Abnormalities involving haem biosynthesis have been postulated as underlying mechanisms in the aetiology of the neural manifestations of acute porphyria and of lead poisoning. This paper reports a study of the enzymes of the haem biosynthetic pathway and their control in mammalian brain. The activity of rat brain 6-aminolaevulinate synthetase (ALA synthetase), 6-aminolaevulinate dehydratase (ALA dehydratase), uroporphyrinogen I synthetase, uroporphyrinogen decarboxylase and ferrochelatase were found to be between 12.5 and 0.002% of the corresponding values for liver. This accords with the lower concentrations of total haem and cytochrome P450 found in brain and with the slower rate of incorporation of [4-14C]ALA into brain haem in uioo. The subcellular distribution of radioactivity following intraventricular injection of [4-I4C]ALA confirmed that the bulk of brain haemoproteins are intramitochondrial in contrast to liver where the major portion is microsomal. Brain haem biosynthesis was apparently unaffected by factors known to influence this pathway in liver, including starvation and treatment with allylisopropylacetamide or phenobarbitone. These findings suggest that brain haem requirements are considerably less than those of liver and are not subject to significant fluctuations under normal circumstances. Apparent non-inducibility of ALA synthetase suggests that deficient haem and consequently haemoprotein production could result where other enzymes in the pathway become rate-limiting due to genetic defects or inhibition by exogenous agents such as lead.
delta-Aminolaevulinic acid (ALA) uptake into neurons and glia in primary culture as well as ALA toxicity and its effects on gamma-aminobutyric acid (GABA) uptake were examined. [4-14C]ALA uptake into neurons and glia was nonsaturable, partially Na+- and temperature-dependent, and appeared to comprise mainly diffusion into the cell. 2,4-Dinitrophenol caused some inhibition of [4-14C]ALA uptake whereas ouabain, KCN, or amino acids at 1 mM concentration were without effect. ALA (1 mM) caused a slight inhibition of [U-14C]GABA uptake into neurons (14%) and glia (9%), but was without effect at lower concentrations. It is unlikely that, in acute porphyria, ALA reaches sufficiently high levels in nervous tissue to interfere with the reuptake of GABA into neurons or glia. ALA was shown to be toxic, judged by the loss of cells, to both neurons and glia at concentrations as low as 10 microM. Such a concentration of ALA may be expected to occur in the CSF of porphyric patients in the acute attack. However, results obtained with dispersed cells in culture may not necessarily reflect the situation in vivo where the cell may have a far greater resistance to the effects of toxic agents.
1. The effect of hexachlorobenzene feeding on liver delta-aminolaevulinate synthase, uroporphyrinogen decarboxylase and cytochrome P 450 was studied at various time-intervals in siderotic and non-siderotic rats. 2 In the non-siderotic group hexachlorobenzene feeding led to a progress decrease in liver uroporphyrinogen decarboxylase activity, accompanied by a progressive increase in delta-aminolaevulinate synthase activity. Cytochrome P 450 concentrations were above normal throughout but fell toward the end of the experiment. 3. Similar but more marked changes were found in the siderotic animals. The fall in uroporphyrinogen decarboxylase activity occurred earlier and was significantly greater in these animals, whereas the increase in delta-aminolaevulinate synthase activity was consistently larger. Liver cytochrome P 450 concentration also rose but to a lesser extent than that in the non-siderotic rats. 4. Hexachlobenzene-induced porphyria would seem to be attributable to inhibition or inactivation of hepatic uroporphyrinogen decarboxylase. Hepatic siderosis has a synergistic effect with hexachlorobenzene on this enzyme and may exert additional effects by promoting cytochrome P 450 turnover.
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