Presence of apo-cytochrome beta 5 in microsomes from rat liver

Recollection of the early years of the research on cytochrome P450

Since the publication of the first paper on "cytochrome P450" in 1962, the biochemical research on this novel hemoprotein expanded rapidly in the 1960s and the 1970s as its principal roles in various important metabolic processes including steroid hormone biosynthesis in the steroidogenic organs and drug metabolism in the liver were elucidated. Establishment of the purification procedures of microsomal and mitochondrial P450s in the middle of the 1970s together with the introduction of molecular biological techniques accelerated the remarkable expansion of the research on P450 in the following years. This review paper summarizes the important developments in the research on P450 in the early years, for about two decades from the beginning, together with my personal recollections.

Studies of cytochrome synthesis in rat liver

The incorporation of radioactive amino acids and of delta-amino[2,3-(3)H(2)]laevulinate into rat liver cytochromes b(5) and c and cytochrome oxidase has been examined with and without protein-synthesis inhibitors. Cycloheximide promptly inhibits labelling of both haem and protein for cytochrome c in parallel fashion. Although incorporation of (14)C-labelled amino acid into microsomal cytochrome b(5) is also rapidly inhibited, cycloheximide incompletely inhibits haem labelling of cytochrome b(5) and cytochrome a+a(3), and inhibition occurs only after repeated antibiotic injections. The possibility of apo-protein pools, or of haem exchange, with a rapidly renewed ;free' haem pool, is considered. Consistent with this model is the observation of non-enzymic haem exchange in vitro between cytochrome b(5) and methaemoglobin. Chloramphenicol, injected intravenously over 5h, results in a 20-40% decrease in incorporation of delta-amino[2,3-(3)H(2)]laevulinate into haem a+a(3) and haem of cytochromes b(5) and c. With the dosage schedule of chloramphenicol studied, amino acid labelling of total liver protein and of cytochrome c was not inhibited. Similarly, ferrochelatase activity was not decreased.

Phenobarbital Confers its Diverse Effects by Activating the Orphan Nuclear Receptor Car

In the early 1960s, phenobarbital (PB) was shown to induce hepatic drug metabolism and the induction was implicated in the molecular mechanism of drug tolerance development. Since then, it has become evident that PB not only induces drug metabolism, but also triggers pleiotropic effects on liver function, such as cell growth and communication, proliferation of the endoplasmic reticulum, tumor promotion, glucose metabolism, steroid/thyroid hormone metabolism, and bile acid synthesis. Upon activation by PB and numerous PB-type inducers, the nuclear receptor CAR mediates those pleiotropic actions by regulating various hepatic genes, utilizing multiple regulatory mechanisms.

Structural consequences of heme removal: molecular dynamics simulations of rat and bovine apocytochrome b5

Molecular dynamics simulations of rat and bovine apocytochrome b5 were performed to investigate the structural and dynamical consequences of heme removal. A crystal structure is available for the bovine holoprotein, while experimental studies of apocytochrome b5 have focused on the rat protein. The rat and bovine proteins are 93% homologous by sequence, and the sequence differences (six residues) appear to have no effect on the structure of the native holoprotein, as seen by the correlation of a bovine simulation with rat holocytochrome b5 experimental data (Storch & Daggett, 1995). There was a marked effect, however, on the structure and dynamics of the apo form. The bovine apocytochrome b5 simulation displayed subtle inconsistencies when compared to the experimental results on the rat apoprotein. Therefore, the rat protein was constructed from the bovine crystal structure coordinates. The MD simulation of the rat apoprotein displayed greater deviations from the crystal structure, yet it was in much closer agreement to the experimental data for the apoprotein. Additionally, the six variant residues fall in the regions where the bovine protein deviated from experiment. The two hydrophobic cores of the rat protein behaved very differently. Core 2 was well maintained, retained native-like structure, and is in good agreement with NMR data (Moore & Lecomte, 1990). Conversely, core 1, which is normally constrained by the prosthetic heme group, exhibited conformational heterogeneity, increased mobility, and some loss of secondary structure. Thus, the model of rat apocytochrome b5 complements past studies by providing structural information about core 1 that has proved difficult to obtain by experiment. The bovine simulation serves as a prediction, since little to no experimental data exist for this form of the apoprotein.

Structural properties of apocytochrome b5: presence of a stable native core

Upon removal of the heme group, the water-soluble fragment of cytochrome b5 adopts a conformation less stable and compact than that of the holoprotein [Huntley, T. E., & Strittmatter, P. (1972) J. Biol. Chem. 247, 4641-4647]. This conformation, imposed by the amino acid sequence alone, has not been described in detail. One- and two-dimensional proton nuclear magnetic resonance spectroscopy techniques were applied to the apoprotein of the soluble fragment of rat liver cytochrome b5 in an effort to characterize the structure of the apoprotein. Nuclear Overhauser spectroscopy revealed a number of short interresidue distances and demonstrated that, in spite of the increased flexibility, at least one cluster of side chains exists on a time scale long enough for study. Several residues participating in the cluster, in particular the only Trp (Trp 22), were identified. Similarities with the spectrum of the reduced holoprotein were observed that led to the inspection of the cytochrome b5 crystal structure for assigning resonances. It appeared that the environment of this residue maintains its integrity in the apoprotein. Since in the holoprotein Trp 22 belongs to a hydrophobic core formed in part by beta-strands, it is proposed that some of this beta-structure is stable in the absence of the heme-protein interactions. Implications for structure and folding are discussed.

Interactions of heme with hepatic microsomal mono-oxygenase. Effect on benzpyrene hydroxylation

The addition of heme (1-10 muM) to liver microsomes from phenobarbital (PB)-treated and 3-methylcholanthrene (MC)-treated male rats increased the rate of benzpyrene (BP) hydroxylation by about 20-40%. On the other hand, protoporphyrin IX caused only inhibition of BP hydroxylation. There was no increase of enzymatic activity by heme when solubilized preparations of liver microsomes were used. This suggested the possibility that an apo-cytochrome P-450 was present in intact microsomes. Higher concentrations of heme inhibited BP hydroxylation by either intact or solubilized microsomes. The inhibition by heme with solubilized microsomal preparations was noncompetitive, "mixed-type". However, with intact microsomes, the lack of linearity, precluded the determination of the type of inhibition. To examine possible effects of heme on the binding of BP to microsomal cytochrome P-450, the spectrum elicited by the addition of BP to microsomes was obtained in the presence or absence of added heme. The addition of heme to liver microsomes produced a marked increase in the trough (419-420 nm) of the difference spectrum formed by the subsequent addition of BP. These findings would suggest that heme increased the binding of BP to microsomes. However, the possibility that BP merely displaces the bound heme of the microsomes to yield, as expected, a trough at 413-416 nm (the addition of heme to microsomes yields a peak of 413-416 nm, unpublished) cannot be ruled out. Nevertheless, independent of our understanding of the mechanism involved in the spectral interactions between heme and BP with liver microsomes it is clear that an effect at their binding site(s) must have been elicited by the presence of both compounds.

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.

Ontogenetic changes of proteins of endoplasmic reticulum

The proteins of the smooth and rough endoplasmic reticulum from fetal, immature, and adult male rats were compared after incorporation of two radioactively labeled precursors, (14)C-labeled amino acids and delta-aminolevulinic acid-(3)H by means of gel electrophoresis. The labeling patterns indicated that protein components present in two major electrophoretic bands underwent significant synthesis in fetal tissue while three actively incorporating protein bands were noted in adult tissue. Although the uptake of the amino acids-(14)C decreased for the smooth and rough elements of the endoplasmic reticulum as a whole during liver development, the qualitative patterns were not significantly different in adult and fetal livers. The over-all incorporation (disintegrations per minute per milligram protein) of the heme precursor into the smooth and rough elements also did not change with development. However, a change was noted in the distributional electrophoretic patterns with development. The estimation of molecular weight (by disc electrophoresis) and the incorporation of the heme precursor suggested the similarity of the two major protein bands to cytochrome P-450 and cytochrome b(5), components of the endoplasmic reticulum, thought to be involved in the mixed-function oxidase system. The evidence indicated that in fetal liver, at a time when the oxidase capability was low, the amino acid incorporation into these two protein groups was the same as in the adult. The incorporation of the heme moiety, however, was different, decreasing in the cytochrome b(5) region and increasing in the cytochrome P-450 region during development. These results correlate with the increase in oxidase activity associated with liver development.