The activity of δ-aminolaevulinate synthetase is generally regarded as rate-limiting for hepatic haem biosynthesis. It has been suggested that cytochrome synthesis may also be regulated by changes in δ-aminolaevulinate synthetase activity. This hypothesis was studied by injecting product, δ-aminolaevulinate, into adult rats over a 4–240h period. The concentrations of hepatic mitochondrial cytochromes a, b, c and c1 were unchanged by treatment with δ-aminolaevulinate, allylisopropylacetamide or phenobarbital. In control animals, total microsomal haem content equalled the sum of cytochromes b5 plus P-450. After δ-aminolaevulinate administration the total amount of microsomal haem, measured as the pyridine haemochromogen, exceeded these components, indicating the formation of a ‘free’ haem pool. Haem synthesis does not appear rate-limiting for hepatic cytochrome synthesis in the adult rat.
