Delta-aminolevulinic Acid Synthetase Activity Research Articles

Preclinical Development of a Subcutaneous ALAS1 RNAi Therapeutic for Treatment of Hepatic Porphyrias Using Circulating RNA Quantification.

The acute hepatic porphyrias are caused by inherited enzymatic deficiencies in the heme biosynthesis pathway. Induction of the first enzyme 5-aminolevulinic acid synthase 1 (ALAS1) by triggers such as fasting or drug exposure can lead to accumulation of neurotoxic heme intermediates that cause disease symptoms. We have demonstrated that hepatic ALAS1 silencing using siRNA in a lipid nanoparticle effectively prevents and treats induced attacks in a mouse model of acute intermittent porphyria. Herein, we report the development of ALN-AS1, an investigational GalNAc-conjugated RNAi therapeutic targeting ALAS1. One challenge in advancing ALN-AS1 to patients is the inability to detect liver ALAS1 mRNA in the absence of liver biopsies. We here describe a less invasive circulating extracellular RNA detection assay to monitor RNAi drug activity in serum and urine. A striking correlation in ALAS1 mRNA was observed across liver, serum, and urine in both rodents and nonhuman primates (NHPs) following treatment with ALN-AS1. Moreover, in donor-matched human urine and serum, we demonstrate a notable correspondence in ALAS1 levels, minimal interday assay variability, low interpatient variability from serial sample collections, and the ability to distinguish between healthy volunteers and porphyria patients with induced ALAS1 levels. The collective data highlight the potential utility of this assay in the clinical development of ALN-AS1, and in broadening our understanding of acute hepatic porphyrias disease pathophysiology.

Toxic effects of arsenic (III) on some hematopoietic and central nervous system variables in rats and guinea pigs.

To evaluate the effects of arsenic (III) exposure on porphyrin metabolism and the central nervous system supplemented with data on the effect of hepatic and renal tissues of rats and guinea pigs. Rats and guinea pigs were exposed to 10 or 25 ppm arsenic in drinking water for 16 weeks. Following chronic arsenic (III) exposure, delta-aminolevulinic acid dehydratase activity in blood showed a significant reduction as did the total cell counts (RBC and WBC) and reduced glutathione with increased urinary delta-aminolevulinic acid. Zinc protoporphyrin, a sensitive indicator of iron deficiency and impairment of heme biosynthesis, showed a significant increase in arsenic exposure. The hepatic delta-aminolevulinic acid dehydratase and delta-aminolevulinic acid synthetase activity increased in chronic arsenic (III) exposure in rats and guinea pigs. Significant changes in the steady-state level of three major neurotransmitters, dopamine, norepinephrine, and 5-hydroxytryptamine, and monoamine oxidase were observed following chronic arsenic (III) exposure. At low doses (10 and 25 ppm in drinking water), the effects of arsenic on hematopoietic indices and whole-brain neurotransmitter concentrations were more prominent in guinea pigs than in rats with some variability in the dose response.

Role of interleukin-1beta in the modulations of cytochrome P450 and heme metabolism in rat liver.

The effect of recombinant human interleukin-1beta (IL-1beta) on the modulation of hepatic cytochrome P450 (P450) was investigated by in vivo subcutaneous dosing studies in male Sprague-Dawley rats. To assess the effect of IL-1beta on heme metabolism, we determined the delta-aminolevulinic acid synthetase (delta-ALAS) and heme oxygenase activities in the liver. IL-1beta suppressed the microsomal total P450 and heme contents and delta-ALAS activity in the liver. In contrast, microsomal heme oxygenase activity was significantly increased by the IL-1beta treatments. Western blot analysis and marker enzyme activities for individual P450 isoforms demonstrated that IL-1beta suppressed CYP2C6, 2C13, 2E1, and 3A2, whereas CYP2A, 2B1/2, 2C11, and 4A1 were not influenced by the treatments. IL-1beta inhibited both allylisopropylamide- and phenobarbital-inducible delta-ALAS activities in the liver. These results indicate that IL-1beta has differential effects on the constitutive P450, and also on delta-ALAS and heme oxygenase activities in rat liver. Thus, the modulation of hepatic P450 by IL-1beta is complex, and IL-1beta may be involved in the regulation of both apoprotein synthesis for each P450 isoform and the heme pools in the liver.

The effect of aluminum chloride on some steps of heme biosynthesis in rats after oral exposure.

Certain disturbances in heme biosynthesis induced by aluminum chloride were examined. The experiment was performed on female rats that received AlCl3 orally at the dose 100 mg Al/kg daily for 21 d. The effects of aluminum on the activity of delta-aminolevulinic acid synthetase (ALA-S), dehydratase (ALA-D), and heme oxygenase (O.H.) were observed on 3, 7, 14, and 21 d in liver and kidneys of rats. Also the activity of ALA-D in blood and the concentration of delta-aminolevulinic acid (ALA-U) in urine were observed. Orally administered aluminum caused increase in the activity of ALA-D in the liver and blood, and parallel decrease of ALA-U in urine (r = -0.85) of rats. Aluminum chloride also induced an increase of ALA-S and O.H. in the liver but not in the kidneys. The changes of the enzymes activity participating in heme biosynthesis after administration of aluminum may be correlated with anemia and iron metabolism in rats.

On the enzyme-inducing action of calcium antagonists.

The effects of three calcium antagonists, nifedipine (NF), verapamil (V) and diltiazem (DL), on rat liver monooxygenases were studied. The drugs were administered in oral doses of 50, 40 and 30 mg/kg daily for 3 weeks in male Wistar rats. NF and V shortened the hexobarbital (HB) sleeping time and increased benzphetamin-N-demethylase (BND), ethylmorphine-N-demethylase (EMND), aniline hydroxylase (AH), ethoxycoumarine-O-deethylase (ECOD), ethoxyresorufin-O-deethylase (EROD) and NADPH-cytochrome c reductase activities and the content of cytochrome P-450 and microsomal heme, but did not change the content of cytochrome b5. The data suggest that these calcium antagonists exert an enzyme-inducing effect on the hepatic monooxygenases. DL significantly increased only the EROD and NADPH-cytochrome c reductase activities and shortened HB sleeping time to a lesser extent, suggesting a weaker enzyme-inducing effect as compared to NF and V. The three drugs increased the delta-aminolevulinic acid (ALA) synthetase activity and decreased heme oxygenase (HO) activity. The increased cytochrome P-450 content is probably due to the increased synthesis and the decreased breakdown of this hemoprotein.

The prolongation effect on the duration of pentobarbital anesthesia by submaxillariectomy in male rats

The present study was performed to investigate the effects of submaxillariectomy (Subx) on the anesthetic effect (sleeping time) of pentobarbital, the concentration-time profile of plasma pentobarbital and the hepatic drug metabolizing enzyme systems in male rats of the Donryu strain. Subx was performed when the rats were 60 days old. Subx prolonged the duration of pentobarbital (30 mg/kg, i.p.) anesthesia at 10, 23 and 43 days after the operation, respectively. At 43 days after Subx, the plasma pentobarbital concentration was higher in the Subx group until 30 min after the administration. There was no significant difference in the hepatic microsomal cytochrome P-450 content between the Subx and control groups. Cytochrome b5 content, aminopyrine N-demethylase and aniline hydroxylase activities were increased significantly in the Subx group. The activity of heme oxygenase, the rate limiting enzyme in the heme catabolic pathway, tended to increase in the Subx group. Furthermore, the activity of delta-aminolevulinic acid synthetase, the rate limiting enzyme in the heme synthetic pathway, was significantly decreased to 51 percent of the control group by Subx. These results may suggest that the prolongation of the duration of pentobarbital anesthesia is caused by Subx due to the change in the pharmacokinetics of pentobarbital, especially the inhibitory effects of the hepatic pentobarbital metabolizing enzyme system.

Disturbances in heme biosynthesis in rabbits after administration per os of low doses of tin or lead.

The experiment was performed on female rabbits that received per os equimolar doses (17 microM Me/kg) of SnCl2 x 2 H2O or Pb (CH3 COO)2 every day for 5 d. The activity of delta-aminolevulinic acid dehydratase (ALA-D) in the whole blood, liver, kidneys, brain, spleen, and bone marrow, concentration of free erythrocyte protoporphyrins (FEP), activity of delta-aminolevulinic acid synthetase (ALA-S) in the liver and bone marrow, urine delta-aminolevulinic acid (ALA-U), and coproporphyrins (CP-U) were determined. Lead and tin concentrations in the blood were estimated. Lead caused a significant inhibition of ALA-D in the blood, increased FEP concentration, and ALA and CP excretion in urine of rabbits. Lead also decreased ALA-D activity in the bone marrow and in the liver, and did not change ALA-S activity in the liver and bone marrow. Tin did not change any of the examined indices. Tin doses applied in the present study, maintained within the limits of permissible standards of metal levels in human diet, did not affect the process of heme biosynthesis in rabbits.

Alterations in hepatic δ-aminolevulinic acid synthetase and heme oxygenase activities after chronic ethanol consumption in rats

In the present study, the effect of chronic ethanol consumption in rats on the hepatic heme metabolism was investigated. Male Wistar rats were fed a nutritionally adequate liquid diet containing ethanol as 36% of the total calories for 5 weeks. After an overnight fast, the livers were excised and centrifuged to obtain mitochondrial and microsomal fractions. Chronic ethanol feeding of rats resulted in about 19% hepatomegaly as represented by the increased liver/body weight ratio. There was no difference in the mitochondrial protein content between the ethanol-treated and control rats, but the microsomal protein content was significantly increased in the ethanol-treated rats. Hepatic microsomal content of cytochrome P-450 (P-450) was markedly enhanced by chronic ethanol ingestion. Microsomal contents of cytochrome b5 (b5) and total heme were also increased to a lesser extent. After chronic ethanol abuse, the hepatic activity of delta-aminolevulinic acid (ALA) synthetase, which is a rate-limiting enzyme for heme production, was significantly increased and that of the heme oxygenase was slightly increased. These data indicate that ALA synthetase activity is induced by the negative feedback mechanism in order to compensate the depletion of heme caused by the utilization of heme for P-450. It is also speculated that, in response to excessive production of heme as described above, heme oxygenase activity is secondarily induced to regulate the amount of heme.

Cloning and nucleotide sequence of the hemA gene of Agrobacterium radiobacter

The hemA gene of Agrobacterium radiobacter ATCC4718 was identified by hybridization with a hemA probe from Rhizobium meliloti and cloned by complementation of a hemA mutant of Escherichia coli K12. E. coli hemA transformants carrying the hemA gene of Agrobacterium showed delta-aminolevulinic acid synthetase (delta-ALAS) activity in vitro. The hemA gene was carried on a 4.4 kb EcoRI fragment which could be reduced to a 2.6 kb EcoRI-SstI fragment without affecting its complementing or delta-ALAS activity. The sequence of the hemA gene showed an open reading frame of 1215 nucleotides, which could code for a protein of 44,361 Da. This is very close to the molecular weight of the HemA protein obtained using an in vitro coupled transcription-translation system (45,000 Da). Comparison of amino acid sequences of the delta-ALAS of A. radiobacter and Bradyrhizobium japonicum showed strong homology between the two enzymes; less, but still significant, homology was observed when A. radiobacter and human delta-ALAS were compared. Primer extension experiments enabled us to identify two promoters for the hemA gene of A. radiobacter. One of these promoters shows some similarity to the first promoter of the hemA gene of R. meliloti.

Effects of sulfur-containing metabolites of hexachlorobenzene on the heme metabolic enzymes in rat liver.

Effects of hexachlorobenzene (HCB) and its sulfur-containing metabolites on the heme metabolic enzymes in rat liver were investigated. A single injection of HCB caused the increase in activities of delta-aminolevulinic acid (ALA) synthetase and heme oxygenase, and contents of cytochrome P-450 and total heme. After a single injection of pentachlorothioanisol (PCTA) or pentachlorophenyl methyl sulfore (PCPSO2Me), ALA synthetase activity was enhanced. Heme oxygenase activity was increased by PCPSO2Me treatment. Cytochrome P-450 and total heme contents were increased by PCPSO2Me or 1,4-bis(methylthio)tetrachlorobenzene (MTTCB). When HCB was injected once daily for 5 weeks, a marked increase in ALA synthetase activity, a significant decrease in ALA dehydratase, almost complete inhibition of uroporphyrinogen decarboxylase activity, and an increased excretion of total porphyrin in the urine were shown. After chronic treatment with its sulfur-containing compounds, PCPSO2Me and MTTCB produced a significant increase in ALA synthetase activity. However, activities of ALA dehydratase and uroporphyrinogen decarboxylase, and excretion of total porphyrin in the urine were unaltered. At this time, the concentrations of the corresponding sulfur-containing compound and related metabolite(s) in blood, liver and kidney were nearly the same as those observed in HCB-treated rats. It is suggested that PCPSO2Me and MTTCB could induce the hepatic ALA synthetase, but, these metabolites, and also PCTA, were not able to induce the porphyria in female rats, and the induction of porphyria by HCB is not attributable to the action of its sulfur-containing compound.

BCNU-induced quantitative and qualitative changes in hepatic cytochrome P-450 can be correlated with cholestasis

Male Sprague-Dawley rats were given single i.p. injections of 1,3-bis(2-chloroethyl)-1-Nitrosourea (BCNU) to investigate changes in hepatic microsomal cytochrome P-450 content and metabolic activity. On day 14 after treatment (20 mg/kg), cytochrome P-450 content had decreased by approximately 25% and ethylmorphine N-demethylase activity (nmol product/nmol P-450/min) had decreased by 36%. In contrast, ethylmorphine O-deethylase and 7-ethoxycoumarin O-deethylase activities were not significantly decreased by BCNU treatment. Hepatic delta-aminolevulinic acid synthetase activity was only 60% of control values, and microsomal heme oxygenase activity was slightly but not statistically elevated. Cytochrome P-450 content in control and BCNU-treated rats increased in a similar manner after phenobarbital or beta-naphthoflavone induction. Electrophoretic analysis of cytochrome P-450 proteins isolated from hepatic endoplasmic reticular membranes of treated and control rats suggested that alterations in these proteins occurred in BCNU-treated rats. These changes in cytochrome P-450 content and activity are very similar to those reported in isolated systems exposed to bile acids or in rats with experimentally produced cholestasis. BCNU has been shown to produce cholestasis, which precedes its effects on microsomal mixed-function oxygenase activity. Thus, the delayed effects of BCNU on microsomal drug metabolism are probably secondary to its interference with bile formation.

Effects of polychlorinated biphenyls on porphyrin synthesis and cytochrome p—450—dependent monooxygenases in small intestine and liver of Japanese quail

The effects of acute exposure to polychlorinated biphenyls (PCBs) on porphyrin synthesis and cytochrome P-450-dependent monooxygenases in the small intestine and liver were studied in male Japanese quail. The birds were dosed orally with the PCB mixture, Aroclor 1242, or the individual PCB isomers, 2,4,2',4'-tetrachlorobiphenyl (2-TCB) and 3,4,3',4'-tetrachlorobiphenyl (3-TCB), and were killed 48 h later. All the PCB compounds caused a significant increase in porphyrin content and delta-aminolevulinic acid synthetase (ALA-S) activity in the small intestine and liver. Increases in porphyrins were greater in the small intestine than in liver. However, a smaller increase in ALA-S activity occurred in the small intestine than in liver, suggesting that ALA-S induction is not a major mechanism for the increased porphyrin content of small intestine. All the test compounds significantly increased the cytochrome P-450 content of liver. In the small intestine, cytochrome P-450 content was increased by Aroclor 1242 and 2-TCB but not by 3-TCB. The activity of 7-ethoxyresorufin O-deethylase, however, was increased by all test compounds in both liver and small intestine. In contrast, there was a striking difference between small intestine and liver in the induction of 7-ethoxycoumarin O-deethylase (ECOD) activity by Aroclor 1242. In the liver, ECOD activity was unchanged or decreased, but in the small intestine, ECOD activity increased linearly with dose. No tissue difference in ECOD activity was observed after treatment with 2-TCB or 3-TCB. These findings suggest that acute exposure to a given PCB results in marked differences between small intestine and liver in porphyrin metabolism and in the induction of cytochrome P-450 isozymes and associated monooxygenases.

Effects of two cannabinoids on hepatic microsomal cytochrome P-450.

The effects of cannabidiol (CBD) and delta 9-tetrahydrocannabinol (delta 9-THC) on the synthesis and degradation of hepatic microsomal cytochrome P-450 were studied in mice. Cannabinoids used (10, 50 and 100 mg/kg, i.p.) did not affect delta-aminolevulinic acid synthetase activity in the liver. delta 9-THC-treatment (10, 50 and 100 mg/kg, i.p.) markedly stimulated heme oxygenase activity in hepatic 18000 X g supernatant fractions in a dose-dependent manner, whereas CBD-treatment was without effect. In vitro experiments, CBD and delta 9-THC (40 to 160 microM) markedly inhibited nicotinamide adenine dinucleotide phosphate (NADPH)-induced lipid peroxidation in hepatic microsomes. When CBD was incubated with the hepatic microsomes in the presence of an NADPH-generating system, cytochrome P-450 content decreased significantly. However, delta 9-THC showed no effects in similar experiments. The rate of decrease in the cytochrome P-450 content using CBD (160 microM) was 0.212 nmol/mg protein/20 min in microsomes from control mice. This value increased significantly in microsomes from phenobarbital-treated mice (0.792 nmol/mg protein/20 min) but not in those from 3-methylcholanthrene-treated mice (0.190 nmol/mg protein/20 min). The metabolic rate (per nmol cytochrome P-450) of CBD was also increased significantly by phenobarbital-treatment but not by 3-methylcholanthrene-treatment. These results suggest that CBD metabolites rather than CBD itself, play some role in the decreasing effect on cytochrome P-450 content in the hepatic microsomes in vitro, and that the microsomal formation of reactive metabolite of CBD is increased by phenobarbital-treatment.

Role of tin on heme and drug biotransformation mechanism of partially hepatectomized rats.

Some parameters of heme and drug-metabolism were measured in sham and partially-hepatectomized rats following a single intraperitoneal injection of tin (II) tartrate (2 mg tin/100 g B. Wt.). One day after treatment, tin, in partially hepatectomized rats produced a depression of activities of delta-aminolevulinic acid synthetase and delta-aminolevulinic acid dehydratase, and induction of heme oxygenase. These changes were accompanied by the marked reduction in the levels of cytochrome P-450 and benzo (a) pyrene hydroxylase. Accumulation of tin measured by incorporation of 113tin was 23% less in liver of hepatectomized rats than sham-operated rats, however the accumulation of 113tin in microsomal fraction was much higher in partially hepatectomized rats.

On the mechanism of the enzyme-inducing action of some heavy metal salts.

The effect of Co(NO3)2, CdSO4, NiSO4, ZnSO4, and HgCl2 (given repeatedly in subtoxic doses in the drinking water for 30 days) on rat liver monooxygenases was studied in experiments on male Wistar rats. The salts of Co, Cd, and Zn increased the activity of benzphetamine-N-demethylase, the content of cytochrome P-450 and microsomal heme. The data suggest that these salts exert an enzyme-inducing effect on the hepatic monooxygenases. The same metal salts (Co, Cd, and Zn) increased the activity of delta-aminolevulinic acid (ALA) synthetase and decreased that of heme oxygenase. The increased cytochrome P-450 content is probably due to the increased synthesis and the decreased breakdown of this hemoprotein. HgCl2 and NiSO4 did not exert an enzyme-inducing action. The lack of change in the activity of NADPH-cytochrome c reductase and cytochrome b5 (except for ZnSO4) suggests that these components of the electron transport chain are not likely to be involved in the enzyme-inducing action of the heavy metal salts.

The effect of tetrachlorohydroquinone on hexachlorobenzene-induced porphyria in Japanese quail.

Female Japanese quail received either hexachlorobenzene (HCB, 100 mg/kg . d) or tetrachlorohydroquinone (TCHQ, 175 mg/kg . d) for 10 d as a primary treatment. Following this, a secondary treatment of HCB (100 mg/kg . d) or TCHQ (175 mg/kg . d), alone or in combination, was administered for 1, 5, 10 or 15 d. The primary HCB treatment caused elevated delta-aminolevulinic acid synthetase (ALA-S) activities and small increases in porphyrin concentrations. Subsequent treatment of these birds with lactose resulted in no further increases in porphyrins or ALA-S. TCHQ treatment caused increases in porphyrins similar to those seen with continued HCB treatment. Apparently, despite the fact that TCHQ alone had no affect on ALA-S or porphyrin levels, this compound is able, in the presence of elevated ALA levels to cause porphyria. A combination of HCB and TCHQ administered to HCB-pretreated animals caused a more severe porphyria than did follow-up treatment with either HCB or TCHQ alone.

Effect of abnormal atmospheric pressure conditions upon mouse liver delta-aminolevulinic acid synthetase activity.

Liver delta-aminolevulinic acid synthetase activity was measured in mice living under abnormal atmospheric pressure conditions for 15 h. In the group living under low atmospheric pressure (51 kPa) the enzymic activity, either basal or induced by starvation and/or allylisopropylacetamide, was significantly (p less than 0.001) lower than that of the control group. In the group living under high atmospheric pressure (153 kPa) the enzymic activity was significantly (p less than 0.001) higher than the one of the controls. Our results might possibly be explained by changes in the cellular redox state, the heme oxygenase activity or the serum erythropoietin levels.

A sustained increase of microsomal heme oxygenase activity following treatment of rats with Bacillus Calmette-Guerin and Corynebacterium parvum: its possible relation to the decrease of cytochrome P-450 content.

The alterations of various enzymes responsible for drug metabolism and heme metabolism were examined in the liver of female rats treated with Bacillus Calmette-Guerin (BCG) and Corynebacterium parvum (CP). Hepatic drug metabolizing enzyme activities and microsomal cytochrome P-450 and b5 content were significantly decreased for up to 15 and 10 d by a single i.v. administration of BCG and CP, respectively. In contrast, microsomal heme oxygenase activity was markedly increased after BCG and CP treatment and the increased enzyme activity was sustained in parallel with the decrease of drug metabolizing enzymes. Both BCG and CP also caused a significant decrease of delta-aminolevulinic acid synthetase activity shortly after their administrations. The decreased enzyme activity returned to normal levels by 12 h after the treatment of rats with BCG and CP. In addition, hepatosplenomegaly was observed in BCG and CP treated rats. Dose related changes of these microsomal enzymes were seen following the administration of BCG and CP. Additionally, there were sex differences in the effects of BCG and CP on the alteration of microsomal enzymes, female rats being more sensitive than male rats. These results suggest that the decrease of cytochrome P-450 and b5 content and drug metabolizing enzyme activities by BCG and CP could be related, at least in part, to the prolonged increase of heme oxygenase activity, that may lead to the increased breakdown of heme available for the synthesis of these hemoproteins.

Alterations in hepatic heme and cytochrome P-450 metabolism in murphy-sturm lymphosarcoma-bearing rats implications for drug metabolism

Previous studies have shown that tumor-bearing rats have significantly decreased hepatic microsomal cytochrome P-450 content and NADPH-cytochrome c reductase activity with, consequently, significantly decreased capacity for microsomal oxidative drug metabolism. Subsequent investigations have revealed that the rates of hepatic cytochrome P-450 apo-protein synthesis and degradation are decreased significantly and hepatic microsomal heme oxygenase activity is increased significantly in rats bearing an extra-hepatic tumor. Further studies have been done to attempt to clarify the pathogenesis and significance of these observations. Hepatic delta-aminolevulinic acid (ALA) synthetase activity in male Wistar rats declined to a nadir of 162 ± 34 (S.E.) pmoles ALA per mg protein per 30 min 6 days following i.m. transplantation of Murphy-Sturm lymphosarcoma (vs control = 218 ± 36 pmoles per mg per 30 min). Turnover of 3H-labeled heme in microsomal CO-binding particles (i.e. cytochrome P-450 heme) was increased significantly 8 days following i.m. transplantation of Murphy-Sturm lymphosarcoma with a T 1 2 of 5.5 hr for the fast phase of hepatic cytochrome P-450 heme disappearance in tumor-bearing rats as compared with a T 1 2 of 7 hr in control rats. Hepatic cytochrome P-450 apo-protein concentration was slightly, but not significantly, increased in Murphy-Sturm lymphosarcoma-bearing rats as compared with control rats up to 10 days following tumor transplantation. These results suggest that, in Murphy-Sturm lymphosarcoma-bearing rats, decreased microsomal cytochrome P-450 concentration is the result of both decreased cytochrome P-450 apo-protein synthesis and increased cytochrome P-450 heme turnover. Apo-cytochrome P-450 concentration was not appreciably altered because increased cytochrome P-450 heme turnover and decreased cytochrome P-450 apo-protein degradation were balanced by decreased cytochrome P-450 apo-protein synthesis. Because of their effects on cytochrome P-450 concentration and action, these alterations in heme and hemoprotein metabolism may be of importance in regulating oxidative drug metabolism in the tumor-bearing state.

Alterations of hepatic delta-aminolevulinic acid synthetase, heme oxygenase, microsomal cytochrome content and drug metabolism in rats bearing ascitic tumors AH 13, AH 66 and AH 414 and a 3-methylcholanthrene induced tumor.

Hepatic microsomal drug-metabolizing enzyme activities, cytochrome content, delta-aminolevulinic acid (ALA) synthetase and heme oxygenase activities were studied in rats bearing ascitic tumors AH 13, AH 66 and AH 414 and a primary, 3-methylcholanthrene (3-MC)-induced, tumor. Hepatic microsomal drug-metabolizing enzyme activities and cytochrome content were decreased in rats transplanted intraperitoneally with 1-2 x 10(6) cells of ascitic tumor cell lines AH 13, AH 66 and AH 414. The extent of the decrease of the microsomal cytochrome content and enzyme activities were dependent on the tumor-bearing periods after inoculation. Hepatic microsomal heme oxygenase activity was significantly increased concurrently with the decrease of microsomal drug-metabolizing enzyme activities and cytochrome content. Hepatic ALA synthetase was not changed appreciably in these tumor-bearing rats. Similar alterations of microsomal enzyme content and activities were observed in the livers of rats transplanted subcutaneously with AH 66 tumor cells and in rats bearing a primary tumor initiated by the subcutaneous injection of 3-MC. When the tumor was surgically removed from the rats bearing AH 66 subcutaneously, these hepatic microsomal parameters returned to normal levels. Microsomal drug-metabolizing enzyme activities and cytochrome content in these ascitic tumor cells were found to be at very low levels. From these results, if appears that there is an inverse relationship between the increase of microsomal heme oxygenase activity and the decrease of cytochrome P-450 and b5 as well as drug-metabolizing enzymes in the liver of tumor bearing rats.