Characterization of microsomal electron transport components from control, phenobarbital- and 3-methylcholanthrene-treated mice. II. Improved resolution and quantitation of major components in ammonium sulfate fractions from total liver microsomes.

Quantitation of microsomal components in ammonium sulfate fractions using a high-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis system, and a comparison of these results with those from similar experiments on total liver microsomes has enabled us to identify and better characterize the interactions between microsomal electron transport components. It was found that: (1) phenobarbital decreased the amount of one protein component of approximately 50 000 molecular weight while increasing a component of very similar molecular weight; (2) only two proteins appeared to be associated with CO binding; (3) another protein of approximately 68 000 molecular weight, one of the glycoproteins found in liver microsomes, appears to be induced by phenobarbital pretreatment; (4) the induction of NADPH-cytochrome c reductase activity after phenobarbital pretreatment is not dependent on an increase in the known NADPH-dependent flavoprotein, but rather on the increase in some component found predominately in our most soluble sub-microsomal fraction. A very good separation of the above components was achieved by ammonium sulfate fractionation, e.g. simply on the basis of their solubility. This and the fact that the more-or-less soluble proteins were induced by phenobarbital or 3-methylcholanthrene respectively indicate that the solubility of membrane proteins plays a major role in the structure and function of microsomal membranes.     

Solubilization and reconstruction of microsomal AMP-deaminase from skeletal muscles]

Microsomal AMP-deaminase was solubilized by 0.5 M KCl after treatment of microsomal membranes with 0.12 M KCl. Using disc-electrophoresis in polyacrylamide gel in the presence of sodium dodecyl sulfate one major protein component (mol. weight about 90 000) and three minor ones with molecular weights of 110 000, 80 000, and 60 000 were found in the soluble fraction. In addition to proteins, the fraction was found in the soluble fraction. In addition to proteins, the fraction was found to contain a small amount of phospholipids. The deaminase found in the solution may be reconstructed into the membranes at a decrease in KCl concentration, part of enzyme being bound in the inactive form under excess of the soluble fraction. Deaminase binding to the membranes is unaffected by the changes within the pH range of 6.2--7.8 and temperature range of 4--10 degrees C. It is assumed that AMP-deaminase is bound to other membrane components by electrostatic bonds.     

Liver cytosolic non-dialysable factor(s) can counteract GTP-dependent Ca2+ release in rat liver microsomal fractions

Readdition to rat liver microsomes of dialysed liver post-microsomal supernatant resulted in an almost complete inhibition of the Ca2+-releasing effect of GTP. Such inhibition was heat-labile, and was associated with non-ultrafiltrable supernatant components with a molecular weight higher than 30,000 D. A preliminary fractionation of liver supernatant showed that the inhibitory effect is recovered in the 40-50% ammonium sulfate-precipitated proteins, with an approx. 10-fold enrichment. The active ammonium sulfate fraction did not modify the GTP-induced Ca2+ increase of passive Ca2+ efflux from microsomes, nor did it affect microsomal GTP hydrolysis, which is likely required for its Ca2+ releasing effect. The active ammonium sulfate fraction appears to markedly favour the translocation of GTP-released Ca2+ into a microsomal GTP-insensitive pool. Separation of liver microsomes in smooth and rough fractions revealed that such GTP-insensitive Ca2+ pool is almost completely associated with smooth microsomes.     

Biogenesis of microsomal membrane glycoproteins in rat liver. II. Purification of soluble glycoproteins and their incorporation into microsomal membranes

Sialoproteins isolated from the soluble fraction of rat liver could be incorporated into microsomal membranes. This incorporation was dependent on protein concentration, time, and temperature. Sodium dodecyl sulfate gel electrophoresis of membrane proteins after in vitro incorporation showed four major sugar-containing peaks and was similar to that found after in vivo labeling. Most of the incorporated protein was tightly bound to the microsomal membrane. Gel filtration and ion-exchange chromatography revealed the presence of several cytosolic glycoproteins that could be incorporated into microsomes. During prolonged centrifugation in a KBr solution with a density of 1.21 a highly labeled ([3H]glucosamine) protein (mole wt approximately to 70,000) that was actively incorporated into microsomes could be recovered in the upper region of the tube. These results demonstrate that several cytoplasmic glycoproteins of rat liver are transferred into microsomal membranes and that one of these is a lipoprotein.     

Transfer of phosphatidic acid between microsomal and mitochondrial outer and inner membranes

A protein fraction from rat liver cytoplasm, precipitable at 50-95% saturation of ammonium sulphate, binds phosphatidic acid from mitochondrial and microsomal membranes. Protein-bound phosphatidic acid was eluted from Sephadex G-75 in fractions corresponding to a molecular weight of about 10 000. No such binding was observed with mitochondrial soluble proteins, either total or precipitated with ammonium sulphate between 50 and 95% saturation. The transfer of phosphatidic acid from microsomes to mitochondria was increased by liver cytoplasmic proteins precipitable at 50-95% saturation of ammonium sulphate but not with mitochondrial soluble proteins. This increase by cytoplasmic proteins was pronounced in 200 mM sucrose but was negligible in 100 mM KCI where the spontaneous transfer was quite high. Cytoplasmic proteins stimulated the synthesis of cardiolipin and phosphatidylglycerol in mitochondria deprived of the outer membrane but not in intact mitochondria when phosphatidic acid was supplied either by microsomes or liposomes. It is suggested that the transfer of phosphatidic acid from the outer to the inner mitochondrial membrane is not mediated by transfer proteins but occurs either by direct contact of the membranes or as free diffusion through the aqueous phase.     

The rabbit pulmonary monooxygenase system. Immunochemical and biochemical characterization of enzyme components.

The enzymatic components of the rabbit pulmonary monooxygenase system, cytochromes P-450I and P-450II and NADPH-cytochrome P-450 reductase, are immunochemically distinct proteins. In pulmonary microsomes, the N-demethylation of benzphetamine, amino-pyrine, and ethylmorphine, and the O-deethylation of 7-ethoxycoumarin are dependent only on cytochrome P-450I, and the hydroxylation of coumarin is apparently catalyzed by both cytochromes. Cytochrome P-450II is immunochemically distinct from the major forms of hepatic cytochrome P-450 induced by phenobarbital or 3-methylcholanthrene, whereas cytochrome P-450I is indistinguishable from the former on the basis of physical and catalytic as well as immunochemical characteristics. Pulmonary and hepatic NADPH-cytochrome P-450 reductases also have identical physical, catalytic, and immunochemical properties. The lack of response of the lung monooxygenase system to phenobarbital, therefore, is apparently not due to an inability of the lung to synthesize the enzymes induced by phenobarbital in the liver. The relatively high proportion of cytochrome P-450I in the lung appears to be responsible for the higher rates (per nmol of P-450) of N-demethylation that have been observed in rabbit pulmonary as compared to hepatic microsomal fractions.     

Formation, size, and solubility in chloroform/methanol of products of protein synthesis in isolated mitochondria of rat liver and Zajdela hepatoma.

The number, size, solubility in chloroform/methanol and some aspects of the formation of the components labeled by radioactive amino acids in isolated mitochondria of rat liver and Zajdela hepatoma were studied. Isolated mitochondria were labeled with radioactive amino acids under various conditions, and the distribution of radioactivity in sodium dodecylsulfate-polyacrylamide gels after electrophoresis of mitochondrial membrane fraction was analysed. 1. Isolated mitochondria of rat liver and Zajdela hepatoma incroporated radioactive amino acids almost exclusively into the membrane fraction. Electrophoretic analysis of this fraction revealed the presence of 15 distinct peaks of radioactivity with corresponding apparent molecular weights of 10 000 to 58 000. The electrophoretic mobility of the labeled components was identical and the general pattern of the radioactivity distribution in the gel for the rat liver and the tumour mitochondria was very similar. 2. Components of the membrane fraction of rat liver mitochondria labeled in vitro displayed an unequal solubility in acidic (2 mM HC1) chloroform/methanol (2/1) mixture; as detected by sodium dodecylsulfate-polyacrylamide gel electrophoresis a single labeled component with apparent molecular weight of 10 000 was soluble in neutral chloroform/methanol. 3. Inverse relation was observed between amino acid incorporation activity of isolated mitochondria and the portion of the label incorporated into the component with apparent molecular weight 10 000. The identity of this component with that soluble in neutral chloroform/methanol mixture has been indicated. 4. The rate of incorporation of [3H]leucine by isolated Zajdela hepatoma mitochondria into the components with lower (10 000-25 000) apparent molecular weights decreased with time, whereas that into components with higher (above 25 000) apparent molecular weight remained approximately constant within the time interval tested (30 min). 5. From the total radioactivity incorporated into the membrane fraction during 5-min pulse labeling of isolated Zajdela hepatoma mitochondria by [3H]leucine up to 25% was recovered in the region of the gel corresponding to a component with apparent molecular weight 10 000. After 25 min chase the radioactivity in this region decreased about 3.5 times while the specific radioactivity of the total membrane fraction did not change significantly. The pattern of radioactivity distribution observed after the pulse was preserved by chloramphenicol. 6. Unlabeled sonicated mitochondria or postribosomal supernatant from rat liver regenerating in the presence of chloramphenicol were incubated with neutral chloroform/methanol extract of in vitro with [14C]leucine labeled rat liver mitochondria. After this incubation several labeled components with apparent molecular weights above 10 000 were recovered in the electrophoreograms of the originally unlabeled fractions.     

Failure of expression of the phenobarbital-induced enhancement of UDP-glycosyltransferases in native, sealed endoplasmic reticulum vesicles from rat liver.

Conflicting data have been published regarding the effects of phenobarbital treatment on bilirubin UDP-glucuronyltransferase activity in native liver microsomes. Recent evidence suggests that the bilirubin UDP-glycosyltransferase system faces the interior of microsomal vesicles, and that expression of its activities in sealed microsomes may be rate-limited by transport of UDP sugars across the membrane. These observations raise the possibility that the reported variability in the effects of phenobarbital may reflect differences in integrity of the membrane in microsomal preparations. We examined the effect of phenobarbital on bilirubin UDP-glucosyltransferase and the UDP-glucuronyltransferase activities towards bilirubin, 4-nitrophenol, and 1-naphthol using native rat liver microsomes with verified vesicle integrity. Phenobarbital-induced microsomes in which the membrane permeability barrier was eliminated by pretreatment with detergent displayed markedly higher UDP-glycosyltransferase activities towards all tested substrates compared with activities in similarly disrupted microsomes from untreated rats. In contrast, none of the transferase activities tested were significantly enhanced by phenobarbital treatment when the enzymic activities were assayed in sealed microsomes. Addition to the enzyme assay mixture of UDPGlcNAc, a presumed physiological activator of the UDP-glucuronyltransferases, failed to expose the enhanced UDP-glucuronyltransferase concentration in phenobarbital-induced sealed microsomes. Our findings are consistent with the idea that transport of UDP sugar across the membrane may be rate-limiting for expression of UDP-glycosyltransferase activities in sealed microsomes. Quantitative assessment of membrane integrity is an essential prerequisite in experiments designed to study the regulation of the microsomal UDP-glycosyltransferase system.     

Purification and characterization of rat hepatic microsomal low molecular weight fatty acid ethyl ester synthase and its relationship to carboxylesterases.

We reported purification of a high molecular weight (HMW) (ca. 180 kD) and a low molecular weight (LMW) (ca. 60 kD) protein fractions from digitonized rat liver microsomes using ammonium sulfate precipitation followed by ion exchange and gel filtration column chromatography. Both fractions expressed fatty acid ethyl ester (FAEE) synthase as well as p-nitrophenyl acetate (PNPA)-hydrolyzing (esterase) activities. The HMW fraction was found to be a trimer with subunit molecular weight ca. 60 kD and structurally and functionally similar to rat hepatic microsomal carboxylesterase (CE, pI 6.1) and adipose tissue FAEE synthase. In this article, we report further purification and characterization of the LMW (minor) fraction expressing FAEE synthase activity and its structural and functional relationship to hepatic microsomal CEs. Using isoelectric focusing (IEF) followed by gel filtration-high-performance liquid chromatography (GF-HPLC), five proteins were purified, which expressed FAEE synthase as well as PNPA-hydrolyzing activity. The isoelectric point values of 6.5, 5.8, 5.6, 5.3, and 5.0 were found for the purified LMW proteins by IEF and each showed a peak corresponding to ca. 60 kD molecular weight by GF-HPLC, which expressed FAEE synthase as well as PNPA-hydrolyzing activity. Sodium dodecyl sulfate-polyacrylamide gel elecrophoresis (SDS-PAGE) analysis of the GF-HPLC purified LMW proteins revealed that these proteins are monomers (ca. 60 kD). All the purified LMW proteins cross-reacted with antibodies to rat adipose tissue FAEE synthase. Coelution of PNPA-hydrolyzing and FAEE synthase activity at each step of purification and cross-reactivity with rat adipose tissue FAEE synthase antibodies suggest that the purified proteins are related to various hepatic microsomal CEs. This conclusion is further supported by the homology of N-terminal amino acid sequence of the purified LMW proteins to various hepatic microsomal CEs and protease precursors. Therefore, LMW FAEE synthase activity most probably is expressed by various isozymes of hepatic microsomal CEs, which are also involved in the biotransformation of xenobiotic alcohols and amines.     

Characterization of microsomal electron transport components from control,phenobarbital, and 3-methylcholanthrene treated mice: I. Distribution of electron transport components in ammonium-sulfate fractions from mouse liver microsomes

Using a modified ammonium sulfate fractionation procedure, seven major components were found in various submicrosomal fractions. Four of these proteins could be assigned to known components of the microsomal electron transport system, cytochromes P-450 and b₅, and the NADH- and NADPH-cytochrome c reductases. The similarity of this system to that seen in the separation of mitochondrial enzymes suggests that the lipo-protein complexes of these fractions are particulate in nature and represent functional subunits of the microsomal membrane.     

Microsomal target proteins of metabolically activated aromatic hydrocarbons.

The specificity of binding to microsomal proteins of metabolically activated hydrocarbons has been studied. Radioactively labelled benzene, phenol, chlorobenzene, BP and MC were incubated with liver microsomes from control, phenobarbital- and MC-treated rats in the presence of an NADPH-generating system. The patterns of metabolite binding to microsomal proteins were examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and fluorography. Benzene, phenol and chlorobenzene metabolites showed one type of binding pattern dominated by a band at 72 000 Mr. This band was strong both in control and induced microsomes. Additional radioactive bands were seen in the 50 000--60 000 Mr region particularly in MC-induced microsomes. BP and MC metabolites showed a different type of binding pattern with incorporation of radioactivity into several fractions in the 50 000--60 000 Mr region of MC-induced microsomes. Two other strongly labelled fractions occurred at 68 000 and 72 000 Mr. The incorporation was low into control and phenobarbital-induced microsomes. Two labelled bands (Mr 56 000 and 72 000) were common for all hydrocarbons in MC-induced microsomes. The 56 000 Mr band had the same mobility in the gel as an MC-induced protein likely to be cytochrome P-448. The NADPH-generating system was essential for metabolite binding and GSH and UDPGA greatly reduced binding. We suggest that differences in metabolite binding patterns reflect differences in the routes of metabolite formation and that activated hydrocarbons are likely to bind to proteins close to their site of formation.     

ON THE SPATIAL ARRANGEMENT OF PROTEINS IN MICROSOMAL MEMBRANES FROM RAT LIVER

Rat liver rough microsomes were labeled enzymatically with ¹²⁵I using lactoperoxidase and glucose oxidase. In intact microsomes only proteins exposed on the outside face of the microsomal membrane were iodinated. Low concentrations of detergent (0.049% deoxycholate) were used to allow entrance of the iodination system into the vesicles without disassembling the membranes. This led to iodination of the soluble content proteins and to an increased labeling of the membrane proteins. The distribution of radioactivity in microsomal proteins was analyzed after separation by sodium dodecyl sulfate acrylamide gel electrophoresis. Most membrane proteins were labeled when intact microsomes were iodinated. No major membrane proteins were exclusively labeled in the presence of low detergent concentrations or after complete membrane disassembly. Therefore it is unlikely that there are major membrane proteins, other than glycoproteins, present only on the inner membrane face or completely embedded within the microsomal membrane. Microsomal proteins were also labeled by incubating rough microsomes with [³H]-NaBH₄ after reaction with pyridoxal phosphate. Microsomal membranes were permeable to these small molecular weight reagents as shown by the fact that proteins in the vesicular cavity as well as membrane proteins were labeled with this system.     

Effects of inducers on the regio- and stereoselective metabolism of benzo[a]pyrene by mouse tissue microsomes

The metabolism of benzo[a]pyrene (BP) by microsomal fractions of the skin, lungs and liver of the mouse, and the effects on this process of pretreatment with the xenobiotics phenobarbital (PB) and 3-methylcholanthrene (3-MC) were examined. Differences between the untreated tissues were found both in terms of the total amounts of diol recovered and in the relative proportions of the individual diols extracted following incubation. Induction with PB or 3-MC significantly altered the profiles of metabolic diols obtained with epidermal and hepatic microsomes compared with their respective controls. Pulmonary microsomes showed similar trends to those obtained with liver microsomes but these were not statistically significant. The optical purity of the BP-7,8-diol that was formed by each microsomal type was examined by direct resolution of the enantiomers on HPLC using a chiral stationary phase. In each case the (-)-7R,8R-enantiomer predominated. Pretreatment with 3-MC significantly decreased the optical purity of BP-7,8-diol recovered from incubations with skin microsomes, but significantly increased the optical purity of the diol extracted from incubations with lung and liver microsomes. In addition to the diols, an unidentified BP metabolite was found that eluted between BP-9,10- and 4,5-diol on a reverse-phase high-performance liquid chromatography (HPLC) system and which represented a major product in extracts of incubations of BP with both induced and uninduced skin and lung microsomal fractions.     

A simple and rapid procedure for the purification of phenobarbital-inducible cytochrome P-450 from rat liver microsomes.

A rapid and simple procedure has been developed for the purification of a phenobarbital-inducible form of cytochrome P-450 from the liver microsomes of phenobarbitalpretreated rats. Within 2 days approximately 1000–1500 nmol of highly purified cytochrome P-450 with a specific content of 16 nmol/mg protein can be recovered from 4 g of microsomal protein. The procedure consists of solubilization of microsomal protein with sodium cholate, fractionation with polyethylene glycol, and column chromatography at room temperature on DEAE-cellulose. The resulting DEAE-cellulose fraction electrophoreses on polyacrylamide gels in the presence of sodium dodecyl sulfate as a major protein band with a minimum molecular weight of 52,000 and a few faint bands. Further chromatography on QAE Sephadex A-25 essentially removes these faint bands and increases the specific content slightly to 17 nmol/mg protein. Relatively low amounts of this form of cytochrome P-450 appear to be present in microsomes of untreated rats since less than 1% can be recovered as the DEAE-cellulose fraction by this procedure. An identical form is inducible by phenobarbital in rats of different ages and sex. In a reconstituted system under optimal assay conditions, this form of cytochrome P-450 catalyses the N-demethylation of benzphetamine with a turnover number greater than 100 and hydroxylates testosterone at the 16α position but not at the 6β or 7α position.     

An electrophoretic analysis of proteolipids from different rat brain subcellular fractions

Proteolipid proteins were extracted from adult rat brain subcellular fractions and purified by chromatography on Sephadex LH-60. Polyacrylamide gel electrophoresis of the delipidized proteins, in the presence or absence of 8 M urea, was carried out with all fractions. The distribution of the various types of proteolipid proteins was studied and their molecular weight calculated by the Ferguson relationship. Several bands of proteolipid proteins were found in the five membrane fractions analyzed. Some of them, such as the 17.5 K and 37 K components were very prominent in mitochondria and synaptosomes. The 30 K component was found in myelin-derived membranes and in microsomes, while the 20 K and 25 K proteolipid proteins were present in all subcellular fractions. The 30 K component (proteolipid protein (PLP)), typical of the purified myelin membranes, showed a similar distribution to that of 2′,3′-cyclic-nucleotide 3′-phosphohydrolase (EC 3.1.4.37) activity, while the other major proteolipid protein present in all subcellular fractions (25 K) did not show such parallelism, indicating that it might not be an exclusive component of myelin. The electrophoretic pattern of microsomal proteolipid proteins did not show the high molecular weight components (aggregates of PLP) which are found in myelin. Furthermore, the 30 K component showed a smaller $\text{Y}{}_0$ value than that of the 30 K found in myelin. Thus the presence of 30 K proteolipid protein in microsomes should not be considered as being due to myelin contamination.     

Induction of drug metabolism enzymes by dihalogenated biphenyls

The effects of pretreatment with symmetrically dihalogenated biphenyls (DXBs, X-F, Cl(C), Br(B) and I) on rat liver drug metabolism enzymes were investigated. 4,4'-DFB, -DCB, and -DBB as well as 2,2'-DFB appeared to be inducers of microsomal cytochrome P-450-linked monoxygenases (N-demethylases of aminopyrine and ethylmorphine). However, no structure-induction relationship was found. 4,4'-DXBs also induced a cytochrome P-448-linked mono-oxygenase (ethoxyresorufin O-deethylase), and their order of induction potential seemed to parallel the increase of the size of the halogen substituent. Therefore, 4,4'-DXB's may be categorized as mixed-type inducers, the cytochrome P-450 component being the more pronounced. Data on the cytochrome P-448 induction by dihalogenated biphenyls with only para substituents may be considered as a refinement of the previously described structure-activity relationship in this respect. All of the DXBs except 3,3'-DCB and 4,4'-DIB, enhanced, like phenobarbital, the activity of UDP-glucuronyltransferase toward 4-hydroxybiphenyl. Only 4,4'-DFB was able to induce the activity of glutathione S-transferase toward 1,2-epoxy-3-(p-nitrophenoxy)propane. Studies after 4,4'-DBB-treatment revealed, like phenobarbital, a preferential induction of ethylmorphine N-demethylase on rough endoplasmic reticulum-derived microsomes, whereas UDP-glucuronyltransferase activity toward 4-hydroxybiphenyl was induced to a larger extent on smooth endoplasmic reticulum microsomes, suggesting a dissimilar enzyme induction in microsomal subfractions.     

A dehydroepiandrosterone sulfatase inhibitory activity in soluble proteins of guinea pig liver

An inhibitor of microsomal dehydroepiandrosterone sulfatase was found in the soluble fraction of non-pregnant guinea pig liver. The extent of inhibitory effect was dependent on the concentration of soluble proteins. The inhibitor was partly purified by gel permeation and hydroxylapatite chromatography with a purification factor of 16.6. The soluble inhibitor was non-dialyzable, not destroyed by RNase or DNase digestion but totally destroyed by pronase digestion. The inhibitor is a soluble protein with a molecular weight of approximately 17,000 (determined by gel permeation chromatography). Inhibition of microsomal dehydroepiandrosterone sulfatase by the soluble inhibitor is a non-competitive inhibition. From this present finding the question arises whether the inhibitor could be involved in the regulation of the hydrolysis of dehydroepiandrosterone sulfate in the guinea pig liver.     

Thyroid microsomal membrane proteins. Effects of solubilization on molecular size.

The molecular size of microsomal membrane proteins from frozen porcine thyroids before and after solubilization by proteolytic and non-proteolytic techniques has been investigated by means of polyacrylamide-gel electrophoresis in the presence of 1% sodium dodecylsulfate. When thyroid microsomal membrane proteins are solubilized by non-proteolytic methods such as high pH, n-butanol, or deoxycholate, no major change in the electrophoretic pattern compared to untreated microsomes has been observed, thereby suggesting that these non-proteolytic methods are capable of extracting membrane proteins from thyroid microsomes without altering their molecular size. However, treatment of microsomes with protein-solubilizing levels of trypsin (1-5 mug trypsin per mg thyroid protein) results in degradation of all major proteins with a molecular weight greater than 30 000. The high-molecular-weight proteins are particularly susceptible to attack by trypsin. Thus, these experiments indicate that the use of trypsin to solubilize thyroid microsomal membrane proteins, particularly thyroid peroxidase, will result in fragmented proteins and should be avoided if intact membrane proteins are desired.     

Induction by Triacetyloleandomycin and partial purification of a LM3 form of cytochrome P-450 from rabbit liver microsomes

Treatment of rabbits with Triacetyloleandomycin (a currently used antibiotic in human therapy) at 1 mmol per kg of body weight daily for 5 days results in a significant induction of liver microsomal cytochrome P-450, (2.6 nmol/mg proteins). Electrophoresis in SDS polyacrylamide gels shows this increase in P-450 is associated to the appearance of a strong band in a zone located between the major bands of microsomes induced by phenobarbital and β-naphtoflavone (LM₃ forms in Coon's terminology). Partial purification of this P-450 LM₃ (TAO) was undertaken by chromatographic procedures (CM cellulose and hydroxylapatite). Its subunit molecular weight is 52 000; the absolute spectra in the oxidized, ferrous and CO-ferrous forms present maxima at 417, 536, and 570 nm; 415 and 548 nm; 450 and 555 nm respectively. Monooxygenase activity of LM₃ (TAO) was compared with that of LM₂ and LM₄ in a reconstituted system containing NADPH cytochrome P-450 reductase and phosphatidylcholine; the activity of P-450 LM₃ (TAO) was higher than that of LM₂ and LM₄ with chlorcyclizine as a substrate. According to these observations, LM₃ (TAO) resembles LM₃ (b), a constitutive form of untreated rabbit liver microsomes.     

Cross-linking studies of the protein topography of rat liver microsomes

A cleavable cross-linking reagent, dithiobis(succinimidyl propionate), DSP, was used to study the topography of the proteins in the endoplasmic reticulum membrane of rat liver. Reaction of untreated (control), phenobarbital- or 3-methylcholanthrene-induced microsomes with 0.5 mM DSP for 30 min at 0°C resulted in the cross-linking of a protein with a molecular weight of about 52 000 to form an apparent dimer. In phenobarbital microsomes, a smaller amount of a 52 000-dalton protein also appeared in a dimer in the absence of DSP if N-ethylmaleimide was not included during homogenization. In phenobarbital and 3-methylcholanthrene microsomes, a 48 000-dalton protein was cross-linked by DSP to a protein of about 57 000. In all three types of microsomes, a protein with an M_I of about 52 000 was also cross-linked to a protein of about 79 000. In phenobarbital and control microsomes, cross-linking resulted in an oligomeric protein of approximate molecular weight 180 000 which contained three proteins, two with M_r of about 52 000 and one about 79 000. Under the cross-linking conditions, little or no denaturation of cytochrome P-450 and NADPH-cytochrome c reductase was observed. The aryl hydrocarbon hydroxylase activity was significantly inhibited by the bifunctional cross-linking reagent, DSP, but not by the monofunctional reagent N-succinimidyl-3-(4-hydroxyphenyl) propionate. However, attempts to regenerate the aryl hydrocarbon hydroxylase activity by cleavage of the disulfide linkage with 2-mercapto-ethanol or dithiothreitol were not successful.