Experimental Studies on Bilirubin and Haem Biosynthesis

It has recently been shown that patients with unconjugated hyperbilirubinaemia due to Gilbert's syndrome have reduced activity of the enzyme protoporphyrinogen oxidase in circulating leucocytes. This may be explained by the further observation that unconjugated bilirubin competes with protoporphyrinogen for binding to protoporphyrinogen oxidase. In order to further investigate the effect of unconjugated hyperbilirubinaemia on porphyrin metabolism and haem biosynthesis studies have been performed in the Gunn rat which has unconjugated hyperbil irubinaemia and raised plasma bilirubin concentrations. Protoporphyrinogen oxidase activity was found to be reduced in the liver of the Gunn rat compared to heterozygous controls with normal plasma bilirubin concentrations. This reduction of protoporphyrinogen oxidase activity was not accompanied by any increase in the activity of the initial and rate-controlling enzyme of the pathway delta-aminolaevulinic acid synthase. In contrast to hepatic tissue, the activities of both protoporphyrinogen oxidase and delta-aminolaevulinic acid synthase were normal in renal tissue of the Gunn rat. Porphyrin excretion was reduced in the Gunn rat compared to that in heterozygous controls. Further studies were performed to investigate the effects of unconjugated hyperbilirubinaemia on haem biosynthesis in the brain of the Gunn rat. This was studied as the major effect of hyperbilirubinaemia is brain damage and the mechanism by which it occurs is unknown. Inherited partial enzyme deficiencies in haem biosynthesis are known to produce neurological damage. Therefore the possibility that bilirubin induced brain damage might be due to inhibition of protoporphryinogen oxidase activity and hence deficient neuronal haem synthesis, was investigated. Optimal conditions for the measurement of protoporphyrinogen oxidase activity were determined in the brain and found to be identical to those which provided optimal activity in hepatic tissue. The activity of protoporphyrinogen oxidase in brain tissue homogenates from Gunn rats was similar to that in heterozygous rats with normal bilirubin concentrations. This was also the case in neonatal Gunn rats in whom kernicterus had been induced by the displacement of bilirubin into the brain by sulphonamide treatment. Delta-aminolaevulinic acid synthase activities were also similar in the homozygous Gunn rat and heterozygous controls. These findings make it unlikely that the brain damage induced by hyperbilirubinaemia is due to inhibition of protoporphyrinogen oxidase activity and impaired neuronal haem biosynthesis. The above studies concerned the effect of disturbed haem catabolism on haem biosynthesis. The latter part of the studies extended the theme of the investigation by examining the effect of the administration of Tin-protoporphyrin, an inhibitor of haem biosynthesis, on haem biosynthesis in acute porphyria. The effect of administering Tin-protoporphyrin and haem arginate alone and in combination on succinyl acetone induced porphyria in rats was studied. This animal model proved to be unsatisfactory as the haem arginate given alone did not significantly reduce the over-production of the porphyrin precursor, delta-aminolaevulinate. This may be explained by inhibition of aminolaevulinic acid dehydratase by succinyl acetone so increasing aminolaevulinic acid overproduction in most body tissues. The effect of the haem arginate is largely confined to the liver. Studies in 1 patient during clinical attacks proved more encouraging. They showed that the coadministration of Tin-protoporphyrin with haem arginate prolonged both the biochemical response and clinical remission induced by haem arginate therapy. These findings indicate that co-administration of Tin-protoporphyrin and haem arginate may be used as prophylactic therapy in patients experiencing recurrent attacks of acute porphyria