STUDIES ON HAEM BIOSYNTHESIS IN RAT BRAIN

Abstract— 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 vivo . The subcellular distribution of radioactivity following intraventricular injection of 4-¹⁴C]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.