Stimulating effect of haemin on the synthesis of cytochrome c by liver slices

Heme: a regulator of rat hepatic tryptophan 2,3-dioxygenase?

The hepatic cytosolic hemoprotein tryptophan 2,3-dioxygenase (TDO) is the rate-limiting enzyme in tryptophan catabolism and thus plays a key role in regulating the physiological flux of tryptophan into relevant metabolic pathways. The TDO protein is induced by corticosteroids such as dexamethasone (DEX) and is stabilized by its prosthetic heme. In rats, acute chemically induced hepatic heme depletion reduces the functional hepatic TDO levels to 25-30% of basal levels within 1 h, and this decrease persists beyond 28 h of heme depletion at which time only 25-30% of the protein is available for heme incorporation. Since this could stem from impaired de novo synthesis and/or instability of the newly synthesized apoTDO protein in the absence of heme, we examined the specific role of heme in these events in a previously validated rat model of acute hepatic heme depletion triggered by the P450 suicide substrate 3, 5-dicarbethoxy 2,6-dimethyl-4-ethyl-1,4-dihydropyridine. We now show that exogenous heme can reverse the functional impairment of the enzyme observed during hepatic heme depletion and fully restore the impaired DEX-mediated induction of the enzyme to normal. Furthermore, through Northern/slot blot analyses coupled with nuclear run-on studies, we now document that this heme regulation of TDO is exerted primarily at the transcriptional level. Immunoblotting analyses also reveal corresponding changes in the TDO protein, thereby establishing that heme is necessary for DEX-inducible TDO mRNA transcription and subsequent translation. Thus, the TDO gene may contain heme-regulatory elements in addition to the reported glucocorticoid-responsive elements. Together, these findings suggest that clinically, hepatic heme deficiency may enhance the tryptophan flux into synthetic (serotonergic) pathways, not only by depriving prosthetic heme for a functionally competent TDO hemoprotein, its primary catabolic enzyme, but also by impairing the de novo synthesis of this enzyme.

Apocytochrome P-450: reconstitution of functional cytochrome with hemin in vitro

Synthesis of microsomal cytochrome P-450 in rat liver requires synthesis of apoprotein in rough endoplasmic reticulum and of heme in mitochondria. Dissociation of apoprotein and heme synthesis by concomitant treatment of rats with inducers of cytochrome P-450 (i.e., phenobarbital) and inhibitors of heme synthesis (i.e., cobalt) resulted in a relative excess of apocytochrome P-450. Under these circumstances, it was possible to reconstitute the holocytochrome by addition of hemin in vitro. The holocytochrome was detected spectrophotometrically by its CO-binding properties and functionally by its increased oxidative activity. Heme-mediated reconstitution was most efficient in cell fractions rich in mitochondria-rough endoplasmic reticulum complexes (640 times g fraction), suggesting that the structural association of these two organelles may represent a functional unit essential for the synthesis of holocytochrome P-450. These findings indicate that phenobarbital-mediated induction of apocytochrome P-450 is independent of heme synthesis. It is suggested that synthesis of the apocytochrome may be the primary and rate-limiting event in the formation of cytochrome P-450.

The cell-free synthesis of cytochrome c by a microsomal fraction from rat liver

Conditions were investigated for demonstrating the synthesis in vitro of the complete molecule of cytochrome c by isolated liver microsomal systems from partially hepatectomized rats. It was first found that in vivo the early labelled cytochrome c associated with the microsomal fraction required, by comparison with the mitochondrial pool, more drastic conditions of extraction and its binding was less affected by freezing and thawing of the subcellular particles. The procedure of extraction and purification of cytochrome c had to be modified accordingly, to assure the recovery of the recently synthesized molecule. Several subcellular fractions were isolated from regenerating liver with a homogenization medium containing either 5 or 10mm-Mg(2+) and most of them were active in the synthesis of the cytochrome c apoprotein. The microsomal fraction, in the presence of either cell sap or pH5.0 fraction, was also able to incorporate [(59)Fe]haemin, delta-amino[(3)H]laevulic acid and (55)Fe into the prosthetic group of cytochrome c. These experiments confirm firmly the conclusions of our previous results obtained in vivo showing that both the apoprotein and the haem moieties are made and linked together on cytoplasmic ribosomes and only then is the complete molecule transferred to the mitochondria.