Highly purified cytochrome P-448 and P=450 from rat liver microsomes

The induction of hepatic and extrahepatic xenobiotic metabolism in the rat and ferret by a polychlorinated biphenyl mixture (Aroclor 1254)

1. The effect of a single i.p. dose (500 mg/kg) of a polychlorinated biphenyl mixture (Aroclor 1254) on hepatic and extrahepatic xenobiotic metabolism in male rat, and male and female ferret, was studied. 2. Aroclor 1254 treatment induced hepatic microsomal N-demethylase activities, and cytochrome P-450 and protein content in both rat and ferret. Liver size and aniline 4-hydroxylase were also increased in rat, but not ferret. The polychlorinated biphenyl mixture appeared to be a mixed-type inducer of hepatic xenobiotic metabolism in both species. 3. Aroclor 1254 treatment produced large increases in activities of benzo(a)pyrene hydroxylase and 7-ethoxycoumarin O-deethylase in whole homogenates of the liver, small intestinal mucosa, kidneys and lungs of both species. Maximal stimulation of xenobiotic metabolism occurred in the kidney of both the rat and ferret. In contrast, UDP-glucuronyltransferase activity was only stimulated in liver, intestine and kidney of the rat and in liver and intestine of the ferret. 4. These results suggest a general species similarity in the response to the polychlorinated biphenyl mixture between the rat and ferret.

Possible correlation between induction modes of hepatic enzymes by PCBs and their toxicity in rats

Acute toxicity of individual PCBs, which were categorized as either phenobarbital (PB)- or 3-methylcholanthrene (MC)-type inducers, was examined in young male Wistar rats, comparing their effects on growth rate, organ weight and liver lipid content, 5 days after a single i.p. injection. PB-type PCBs (2,4,3',4'- and 2,5,2'5'-tetrachlorobiphenyl), which slightly increased a content of cytochrome P450, did not show any significant toxicity at a dose of 100 mg/kg. On the contrary, MC-type PCBs (3,4,5,3',4'-pentachloro- and 3,4,5,3',4',5'-hexachlorobiphenyl), which markedly increased a content of cytochrome P448, strongly reduced growth rate and weights of thymus and spleen at a dose of 10 mg/kg. Liver enlargement accompanied by fatty liver was also observed only with MC-type PCBs. 3,4,3',4'-Tetrachlorobiphenyl was also toxic at a dose of 50 mg/kg, in keeping with its weak MC-type-inducing ability. Pretreatment with MC affected neither growth rate, spleen weight, nor liver lipid content. These results suggest that the toxic potency of PCBs is related to their MC-type inducing ability, but the toxic characteristics are different from those of MC itself.

The role of components of the endoplasmic reticulum in the biosynthesis of cytochrome P-450

1. Antibodies have been prepared to rat hepatic cytochrome P-450 and their specificity demonstrated. These antibodies have been used to investigate the biosynthesis of cytochrome P-450 in vitro and in situ in various components of the endoplasmic reticulum. 2. A preparation of heavy rough endoplasmic reticulum translocates proteins newly biosynthesized in vitro vectorially into the luminal space and these are released by low concentrations of deoxycholate. A significant proportion of the radioactivity found in this released fraction is incorporated into cytochrome P-450. 3. Following incorporation of [14C]leucine by perfused rat liver, radioactively labelled cytochrome P-450 can be found in the intrascisternal content of heavy rough, light rough and smooth endopalsmic reticulum and also in a solublized Golgi preparation. 4. We suggest that at least part of the newly biosynthesized cytochrome P-450 is translocated into the intracisternal space of the rough endoplasmic and then passes through the other components of the endoplasmic reticulum before insertion at its ultimate membrane locus.

Interaction between NADPH-cytochrome P-450 reductase and cytochrome P-450 in the membrane of phosphatidylcholine vesicles

Cytochrome P-450 and NADPH-cytochrome P-450 REDUctase, both purified from liver microsomes of phenobarbital-pretreated rabbits, have been incorporated into the membrane of phosphoaditylcholine vesicles by the cholate dialysis method. The reduction of cytochrome P-450 by NADPH in this system is biphasic, consisting of two first-order reactions. The rate constant of the fast phase, in which 80--90% of the total cytochrome is reduced, increases as the molar ratio of the reductase to the cytochrome is increased at a fixed ratio of the cytochrome to phosphatidylcholine, suggesting that the rate-limiting step of the fast phase is the interaction between the reductase and the cytochrome. The rate constant of the fast phase also increases when the amount of phosphatidylcholine, relative to those of the two proteins, is decreased. This latter observation suggests that the interaction between the two proteins is effected by their random collision caused by their lateral mobilities on the plane of the membrane of phosphatidylcholine vesicles. The rate constant of the slow phase as well as the fraction of cytochrome P-450 reducible in the slow phase, on the other hand, remains essentially constant even upon alteration in the ratio of the reductase to the cytochrome or in that of the two proteins to phosphatidylcholine. No satisfactory explanation is as yet available for the cause of the slow-phase reduction of cytochrome P-450. The overall activity of benzphetamine N-demethylation catalyzed by the reconstituted vesicles responds to changes in the composition of the sysTEM IN A SIMILAR WAY TO THE FAST-PHASE REDUCTION OF CYTOCHROME P-450, though the latter is not the rate-limiting step of the overall reaction.

The drug metabolism systems of liver and liver tumors: a comparison of activities and characteristics

Transplantable rat liver tumors 5123 t.c., 7288 ct.c., 5123 t.c.(H) and the Novikoff hepatoma have active mixed function oxidase systems capable of metabolizing a variety of drug and polycyclic hydrocarbon substrates. The tumor drug metabolism systems are at best 20% as active as rat liver. The tumor drug metabolism activities are induced by pretreatment with phenobarbital or beta-naphthoflavone and can be inhibited with specific inhibitors such as carbon monoxide or 7,8-benzoflavone. Tumor drug metabolism systems appear to consist of cytochrome P-450 and cytochrome P-450 reductase. The properties of the two protein components from tumors are highly similar to the corresponding components of the liver drug metabolism system. Cytochrome P-450 reductase has been at least partially purified from the Novikoff hepatoma and hepatoma 5123 t.c.(H). The kinetic and physical properties of the tumor reductases are similar to those of the liver reductase except that the Km of hepatoma 5123 t.c.(H) reductase, but not of the Novikoff hepatoma reductase for NADPH, is elevated an order of magnitude over the Km of the liver reductase. The mechanism for the interaction of electron donor and electron acceptor with liver or tumor reductases seems to be a sequential reaction mechanism. Experiments on the NADP-inhibition of the interaction of NADPH and cytochrome c with liver reductase indicate that NADP is competitive with NADPH and noncompetitive with cytochrome c. This result is consistent with the postulate of a sequential reaction for NADPH-cytochrome P-450 reductases of liver and tumors. These data support the conclusions that an active drug metabolism system is present in liver tumors and that the tumor systems are constituted like the liver system.

Spatial orientation of glycoproteins in membranes of rat liver rough microsomes. II. Transmembrane disposition and characterization of glycoproteins

Rat liver microsomal glycoproteins were purified by affinity chromatography on concanavalin A Sepharose columns from membrane and content fractions, separated from rough microsomes (RM) treated with low concentrations of deoxycholate (DOC). All periodic acid-Schiff (PAS)-positive glycoproteins of RM showed affinity for concanavalin A Sepharose; even after sodium dodecyl sulfate (SDS) acrylamide gel electrophoresis, most of the microsomal glycoproteins bound [125I]concanavalin A added to the gels, as detected by autoradiography. Two distinct sets of glycoproteins are present in the membrane and content fractions derived from RM. SDS acrylamide gel electrophoresis showed that RM membranes contain 15--20 glycoproteins (15--22% of the total microsomal protein) which range in apparent mol wt from 23,000 to 240,000 daltons. A smaller set of glycoproteins (five to seven polypeptides), with apparent mol wt between 60,000 and 200,000 daltons, was present in the microsomal content fraction. The disposition of the membrane glycoproteins with respect to the membrane plane was determined by selective iodination with the lactoperoxidase (LPO) technique. Intact RM were labeled on their outer face with 131I and, after opening of the vesicles with 0.05% DOC, in both faces with 125I. An analysis of iodination ratios for individual proteins separated electrophoretically showed that in most membrane glycoproteins, tyrosine residues are predominantly exposed on the luminal face of the vesicles, which is the same face on which the carbohydrate moieties are exposed. Several membrane glycoproteins are also exposed on the cytoplasmic surface and therefore have a transmembrane disposition. In this study, ribophorins I and II, two integral membrane proteins (mol wt 65,000 and 63,000) characteristic of RM, were found to be transmembrane glycoproteins. It is suggested that the transmembrane disposition of the ribophorins may be related to their possible role in ribosome binding and in the vectorial transfer of nascent polypeptides into the microsomal lumen.

The rabbit pulmonary monooxygenase system: characteristics and activities of two forms of pulmonary cytochrome P-450

Two forms of rabbit pulmonary cytochrome P-450 have been characterized spectrally and their activities in reconstituted monooxygenase systems investigated. The presence of both microsomal phospholipids and sodium cholate was required to obtain optimum activity. Only one of the cytochromes (I) was active in the N-demethylation of benzphetamine and the O-deethylation of 7-ethoxycoumarin. However, cytochrome II was 20% more active than cytochrome I in the metabolism of benzo[a]pyrene. The profile of the metabolites formed from benzo[a]pyrene indicated that metabolism at the 9 and 10 positions was insignificant in the case of cytochrome I but represented about 40% of the metabolites produced by cytochrome II. The two forms of the cytochrome are present in pulmonary microsomes in approximately equal amounts.