Multiple genotypes, multiple phenotypes, and partial defects

In recent years, the following ideas have been expressed: (a) that all cases of a discrete, inherited neuromuscular syndrome should prove to be due to a single biochemical defect, (b) that any single biochemical defect should give rise only to one syndrome, and (c) that an enzymatic defect cannot give rise to a disease unless there is virtual absence of activity, that is, less than 5% or 10% of the normal value. We review evidence from research in neuromuscular, neurological, and other genetic diseases of humans that suggest the contrary. There are now examples of single clinical syndromes related to each of several defects, of defects of one biochemical reaction related to two or more distinct clinical syndromes, and of partial defects associated with disease in a way that suggests a causal relationship.

The effect of delta-aminolevulinic acid on the synthesis and metabolism of GABA in rabbit brain homogenates

The porphyrin precursor delta-aminolevulinic acid (delta-ALA) is a structural analogue of the putative amino acid neurotransmitter, gamma-aminobutyric acid (GABA). This study has demonstrated that delta-ALA has no effect on glutamate decarboxylase activity and only a small inhibitory effect of GABA aminotransferase activity. This would suggest that if accumulation of delta-ALA is related to development of the acute attack of porphyria, it is not via an effect on GABA synthesis and metabolism.

Reduced ferrochelatase activity in fibroblasts from patients with porphyria variegata

Ferrochelatase deficiency has been shown in both porphyria variegata (PV) and erythropoietic protoporphyria (EPP). It has been suggested that in PV there is a decrease in the enzyme, whereas in EPP the enzyme is unstable. In the present study ferrochelatase activity was measured in skin fibroblasts from three patients with PV and three normal subjects. The enzymatic activity in the patients with PV (17.5 +/- 4.5 pmoles heme formed per 10(7) fibroblasts per hour) was 50% of that of the control group (31.0 +/- 3.2 pmoles heme formed per 10(7) fibroblasts per hour). This supports the contention that the enzyme is deficient in PV and that an inactive ferrochelatase is the primary deficiency in this type of porphyria.

Porphyria variegata--studies of an affected couple and their children

Porphyria variegata affects approximately 1 in 200 Afrikaans-speaking people in South Africa. This paper reports the first case of a marriage between 2 people with porphyria variegata and describes investigations carried out on their 2 children, who do not exhibit any signs of the disease. The wife had suffered 1 miscarriage at 4 months' gestation.

The neurological manifestations of porphyria: a review

The hereditary hepatic porphyrias, PV, AIP and HC, are characterized biochemically by increased excretion of porphyrins and the porphyrin precursors ALA and PBG. They are characterized clinically by episodes of acute neurological involvement. The increased production of porphyrins and porphyrin precursors has been shown to be due to partial enzyme blocks along the heme biosynthetic pathway which results in secondary depression of the key enzyme ALA-synthetase. The neurological manifestations could therefore be related to either a decrease in essential heme-proteins or other heme-containing compounds within the nervous system, or to a toxic effect of the over-production of the porphyrin precursors ALA and PBG. There is evidence for and against both theories. Recent work from a number of research groups has shown the porphyrin precursors to have potent pharmacological effects on the nervous system, and these are possibly related to the GABA receptor and binding site-porphyrin precursor interactions. Current studies on therapy of the acute attack have concentrated on suppression of ALA-synthetase activity, and consequently, on reduced ALA and PBG production. A number of such methods of therapy have met with remarkable success and hold promise for the future treatment of the acute attack.

Reduced ferrochelatase activity: a defect common to porphyria variegata and protoporphyria

Erythroid ferrochelatase activity has been studied in the normoblasts of patients with porphyria variegata and protoporphyria. Two methods were used for the investigation: one using intact cells and the other lysed cells, each measuring the amount of haem synthesized by normoblasts. In patients with porphyria variegata, ferrochelatase activity estimated by both methods was approximately 50% of the normal, and in protoporphyria the ferrochelatase activity was normal in intact normoblasts but was 20% of the normal in sonicated normoblasts (marrow lysates). It is suggested therefore that in porphyria variegata a dominantly inherited structural gene mutation results in an active ferrochelatase whereas in protoporphyria the genetic mutation results in an unstable ferrochelatase. The mechanism of the enzyme instability is not known though a number of postulates are discussed.

Effect of delta-aminolaevulinic acid on the resting membrane potential of frog sartorius muscle

The porphyrin precursor delta-aminolaevulinic acid can significantly decrease the resting membrane potential in a single fibre from the sartorius muscle of the frog Rana angolensis. This effect is dose-dependent and reversible.

The effect of porphyrin precursors on monosynaptic reflex activity in the isolated hemisected frog spinal cord

The porphyrin precursors beta-aminolevulinic acid (beta-ALA) and porphobilinogen (PBG) which accumulate, and are excreted in the urine in increased amounts during acute attacks of porphyria, were tested for their effects on reflex activity in the isolated hemisected spinal cords of Xenopus laevis. The two compounds were found to exert an inhibitory effect on monosynaptic ventral root responses, as well as on dorsal root responses (DRR) and dorsal root potentials (DRP). The latent period for inhibition of the monosynaptic response was longer than that for the DRR and DRP. The sensitivity of the preparations to the effect of the porphyrin precursors was subject to some seasonal variation. BETA-ALA and PBG did not effect conduction in isolated sciatic nerves at a concentration of 1 mg/ml.