marrow. Hepatic heme biosynthesis is rate-limited and primarily
regulated by the first enzyme, ALA synthase. This enzyme's
activity in liver cells is normally very low but increases
dramatically by induction of enzyme synthesis when the liver
produces more heme. Certain drugs and hormones induce hepatocytes
to make more ALA synthase, heme, and cytochrome P-450.
In the bone marrow, heme is made in erythroblasts and
reticulocytes that still contain mitochondria, whereas
circulating erythrocytes lack mitochondria and cannot form heme.
Bone marrow cells express erythroid-specific forms of some
pathway enzymes.
Porphyrias and related disorders are associated with deficiencies
of the other seven enzymes, and mutations in genes for these
enzymes have been characterized in detail in previous PES
bulletins.
Although each type of hereditary porphyria is associated with
deficiency of a particular enzyme, unless people with that enzyme
deficiency come from the same family, they are likely to have
different mutations in the gene for that enzyme. Because of this
factors these diseases are heterogeneous at the molecular level.
Some porphyrias, especially those that increase the early
precursors ALA and PBG, damage nerves, leading to a variety of
symptoms such as abdominal pain and muscle weakness, which can
even progress to paralysis.
Mechanisms for the neurologic disturbances have been speculated,
such as effects of excessive heme pathway intermediates, or
deficient heme synthesis, in the nervous system. ALA and other
products of the heme biosynthetic pathway have not been proven to
be neurotoxic, and heme deficiency in nervous tissue in these
disorders is also unproven. Therefore, the exact cause remains
unclear.