| Popis: |
In the liver most of the heme synthesized by the parenchymal cells is apparently destined for the cytochromes of the P450 group, a family of related but distinct hemoproteins under the control of separate genes. Protoporphyrinogen is then oxidized to protoporphyrin by another oxidase and the last enzyme of heme biosynthesis, ferrochelatase, inserts ferrous iron into protoporphyrin to produce heme. The most important regulatory mechanism exercised by heme on 5-aminolevulinate (ALA)-synthetase involves repression of the synthesis of the mRNA for this enzyme, and drugs which stimulate the biosynthesis of ALA-synthetase do so by accelerating the transcription and increasing the amount of the mRNA for ALA-synthetase. Under normal conditions the biosynthesis of ALA is tightly coupled to formation of heme, as ALA and other intermediates proceed unhindered along the biosynthetic chain with little accumulation. The increased supply of ALA thus resulting, a compensatory response intended to overcome the enzymatic block, is an important contributing factor to the accumulation of protoporphyrin. |
