- limited availability of heme in the CNS
- limited heme saturation of tryptophan pyrrolase
- toxic effects of porphyrins and/or porphyrin precursors on CNS and/or peripheral nerves
- limited availability of heme as a cofactor of nitric oxide synthase (nNOS and eNOS)
In the course of porphyria research, many experimental porphyrias have been developed, including PBG-DE deficient mice and ferrochelatase deficient mice. At this Institute, we are still using old fashioned in vivo models - animals treated with an inhibitor of heme biosynthesis. It is felt that these models could - in spite of their simple design - still throw some light on the pathophysiology of porphyria. Next, we will present some results obtained in mice treated with a competitive inhibitor of protoporphyrinogen oxidase, which form a basis for our porphyria pathophysiology theory.
Treatment of mice with oxadiazon, an inhibitor of protoporphyrinogen oxidase, produces a dose-related inhibition of the liver enzyme. The condition resembles variegate porphyria: the animals excrete protoporphyrin in the feces and, if a sufficiently high dose of oxadiazon is used, they also excrete large amounts of porphobilinogen in urine. Liver contains elevated amounts of protoporphyrinogen, the colourless precursor of protoporphyrin. Interestingly, at high oxadiazon concentrations, we observed porphyrin fluorescence in the peripheral nerves.
The liver accumulation of protoporphyrinogen (protogen) suggests that porphyrinogens can be stable in vivo. In the majority of studies concerning the possible toxic effects of porphyrin precursors on nerve cells, PORPHYRINS were used instead of PORPHYRINOGENS. Is it possible that porphyrinogens circulate in plasma and are taken up by, for example, peripheral nerve cells? IF they really enter the circulation, they are certainly not taken up by the brain: there is no increase in brain porphyrin content following oxadiazon treatment.
Thus, we have set up the following hypothesis:
The excess precursor production by the liver, and consequent conversion to porphyrins in other organs, is a key to the whole story. It explains the seemingly paradoxical excretion of excess ISOMER III series in urine of acute intermittent porphyria patients (remember that the PBG-deaminase activity is blocked in AIP, and non-enzymatic condensation of excess PBG should result in ISOMER I excretion). There are sporadic autopsy reports describing fluorescence in the peripheral nerves of porphyria patients. There is also one paper on increased trigeminal nerve porphyrin content in DDC-induced porphyria.
For any protoporphyrinogen accumulation, the relative activity ratios between the rate-limiting ALAS activity and PROTOX activity would be crucial. And here the liver is almost unique - liver ALAS is INDUCIBLE!
Although brain ALAS activity is usually reported to be lower than liver ALAS, we did not find any such difference with PROTOX - on the contrary, brain PROTOX activity was slightly higher than the liver activity under normal contitions. This again suggests that the nerve cell PROTOX could easy handle large quantities of protoporphyrinogens, derived from plasma porphyrin precursor, as long as these precursors are able to enter the nerve cell. While the blood brain barrier appears to effectively block ALA entry in the brain, peripheral nerves could be more susceptible.
On the other hand, plasma of oxadiazon-treated mice, as well as plasma from variegate porphyria patients, also contains FLUORESCENT porphyrins, as evidenced by fluorescence emission scan. The maximum typical for both oxadiazon porphyria and variegate porphyria is at about 625 nm. Thus the trigeminal porphyrin accumulation could be caused simply by porphyrin uptake from plasma, and here one could immediately ask: why, then, are acute porphyric attacks not encountered in EVERY type of porphyria? Why only in the acute types?
We believe that passive uptake of porphyrins from plasma is NOT
the major explanation for peripheral nerve porphyrin accumulation. According
to our HPLC results, plasma contains mainly protoporphyrin - other porphyrins
were bellow detection limits. On the contrary, the trigeminus showed a
very atypical porphyrin pattern: 80% protoporphyrin and 20% uro I. This
seems to indicate that the trigeminus porphyrins originate in the trigeminus,
and are not just passively taken up from plasma.
Last modified December 15, 1999