Incorporation of haemoglobin haem into the rat hepatic haemoproteins tryptophan pyrrolase and cytochrome P-450

Abstract

After its administration to intact rats, haemoglobin haem was incorporated into hepatic tryptophan pyrrolase as shown by the marked increase in functional constitution of this enzyme. Incorporation of haemoglobin haem into cytochrome P-450 was demonstrated in intact rats and in the isolated rat liver perfused with haemoglobin-free medium. In both systems, haemoglobin haem restored cytochrome P-450 content and its dependent mixed-function-oxidase activity after substrate-induced destruction of the cytochrome P-450 haem moiety. Further confirmation that haemoglobin haem could be incorporated prosthetically into cytochrome P-450 was achieved by administration of [3H]haemoglobin to rats and subsequent isolation and characterization of radiolabelled substrate-alkylated products of cytochrome P-450 haem. Our findings indicate that, although hepatic uptake of parenteral haemoglobin is slower than that of haem, it appears to serve as an effective haem donor to the intrahepatic 'free' haem pool. Thus parenteral haemoglobin may warrant consideration as a therapeutic alternative to haem in the acute hepatic porphyrias.

Cytochrome P450 regulation: the interplay between its heme and apoprotein moieties in synthesis, assembly, repair, and disposal

“…This suggests that the newly synthesized heme from the ALA chase readily equilibrated with the prelabeled microsomal heme (largely P450 heme), effectively diluting out the radioactive heme from the microsomal pool. (b) Similarly, a transfer of radioactivity to P450 could be demonstrated from labeled hemoglobin heme in vivo (Wyman et al, 1986) or when a P450 enzyme containing prelabeled heme was mixed with another unlabeled P450 enzyme in vitro and the radioactivity was then determined in both after resolving them from each other (Sadano and Omura, 1983; 1985). (c) As discussed above, the incorporation of exogenously administered labeled heme into P450 in the presence of a suicide inactivator, consequent generation of an inactivator-modified labeled heme adduct, followed by repeated cycles of P450 heme reconstitution and destruction, also argues for freely exchangeable heme pools (Unseld and De Matteis, 1978; Correia et al ., 1979; 1981); as does the secondary induction of ALAS1 seen with AIA and similar P450 suicide inactivators, a response compatible with “regulatory” cellular heme depletion (see sections 1.1.2 and 2.2.1 for further discussion of these aspects).…”