The Acute Porphyria Theories

From pathophysiological point of view, the acute porphyrias are perhaps more puzzling than nonacute porphyrias. A major clinical complication of an acute attack of porphyria is severe peripheral neuropathy. According to current concepts, the pathophysiology of porphyric attack  could be related to:

 - 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:

Porphyric Peripheral Neuropathy - a Hypothesis

A 50% reduction of any enzymatic activity in the pathway is not critical, and most tissues can easy copy with it. The liver is an exception, due to its high demand for cytochrome P450 and tryptophan pyrrolase heme. During an attack, the liver heme stores are depleted (for example as a result of P450 induction by phenobarbital) and the activity of ALAS, the first enzyme in the pathway, increases by an order of magnitude. The excess precursors - ALA, PBG and perhaps porphyrinogens in the case of VP - are released into the circulation. They are taken up by peripheral nerves, converted to protoporphyrin (lack of iron would prevent conversion to heme) and accumulate in the nerve cells.

 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.

Discussion of experimental data

The presented results show that liver accumulates a significant amount of colorless porphyrinogens, whereas the trigeminus contains fluorescent porphyrins. It thus seems that the machanisms of proto accumulation differ in both tissues. The residual liver PPO activity, either inhibited by oxadiazon or decreased due to an inborn metabolic error (as in VP),  simply can not handle the tenfold (or more) increase in ALAS activity, and the majority of protoporphyrin precursors (produced in the hepatocyte thanks to increased ALAS activity) enter the circulation. On the other hand, nerve ALAS is probably not increased, at least not that dramatically, and nerve PROTOX can handle both endogenously synthetised ALA, as well as ALA taken up from the circulation. However, the neuronal heme synthesis stops at the next step: iron insertion in the protoporphyrin ring.

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