Studies on the synthesis of cytochrome P-450 and cytochrome P-448 in rat liver

Dimethylnitrosamine-induced liver injury in rats: the early deposition of collagen

Dimethylnitrosamine (DMN) is a potent hepatotoxin that can cause fibrosis of the liver. It's ability to provide a suitable rapid experimental murine model for early human cirrhosis was examined. The drug was administered to adult male albino rats in order to document sequential pathological and biochemical alterations. Injury was produced by intraperitoneal injections of DMN on three consecutive days of each week over a 3-week period. A rapid increase in collagen content was documented, with linear increases occurring from days 7 to 21. Livers were examined for histopathological changes on days 7, 14 and 21 following the beginning of exposure. Severe centrilobular congestion and haemorrhagic necrosis could be observed on day 7. Centrilobular necrosis and intense neutrophilic infiltration were observed on day 14. By day 21, collagen fiber deposition could be observed, together with severe centrilobular necrosis, with focal fatty changes, bile duct proliferation, bridging necrosis and fibrosis surrounding the central veins. A decrease in total protein and increase in DNA were also documented. DMN-induced liver injury in rats appears to be a potential animal model for early human cirrhosis and the rapid deposition of collagen, and may serve as a convenient procedure for screening antifibrotic agents.

Relative induction of molecular forms of cytochrome P-450 in gamma-hexachlorocyclohexane exposed rat liver microsomes

The effect of gamma-hexachlorocyclohexane (HCH), (25 mg/kg body weight, i.p., administered for 4 consecutive days) on the induction of types of cytochrome P-450 in rat liver microsomes was studied. Induced proteins were characterized by "Western" blot analysis, using anticytochrome P-450 antibodies. It was observed that HCH is a "mixed-type" inducer and mediates induction of cytochrome P-450 b/e forms by several fold and of cytochrome P-450 c and d forms by nearly three fold.

Genes for cytochrome P-450 and their regulation

The capacity of the liver microsomal mixed-function oxidase system to metabolize a wide variety of exogenous as well as endogenous compounds reflects the participation of multiple forms of the terminal oxidase, cytochrome P-450, which have different broad, but overlapping, substrate specificities. Several of these isozymes accumulate in the liver after exposure of animals to specific inducing agents. Recent studies employing recombinant DNA techniques to investigate the genetic and evolutionary relatedness of various cytochrome P-450 isozymes as well as the molecular basis for the induction phenomenon are described. The conclusions from these investigations are presented in the context of the substantial body of data obtained from the characterization of specific cytochrome P-450 isozymes and from studies on the induction of specific isozymes or enzymatic activities during development or after treatment of animals with various inducing agents.

Evaluation of monooxygenase induction as a means of enhancing the yield of the rat liver cytochrome P-450 isoenzyme with high affinity for cimetidine

The binding of cimetidine to liver microsomes prepared from untreated rats and rats pretreated with phenobarbitone or 3-methylcholanthrene has been investigated by difference spectroscopy and equilibrium partition methods. In M/15 phosphate buffer, pH 7.9, microsomes from each group of rats yielded markedly biphasic spectral binding curves, which have been interpreted in terms of two independent classes of cytochrome P-450 site with widely differing binding affinities for the drug. Support for such interpretation was provided by the finding that the spectral binding curve for a purified sample of the principal cytochrome P-450 isoenzyme from liver microsomes of phenobarbitone-pretreated rats could be described adequately by a single rectangular hyperbolic relationship, the spectral dissociation constant being indistinguishable experimentally from that for the weaker class of cytochrome P-450 binding site in the corresponding microsomes. The spectral dissociation constants were 2 microM and 80 microM for microsomes from untreated rats; 44 microM and 540 microM for those from phenobarbitone-pretreated rats; and 34 microM and 540 microM for microsomes from rats pretreated with 3-methylcholanthrene. On this basis, both classes of P-450 site in the microsomes from rats subjected to either pretreatment exhibited lower affinity for cimetidine than their counterparts in microsomes from untreated rats. Equilibrium partition studies of the higher-affinity class of microsomal binding site for cimetidine showed that the twofold increase in the cytochrome P-450 content of microsomes effected by 3-methylcholanthrene pretreatment was more than offset by a diminished proportion of the total cimetidine-binding capacity present as the higher-affinity, pharmacologically significant, receptor (18%, cf. 48% in control microsomes); and that phenobarbitone pretreatment resulted in replacement of the high-affinity receptor by one with a threefold weaker cimetidine-binding affinity. Thus the use of these monooxyginase inducers to enhance the cytochrome P-450 content of liver microsomes would seem to offer little potential in the isolation of the isoenzyme with high affinity for cimetidine.

Purification and characterization of microsomal cytochrome P-450s

1. Eight different forms of cytochrome P-450 have been purified to electrophoretic homogeneity. Electrophoretic, spectral and catalytic properties of these cytochrome P-450s are presented and comparison is made with preparations presented elsewhere in the literature. 2. The levels of these forms of cytochrome P-450 present in liver microsomes of rats treated with various compounds have now been quantified. Several forms of cytochrome P-450 are induced, in a more or less coordinate manner, while levels of other cytochrome P-450s are lowered, during administration of commonly used inducing agents. 3. The role of cytochrome P-450 purification and characterization studies in the understanding of the total field is discussed, along with directions in which future research is needed.

Further characterization of the effects of 3-methylcholanthrene administration upon hepatic ribonucleic acid polymerase activities

The administration of 3-methylcholanthrene (MC) to rats results in a marked increase in the specific activities of hepatic RNA polymerases I and II. In the present study, we were able to show that this increase was not caused by a shift in the ratio of 'free' to 'template-engaged' RNA polymerase. By means of binding studies with [3H]amatoxin, we were unable to demonstrate any increase in the number of RNA polymerase II molecules in liver after MC administration to the rats. RNA polymerase I was purified in excess of 3000-fold from hepatic nuclei isolated both from control and MC-treated rats. The stimulation in activity was demonstrated at each step in the purification scheme until glycerol sedimentation analysis. Results from cation-exchange chromatography on phosphocellulose indicated that the polycyclic hydrocarbon increased the enzyme activity of RNA polymerase Ib somewhat specifically. Subsequent to glycerol gradient centrifugation, this stimulatory advantage was no longer evident. Reconstitution experiments revealed the presence of a stimulatory component, which was demonstrated in low molecular weight fractions from both control and experimental preparations.

Zinc . protoporphyrin is a selective inhibitor of heme oxygenase activity in the neonatal rat

The present study was undertaken to examine the liver, spleen and kidney heme oxygenase activity in the rat, and also to investigate the response of the enzyme to a variety of metalloporphyrin complexes. The enzyme activity in the liver and the kidney of 3--4 day-old rats was several-fold greater than the corresponding values in the adult animals; however, the splenic enzyme activity was markedly depressed in comparison to that of adult rats. During the first 2--3 weeks post-parturation period, the activity of heme oxygenase in the spleen progressively increased, and in 4 weeks approached the adult values. The treatment of the newborn animals with the metalloporphyrin complex. Zn . protoporphyrin-IX, inhibited heme oxygenase activity in the spleen, liver and the kidney. Sn . protoporphyrin treatment also inhibited the activity of the enzyme in the liver and the spleen. The mechanism of the inhibition appeared to be competitive in nature. In contrast, the treatment of the newborn animals with Co . protoporphyrin increased the activity of the enzyme in the tested organs. The treatment of newborn animals with Fe . protoporphyrin (heme) also increased heme oxygenase activity in the spleen and the kidney. In addition, Co . and Fe . protoporphyrin complexes inhibited the activity of delta-aminolevulinate synthetase in the spleen; Sn . protoporphyrin and Zn . protoporphyrin, however, did not alter the activity of this enzyme. The effects of Co . protoporphyrin and Zn. protoporphyrin on the microsomal contents of cytochromes P-450, b5, the total heme, and the microsomal drug metabolism activity in the liver were compared. Zn . protoporphyrin was ineffective in altering the indicated cellular variables. According to these findings Zn . protoporphyrin may be useful as an experimental tool for the selective suppression of heme degradation activity.