4'-Ethynyl-2'-Deoxycytidine (EdC) Preferentially Targets Lymphoma and Leukemia Subtypes by Inducing Replicative Stress

Anticancer nucleosides are effective against solid tumors and hematologic malignancies, but typically are prone to nucleoside metabolism resistance mechanisms. Using a nucleoside-specific multiplexed high-throughput screening approach, we discovered 4'-ethynyl-2'-deoxycytidine (EdC) as a third-generation anticancer nucleoside prodrug with preferential activity against diffuse large B-cell lymphoma (DLBCL) and acute lymphoblastic leukemia (ALL). EdC requires deoxycytidine kinase (DCK) phosphorylation for its activity and induces replication fork arrest and accumulation of cells in S-phase, indicating it acts as a chain terminator. A 2.1Å cocrystal structure of DCK bound to EdC and UDP reveals how the rigid 4'-alkyne of EdC fits within the active site of DCK. Remarkably, EdC was resistant to cytidine deamination and SAMHD1 metabolism mechanisms and exhibited higher potency against ALL compared with FDA-approved nelarabine. Finally, EdC was highly effective against DLBCL tumors and B-ALL in vivo. These data characterize EdC as a preclinical nucleoside prodrug candidate for DLBCL and ALL.

Plasma Markers of Disrupted Gut Permeability in Severe COVID-19 Patients

A disruption of the crosstalk between the gut and the lung has been implicated as a driver of severity during respiratory-related diseases. Lung injury causes systemic inflammation, which disrupts gut barrier integrity, increasing the permeability to gut microbes and their products. This exacerbates inflammation, resulting in positive feedback. We aimed to test whether severe Coronavirus disease 2019 (COVID-19) is associated with markers of disrupted gut permeability. We applied a multi-omic systems biology approach to analyze plasma samples from COVID-19 patients with varying disease severity and SARS-CoV-2 negative controls. We investigated the potential links between plasma markers of gut barrier integrity, microbial translocation, systemic inflammation, metabolome, lipidome, and glycome, and COVID-19 severity. We found that severe COVID-19 is associated with high levels of markers of tight junction permeability and translocation of bacterial and fungal products into the blood. These markers of disrupted intestinal barrier integrity and microbial translocation correlate strongly with higher levels of markers of systemic inflammation and immune activation, lower levels of markers of intestinal function, disrupted plasma metabolome and glycome, and higher mortality rate. Our study highlights an underappreciated factor with significant clinical implications, disruption in gut functions, as a potential force that may contribute to COVID-19 severity.

Steric-Free Bioorthogonal Labeling of Acetylation Substrates Based on a Fluorine-Thiol Displacement Reaction

We have developed a novel bioorthogonal reaction that can selectively displace fluorine substitutions alpha to amide bonds. This fluorine-thiol displacement reaction (FTDR) allows for fluorinated cofactors or precursors to be utilized as chemical reporters, hijacking acetyltransferase-mediated acetylation both in vitro and in live cells, which cannot be achieved with azide- or alkyne-based chemical reporters. The fluoroacetamide labels can be further converted to biotin or fluorophore tags using FTDR, enabling the general detection and imaging of acetyl substrates. This strategy may lead to a steric-free labeling platform for substrate proteins, expanding our chemical toolbox for functional annotation of post-translational modifications in a systematic manner.

MYC regulates fatty acid metabolism through a multigenic program in claudin-low triple negative breast cancer

Background

Recent studies have suggested that fatty acid oxidation (FAO) is a key metabolic pathway for the growth of triple negative breast cancers (TNBCs), particularly those that have high expression of MYC. However, the underlying mechanism by which MYC promotes FAO remains poorly understood.

Methods

We used a combination of metabolomics, transcriptomics, bioinformatics, and microscopy to elucidate a potential mechanism by which MYC regulates FAO in TNBC.

Results

We propose that MYC induces a multigenic program that involves changes in intracellular calcium signalling and fatty acid metabolism. We determined key roles for fatty acid transporters (CD36), lipases (LPL), and kinases (PDGFRB, CAMKK2, and AMPK) that each contribute to promoting FAO in human mammary epithelial cells that express oncogenic levels of MYC. Bioinformatic analysis further showed that this multigenic program is highly expressed and predicts poor survival in the claudin-low molecular subtype of TNBC, but not other subtypes of TNBCs, suggesting that efforts to target FAO in the clinic may best serve claudin-low TNBC patients.

Conclusion

We identified critical pieces of the FAO machinery that have the potential to be targeted for improved treatment of patients with TNBC, especially the claudin-low molecular subtype.

Planning Telehealth for Older Adults With Atrial Fibrillation in Rural Communities: Understanding Stakeholder Perspectives

Older adults with atrial fibrillation (AF) in rural communities have less access to cardiac specialty care. Telehealth offers a viable approach to provide cardiac care, yet little is known about patients’ and providers’ views on telehealth’s potential to support rural patients with AF. This qualitative descriptive study examines patient and health providers’ perspectives, an important first step in planning a telehealth initiative. Eight patients with AF, along with one partner from rural communities, were recruited through an urban-based AF clinic. Five providers were recruited through professional practice leads in the health region. Semistructured telephone interviews were conducted with both stakeholder groups. The overriding theme was variability in patient and provider receptiveness to telehealth. Receptiveness reflected differences in past experience with telehealth, in perceived adequacy of rural health services, and in perceived gaps in AF care. These are important considerations in planning effective and sustainable telehealth in rural communities.

The Stressors and Coping Strategies of Older Adults With Persistent Atrial Fibrillation Prior to and Following Direct Current Cardioversion

Objective:The purpose of this study was to explore the stressors and coping strategies of older adults with persistent atrial fibrillation (AF) before and after direct current cardioversion. Method: The study used a qualitative descriptive design. Sixteen patients were recruited through an AF clinic to participate in individual interviews prior to the cardioversion and at 6 and 12 weeks post procedure. Results: Pre-cardioversion, older adults experienced symptom and health care–related stressors superimposed on existing non-AF stressors. They used a range of emotion and problem-focused coping. Non-AF stressors increased post procedure at the same time that participants perceived less need for coping strategies with a return to regular rhythm. Discussion: There was a shift from AF to non-AF related stressors following the cardioversion but a decrease in coping strategies. Older adults with AF should be encouraged to maintain use of coping strategies to manage ongoing stress and reduce the risk of AF recurrence.

Measurement of heme concentration

Heme (iron protoporphyrin IX) is a prosthetic group for a number of hemoproteins in different tissues (e.g., hemoglobin, myoglobin, cytochrome P-450s, mitochondrial cytochromes, catalases, and peroxidases). Mutations in the biosynthetic pathway can affect the synthesis and/or degradation of heme. Several assays are provided in this unit for quantifying heme: a spectrophotometric assay based on the characteristic absorption spectrum of oxidized and reduced form of the hemochrome formed by replacing the nitrogen ligands with pyridine; a fluorescence assay based on removal of the iron by a heated, strong oxalic acid solution to produce fluorescent protoporphyrin; a reversed-phase HPLC assay to measure heme and intermediates in the synthetic pathway; and a radiometric assay to measure newly synthesized heme in tissue culture cells.

N-terminal sequence analysis of proteins and peptides

Automated N-terminal sequence analysis involves a series of chemical reactions that derivatize and remove one amino acid at a time from the N-terminus of purified peptides or intact proteins. At least several picomoles of a purified protein or 10 to 20 pmol of a purified peptide with an unmodified N-terminus is required to obtain useful sequence information. In recent years, the demand for N-terminal sequencing has decreased substantially as some applications for protein identification and characterization can now be more effectively performed using mass spectrometry. However, N-terminal sequencing remains the method of choice for verifying the N-terminal boundary of recombinant proteins, determining the N-terminus of protease-resistant domains, identifying proteins isolated from species where most of the genome has not yet been sequenced, and mapping modified or crosslinked sites in proteins that prove to be refractory to analysis by mass spectrometry.

Measurement of ALA synthase activity

In most cells, δ-aminolevulinate (ALA) synthase is the rate-limiting enzyme in heme synthesis. It is inducible by drugs and toxins and is feedback regulated by heme. This unit describes a radiometric assay using [¹⁴C]succinate as a substrate and a colorimetric assay based on the conversion of ALA to a pyrrole.

Role of cytochrome P450 1A2 in bilirubin degradation Studies in Cyp1a2 (-/-) mutant mice

In congenital jaundice, which is due to defects of bilirubin gluruconidation, bilirubin is degraded by an alternative pathway into unidentified products. Previously, it was shown that plasma bilirubin levels can be decreased in rats with this defect by inducers of CYP1A enzymes. Here, liver microsomes from rats or mice treated with beta-naphthoflavone (BNF) or 3-methylcholanthrene (3 MC) had increased activity for bilirubin degradation. The activity was further stimulated by addition of the coplanar molecule 3,4,3',4'-tetrachlorobiphenyl (TCB). There was more stimulation of bilirubin degradation by TCB in microsomes from BNF-treated rats than in microsomes from BNF-treated mice. CYP1A1 to CYP1A2 ratios were greater in rats treated with BNF. In Cyp1a2 (-/-) mutant mice, 3-MC treatment did not increase the rate of bilirubin degradation, but TCB increased this degradation severalfold. Between SWR and C57BL/6 inbred mouse strains that have a 2-fold difference in hepatic constitutive CYP1A2 levels, there was also a 2-fold difference in bilirubin degradation; TCB did not stimulate in either strain. We conclude that CYP1A2 is responsible for microsomal bilirubin degradation in the absence of TCB. TCB was required for bilirubin degradation by CYP1A1. Manipulation of CYP1A2 may be of therapeutic benefit in patients with these diseases of bilirubin conjugation.

Uroporphyria in Hfe mutant mice given 5-aminolevulinate: a new model of Fe-mediated porphyria cutanea tarda

Porphyria cutanea tarda (PCT), a liver disease with skin lesions caused by excess liver production of uroporphyrin (URO), is associated with consumption of alcoholic beverages or estrogens, and moderate iron overload. Recently, it has been shown that many PCT patients carry mutations in the HFE gene, which is responsible for hereditary hemochromatosis. Mice homozygous for either the null mutation in the Hfe gene or the C282Y missense mutation rapidly accumulate hepatic parenchymal iron similar to patients with hemochromatosis. Here we investigated whether disruption of the murine Hfe gene would result in hepatic uroporphyria. Mice homozygous for the Hfe-null mutation accumulated high levels of hepatic URO when fed 5-aminolevulinate (ALA). Hfe (+/-) mice also accumulated hepatic URO when fed ALA, but at a much slower rate. The amount of accumulated URO in the null mutant mice was similar to that in wild-type mice treated with iron carbonyl in the diet, or injected with iron dextran. Iron in both wild-type and Hfe (+/-) mice was mostly in Kupffer cells. In contrast, Hfe (-/-) mice had considerable parenchymal iron deposition as well, in a pattern similar to that observed in wild-type mice treated with iron carbonyl. URO accumulation was accompanied by 84% and 33% decreases in hepatic uroporphyrinogen decarboxylase activities in Hfe (-/-) and Hfe (+/-) mice, respectively. No increases in CYP1A2 or other cytochrome P450s were detected in the Hfe-null mutant mice. We conclude that this experimental model of uroporphyria is a valid model for further investigations into the mechanism of PCT.

Relative Roles of CYP2E1 and CYP1A2 in Mouse Uroporphyria Caused by Acetone

Porphyria cutanea tarda is a liver disease characterized by excess production of uroporphyrin. We previously reported that acetone, an inducer of CYP2E1, enhances hepatic uroporphyrin accumulation in mice treated with iron dextran (Fe) and 5-aminolevulinic acid (ALA). Cyp2e1(-/-) mice treated with Fe and ALA were used to investigate whether CYP2E1 is required for the acetone effect. Hepatic uroporphyrin accumulation was stimulated by acetone in Cyp2e1(-/-) mice to the same extent as in wild-type mice. In the absence of acetone, uroporphyrin accumulated in Cyp2e1(-/-) mice treated with Fe and ALA, but less than in wildtype mice. However, in Cypla2(-/-) mice, uroporphyrin accumulation caused by Fe and ALA, with or without acetone, was completely prevented. Acetone was not an inducer of hepatic CYP1A2 in the wild-type mice. Although acetone is an inducer of CYP2E1, CYP1A2 appears to have the essential role in acetone-enhancement of uroporphyria.

Effect of arsenite on the induction of CYP1A4 and CYP1A5 in cultured chick embryo hepatocytes

We had reported previously that 2.5-5 microM sodium arsenite decreased the phenobarbital-mediated induction of CYP2H activity and protein but not CYP2H1 mRNA in chick-embryo hepatocyte cultures. Induction of a CYP1A activity and protein by 3-methylcholanthrene was also decreased by low arsenite concentrations; however, CYP1A mRNAs were not measured in those studies. We report here that low concentrations of arsenite decreased induction of activities and mRNAs of two chicken cytochromes P450, CYP1A (1A4 and 1A5), by 3-methylcholanthrene in chick-embryo hepatocyte cultures. Arsenite treatment did not affect the turnover of either mRNA, nor did it decrease the superinduction of each mRNA caused by treatment with cycloheximide in addition to 3-methylcholanthrene. Glutathione depletion enhanced the effect of arsenite to decrease induction of CYP1A4. These results indicate the induction of CYP1A4 and 1A5 is inhibited by sodium arsenite at the level of transcription, suggesting that the Ah receptor complex may be involved.

CYP1A2 is essential in murine uroporphyria caused by hexachlorobenzene and iron

Using Cyp1a2(-/-) mice we previously showed that CYP1A2 is absolutely required for hepatic uroporphyrin accumulation caused by iron and 5-aminolevulinate (ALA) treatment, both in the presence and absence of an inducer of CYP1A2. In this study we have used these mice to investigate whether CYP1A2 has an obligatory role in hepatic uroporphyria caused by hexachlorobenzene (HCBZ), an inducer of CYP2B and CYP3A, as well as CYP1A2. Here we treated mice with HCBZ and iron, with and without the porphyrin precursor, ALA, in the drinking water. In iron-loaded wild-type mice given a single dose of HCBZ and ALA, hepatic uroporphyrin (URO) accumulated to 300 nmol/g liver after 37 days, whereas in Cyp1a2(-/-) mice, there was no hepatic URO, even after an additional dose of HCBZ, and a further 29 days of ALA treatment. A similar requirement for CYP1A2 was found in uroporphyria produced in HCBZ and iron-treated mice in the absence of ALA. As detected by Western immunoblotting, HCBZ induced small increases in CYP2B and CYP3A in the livers of all animals. In the wild-type animals, HCBZ also induced CYP1A2 and associated enzyme activities, including uroporphyrinogen oxidation, by about 2-3-fold. In the Cyp1a2(-/-) mice, HCBZ did not increase hepatic microsomal uroporphyrinogen oxidation. These results indicate that, in mice, CYP1A2 is essential in the process leading to HCBZ-induced uroporphyria. Contributions by other CYP forms induced by HCBZ appear to be minimal.

Role of small differences in CYP1A2 in the development of uroporphyria produced by iron and 5-aminolevulinate in C57BL/6 and SWR strains of mice

Previous work has implicated CYP1A2 in experimental uroporphyria caused by polyhalogenated aromatic compounds, and in uroporphyria caused by iron and 5-aminolevulinate (ALA) in the absence of inducers of CYP1A2. Here we examined whether the different susceptibilities of SWR and C57BL/6 strains of mice to uroporphyria in the absence of inducers of CYP1A2 are related to different levels of CYP1A2. Enzymological assays (ethoxy- and methoxyresorufin dealkylases, and uroporphyrinogen oxidation) and immunoblots indicated that there was about twice the amount of hepatic CYP1A2 in SWR mice compared with C57BL/6 mice. Immunohistochemistry revealed that CYP1A2 was located centrilobularly in the liver, and the staining was more intense in SWR mice than in C57BL/6 mice. Hepatic non-heme iron was about double in SWR compared with C57BL/6 mice. In SWR mice given iron dextran, hepatic iron was 1.7-fold that of C57BL/6 mice given iron dextran. SWR mice administered ALA in the drinking water accumulated much less hepatic protoporphyrin than did C57BL/6 mice. To confirm the importance of small increases in CYP1A2, C57BL/6 mice were given a low dose of 3-methylcholanthrene (MC) (15 mg/kg), as well as iron and ALA. There was about a 5- to 6-fold increase in hepatic uroporphyrin accumulation after 32 days on ALA compared with animals not given MC. In these animals, CYP1A2 was increased by 10-fold at 2 days, but returned to basal levels by 14 days. We conclude that small and transient differences in CYP1A2 may be important in the development of uroporphyria.

CYP1A-catalyzed uroporphyrinogen oxidation in hepatic microsomes from non-mammalian vertebrates (chick and duck embryos, scup and alligator)

Uroporphyrin (URO) accumulation in the liver of animals treated with polyhalogenated aromatic hydrocarbons (PHAH) is associated with increased microsomal oxidation of uroporphyrinogen catalyzed by rodent CYP1A2 and by a similar form in chicken, CYP1A5. The planar biphenyl, 3,3',4,4'-tetrachlorobiphenyl (TCB) stimulates uroporphyrinogen oxidation (UROX) in chick hepatic microsomes, but inhibits UROX activity in hepatic microsomes from mice and rats pre-induced by CYP1A2. Here we investigated whether TCB would stimulate or inhibit UROX in other non-mammalian species. UROX was stimulated 1.5-3-fold by TCB and 2-4-fold by 3,3',4,4',5,5'-hexachlorobiphenyl in hepatic microsomes from duck, alligator and scup treated with inducers of CYP1A. Hexachlorobenzene stimulated chick UROX, but was ineffective with microsomes from the other species. The stimulation of UROX by TCB was also observed in chick hepatocyte cultures. Pretreatment with up to 5 nM TCB induced CYP1A, but did not result in accumulation of URO. However, URO did accumulate if additional (post-induction) TCB was added along with 5-aminolevulinic acid. In this post-inductional TCB treatment, cycloheximide was included to prevent further induction of CYP1A. In duck hepatocytes, pretreatment with 25 nM TCB resulted in URO accumulation from 5-aminolevulinic acid. Post-induction TCB was not required and caused no further increase in URO accumulation. The differences in PHAH stimulation of UROX among the non-mammalian species have implications in the evolutionary changes in CYP1A, as well as the mechanism of development of PHAH-stimulated uroporphyria in different species.

Uroporphyrinogen oxidation catalyzed by human cytochromes P450

Porphyria cutanea tarda is associated with excess hepatic production of uroporphyrin. Oxidation of uroporphyrinogen to uroporphyrin was previously demonstrated to be specifically catalyzed by cytochrome P450 (CYP) 1A2. Here, we investigated the ability of human CYP1A2 to catalyze uroporphyrinogen oxidation (UROX). UROX activity in human liver microsomes was maximally only 10% of the activity in microsomes from livers of untreated mice. There was a poor correlation of UROX activity with methoxyresorufin demethylation, an activity catalyzed predominantly by CYP1A2 and strongly correlated with immunodetectable CYP1A2. With CYP forms expressed in HepG2 cells, the methoxyresorufin demethylation and (ethoxyresorufin deethylation) activities of murine and human CYP1A2 forms were similar, but UROX activity catalyzed by human CYP1A2 was only 15-20% of the activity catalyzed by murine CYP1A2. Human CYP1A1, CYP1A2, and CYP3A4 expressed in lymphoblastoid cells all catalyzed UROX. In insect cells, CYP1A2 was more active in catalyzing UROX than was CYP1A1, CYP2E, CYP3A4, or CYP3A5. Human CYP1A2 expressed in Escherichia coli as a fusion protein with rat CYP oxidoreductase also catalyzed UROX. Reconstituted human CYP1A2 and CYP3A4 were active in catalyzing UROX, with reconstituted CYP1A2 having the highest specific activity obtained in this study. From inhibitor studies, it was concluded that some of the UROX activity in the insect cell microsomes was attributable to expressed CYP and some to an unidentified source. These results indicate that human CYP1A2 is active in catalyzing UROX but has lower activity than the murine orthologue. The results also indicate that most of the UROX activity found in human liver microsomes is not due to CYP1A2.

Uroporphyria produced in mice by iron and 5-aminolaevulinic acid does not occur in Cyp1a2(-/-) null mutant mice

In the present study we have investigated the putative requirement for the cytochrome P-450 isoform CYP1A2 in murine uroporphyria, by comparing Cyp1a2(-/-) knockout mice with Cyp1a2(+/+) wild-type mice. Uroporphyria was produced by injecting animals with iron-dextran and giving the porphyrin precursor 5-aminolaevulinic acid in the drinking water. Some animals also received 3-methylcholanthrene (MC) to induce hepatic CYP1A2. In both protocols, uroporphyria was elicited by these treatments in the Cyp1a2(+/+) wild-type mice, but not in the null mutant mice. Uroporphyrinogen oxidation activity in hepatic microsomes from untreated Cyp1a2(+/+) mice was 2.5-fold higher than in Cyp1a2(-/-) mice. Treatment with MC increased hepatic CYP1A1 in both mouse lines and hepatic CYP1A2 only in the Cyp1a2(+/+) line, as determined by Western immunoblotting. MC increased hepatic ethoxy- and methoxy-resorufin O-dealkylase activities in both mouse lines, but increased uroporphyrinogen oxidation activity in the Cyp1a2(+/+) wild-type mice only. These results indicate the absolute requirement for hepatic CYP1A2 in causing experimental uroporphyria under the conditions used.

Formation of zinc protoporphyrin in cultured hepatocytes: effects of ferrochelatase inhibition, iron chelation or lead

The formation of zinc protoporphyrin in response to lead or iron depletion has previously been investigated in erythroid systems. Because of its possible metabolic role in non-erythroid tissue, we investigated the formation of zinc protoporphyrin in cultured hepatocytes. The effects of lead and inhibitors of ferrochelatase, the iron insertion step of heme synthesis, on the conversion of 5-aminolevulinic acid to zinc protoporphyrin, protoporphyrin and heme were compared in rat and chick embryo hepatocyte cultures. In rat cultures, zinc protoporphyrin was synthesized enzymatically by ferrochelatase, since N-methylmesoporphyrin, an inhibitor of ferrochelatase. caused 40% or greater decreases in both heme and zinc protoporphyrin accumulation and markedly stimulated protoporphyrin accumulation. In addition, chelation of ferrous iron with 2,2'-dipyridyl decreased heme accumulation by 50%, but increased ZPP accumulation by 200%. Zinc protoporphyrin formation in chick embryo hepatocytes required the addition of zinc as well as 5-aminolevulinic acid and apparently was non-enzymatic, since it was not inhibited by N-methylmesoporphyrin nor increased by iron chelation. In the presence of 5-aminolevulinic acid, lead had no effect on zinc protoporphyrin, protoporphyrin or heme accumulation in chick hepatocytes, but decreased all three in rat hepatocytes, with the decrease in protoporphyrin being far greater than that of zinc protoporphyrin or heme. These findings indicate that, in contrast to the effect of lead in erythroid tissue, it did not specifically increase zinc protoporphyrin accumulation or alter iron availability in cultured hepatocytes.

Multiple roles of polyhalogenated biphenyls in causing increases in cytochrome P450 and uroporphyrin accumulation in cultured hepatocytes

Uroporphyrin (URO) accumulation occurs in chick embryo hepatocytes treated with a number of polyhalogenated aromatic hydrocarbons (PHAHs) that are known inducers of cytochrome P4501As (CYP1A). Previous dose response studies had shown that URO accumulation does not begin until CYP1A, as indicated by ethoxyresorufin O-deethylase (EROD) activity, is maximally induced. The reason why the concentrations of PHAHs required for URO accumulation were higher than those required to induce EROD had not been explained. PHAHs, such as 3,3',4,4'-tetrachlorobiphenyl (PCB77, IUPAC nomenclature, TCB) stimulate uroporphyrinogen (UROGEN) oxidation by microsomes from 3-methylcholanthrene (MC)-treated chick embryos. Here we used a new protocol to investigate whether the requirement for more TCB to stimulate in vitro microsomal UROGEN oxidation extended to TCB-induced URO accumulation in intact cultured hepatocytes. Cultures were treated with increasing concentrations of TCB or other PHAHs to induce CYP1As, then with cycloheximide (CX) to prevent further P450 synthesis. The CX treatment was shown to block any further increases in CYP1A as determined by immunoblots. 5-Aminolevulinic acid and a high concentration of TCB ("postinduction TCB") were then added to stimulate intracellular UROGEN oxidation. Using the protocol with postinduction TCB, the inducing concentrations of TCB which caused URO to begin to accumulate were now much lower than in the absence of postinduction TCB. Increases in CYP1A proteins, measured immunochemically, were detected at about the same inducing TCB concentrations that began to increase URO accumulation. The new protocol, with postinduction TCB, using URO accumulation as the end point, greatly increased the sensitivity of the culture system for detection of PHAHs with EC50s (nM) for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), TCB, 3,3',4,4',5,5'-hexachlorobiphenyl, MC, and hexachlorobenzene being about 0.003, 0.11, 0.75, 3.5, and 30, respectively. As little as 2-4 fmol TCDD per culture dish caused detectible increases in URO accumulation. We conclude that URO accumulation in chick hepatocyte cultures is limited not only by the induction of CYP1A, but also by the stimulation of intracellular UROGEN oxidation.

Ascorbic acid deficiency in porphyria cutanea tarda

Porphyria cutanea tarda (PCT), the most common form of porphyria, is manifested as skin photosensitivity caused by excess hepatic production of uroporphyrin and heptacarboxylporphyrin. In experimental animal models, ascorbic acid modulates chemically induced uroporphyrin accumulation. The purpose of this study was to determine whether ascorbic acid is decreased in the plasma of patients with PCT. Plasma was obtained after an overnight fast from 21 PCT patients, 16 of whom were infected with hepatitis C virus (HCV), and from a separate group of 9 patients with HCV infection but not PCT. Thirteen PCT patients were studied when they had active disease and 8 after treatment-induced remission. Plasma ascorbic acid was low (<23 micromol/L) in 11 (85%) of the 13 untreated PCT patients and deficient (<11 micromol/L) in 8 (62%). Two patients with normal ascorbic acid levels (45 and 62 micrommol/L) had consumed multivitamins. In 2 patients with deficient ascorbic acid, plasma levels returned to normal after phlebotomy treatment. Of the 8 patients studied during remission, 4 had normal ascorbic acid values and 4 were deficient (5 to 8 micromol/L). Plasma ascorbic acid values were normal for all patients who had HCV but no PCT. These data suggest that plasma ascorbic acid concentrations are commonly low in PCT, but this decrease is unrelated to HCV infection. Ascorbic acid deficiency may be one of the factors that contributes to the pathogenesis of PCT.

Identification of CYP1A5 as the CYP1A enzyme mainly responsible for uroporphyrinogen oxidation induced by AH receptor ligands in chicken liver and kidney

Uroporphyrinogen is an intermediate of the heme biosynthetic pathway. The oxidation of uroporphyrinogen to uroporphyrin (UROX) has been demonstrated to be catalyzed by mammalian CYP1A2. This reaction has an important role in uroporphyria caused by halogenated aromatic compounds. Two CYP enzymes induced by Ah receptor ligands were purified recently from chick embryo liver. One, designated CYP1A5, was preferentially active in arachidonic acid epoxygenation and the other, designated CYP1A4, in 7-ethoxyresorufin deethylase (EROD) and aryl hydrocarbon hydroxylase (AHH), reactions mainly catalyzed by CYP1A1 in rodents. The amino acid sequences of both CYP1A5 and CYP1A4 are more similar to CYP1A1 than to 1A2, and neither can be classified as an ortholog of mammalian CYP1A1 or 1A2. Here we report that reconstituted purified CYP1A5 was eight times more active than CYP1A4 in catalyzing UROX. The stimulation of UROX by 3,4,3',4'-tetrachlorobiphenyl that has been observed in microsomes was also observed with the reconstituted enzymes. Similar dose response relationships were found for induction of UROX and EROD in both chick embryo liver microsomes and in cultured chick hepatocytes, indicating coinduction of CYP1A5 and CYP1A4. UROX was induced by the Ah receptor ligand, 3-methylcholanthrene, in chicken kidney as well as liver. The findings reported here and other evidence that CYP1A4 and CYP1A5 tend to exhibit CYP1A1 and 1A2-like enzyme activites, respectively, indicate that the division of some enzyme activities among CYP1A enzymes applies to different vertebrate classes.

Cloning and identification of HEM14, the yeast gene for mitochondrial protoporphyrinogen oxidase

A respiratory-defective mutant (C54) of Saccharomyces cerevisiae was found to have a phenotype consistent with a mutation in either mitochondrial protoporphyrinogen oxidase or ferrochelatase. The mutant is grossly deficient in hemes, accumulates protoporphyrin and is rescued by exogenous heme. The increased levels of protoporphyrin at the expense of heme is indicative of a block in one of the two last steps of the heme biosynthetic pathway. Complementation of C54 by a known ferrochelatase mutant suggested that the defect was most likely in HEM14 encoding protoporphyrinogen oxidase. A plasmid capable of complementing C54 was obtained by transformation with a yeast genomic plasmid library. A partial sequence of the insert identified the gene as reading frame YER014 of yeast chromosome V (GenBank Accession Number U18778). This reading frame codes for a protein homologous to human protoporphyrinogen oxidase. Disruption of this gene elicits a respiratory defect and accumulation of protoporphyrin. The phenotype of the null mutant together with the homology of YER014p to human protoporphyrinogen oxidase provide compelling evidence that YER014 is HEM14.

Oxidation of porphyrinogens by horseradish peroxidase and formation of a green pyrrole pigment

When humans or plants are exposed to certain chemicals which interfere with heme biosynthetic enzymes, porphyrinogen intermediates accumulate and are oxidized to cytotoxic porphyrins. Here we have investigated the role of peroxidases in porphyrinogen oxidation. Horseradish peroxidase (HRP) rapidly oxidizes uroporphyrinogen to uroporphyrin and this is inhibited by ascorbic acid. HRP also oxidizes deuteroporphyrinogen (a synthetic porphyrin similar to protoporphyrinogen), but the yield of porphyrin is lower than with uroporphyrinogen as substrate. This low yield is in part due to a rapid, HRP-dependent conversion of deuteroporphyrin (but not uroporphyrin) to a green compound with spectral characteristics of a chlorin with a large peak at 638 nm. This reaction requires addition of a sulfhydryl reductant such as glutathione and is inhibited by ascorbic acid. These findings suggest that cellular peroxidases and ascorbic acid levels may play a role in modifying the phototoxic tetrapyrroles which accumulate in plants and humans after certain environmental exposures.

Ascorbic acid inhibits chemically induced uroporphyria in ascorbate-requiring rats

Ascorbate was previously shown to suppress accumulation of uroporphyrin (URO) in cultured chick embryo hepatocytes and to competitively inhibit microsomal oxidation of uroporphyrinogen catalyzed by cytochrome P4501A2. Here we used the Osteogenic Disorder Shionogi (ODS) mutant rat, which cannot synthesize ascorbic acid, to examine the in vivo effect of ascorbic acid on hepatic URO accumulation caused by treatment with 3-methylcholanthrene (MC) and 5-aminolevulinate (ALA). Female mutant rats maintained on three levels of dietary ascorbate (15,200, and 800 ppm) were treated for a total of 24 days. On the 11th and 16th days, rats were administered 3-methylcholanthrene, and 5-aminolevulinate was present continuously in the drinking water from day 14. Hepatic URO accumulated at the two lowest ascorbate levels, but not at 800 ppm ascorbate. The latter dose produced normal hepatic ascorbate levels. Plasma ascorbate levels were proportional to the hepatic values. Male rats also accumulated URO at the low dietary dose of ascorbic acid. The methylcholanthrene-induced increase in microsomal levels of CYP1A1 and CYP1A2, total cytochrome P450, and activities of uroporphyrinogen oxidation and ethoxyresorufin deethylase were not affected by the dietary level of ascorbate. Neither male nor female Fischer 344 rats accumulated URO when treated with the MC/ALA regime. Hepatic ascorbate concentrations in these rats were five-fold to seven-fold higher than they were in mutant rats that developed uroporphyria on 150 ppm dietary ascorbate. In ODS rats fed ascorbate at 90 but not 900 ppm in the diet, hexachlorobenzene caused hepatic URO accumulation, indicating that the effect of ascorbic acid is not unique to the regimen using methylcholanthrene.(ABSTRACT TRUNCATED AT 250 WORDS)

Protoporphyrinogen accumulation in cultured hepatocytes treated with the diphenyl ether herbicide, acifluorfen

Diphenyl ether (DPE) herbicides such as acifluorfen inhibit both the plant and mammalian forms of protoporphyrinogen oxidase, a heme biosynthetic enzyme. Only small amounts of protoporphyrin accumulated in primary cultures of chick embryo and rat hepatocytes treated with acifluorfen and the porphyrin precursor, 5-aminolevulinic acid. However, there was a large accumulation of the porphyrin precursor, protoporphyrinogen, which was detected after oxidation to protoporphyrin by an E. coli membrane enzyme. In contrast, conventional methods of porphyrin analysis which depend on quantitative autoxidation of protoporphyrinogen failed to detect this accumulation of protoporphyrinogen. This is the first demonstration that protoporphyrinogen can accumulate to high levels and remain stable in liver cells. In addition, we found that the effect of a protoporphyrinogen oxidase inhibitor such as acifluorfen on the regulation of heme synthesis in hepatocyte cultures differed from that of an iron chelator.

Effects of hemopexin on heme-mediated repression of 5-aminolevulinate synthase and induction of heme oxygenase in cultured hepatocytes

The serum protein hemopexin is considered to have a major role in the mechanism of the uptake of heme by hepatocytes by means of a heme-hemopexin receptor. Therefore, we examined in primary cultures of adult rat and embryonic chick hepatocytes whether the presence of hemopexin would affect the heme-mediated repression of 5-aminolevulinate synthase activity (the rate-limiting enzyme of heme biosynthesis) and the heme-induced increase of heme oxygenase activity (the rate-limiting step of heme degradation). Both of these heme-mediated effects were partly or entirely prevented by the presence of hemopexin. We conclude that homologous hemopexin, at molar concentrations exceeding that of heme, inhibited the uptake of heme into hepatocytes. These results suggest that heme, in amounts sufficient to affect the rate-limiting steps of heme synthesis and degradation, can only enter hepatocytes in primary culture when the binding capacity of hemopexin for heme has been exceeded or altered.

On the functions of the yeast COX10 and COX11 gene products

COX10 and COX11 are nuclear genes of Saccharomyces cerevisiae whose products are localized in mitochondria and are required for the synthesis of cytochrome oxidase. Genes homologous to COX10 are present in at least four different bacterial cytochrome oxidase operons. The bacterial gene, termed cyoE, has recently been proposed to code for a farnesyl transferase that converts protoheme to heme O (Saiki et al. (1992), Biochem. Biophys. Res. Commun. 189, 1491-1497). In this communication we report that the COX10 protein, like the product of cyoE is needed for heme A synthesis. Analyses of the heme constituents in a cox11 mutant indicate the absence of heme A and presence of a novel heme with chromatographic properties indistinguishable from those of heme O. This evidence suggests that the COX11 protein may be another heme A biosynthetic enzyme involved in forming the formyl group at position 8 of the porphyrin ring.

Ascorbic acid inhibition of cytochrome P450-catalyzed uroporphyrin accumulation

Previous studies on the mechanism of the uroporphyria caused by polyhalogenated aromatic hydrocarbons have indicated a key role of cytochrome P450 of the 1A subfamily in catalyzing uroporphyrinogen (UROgen) oxidation. Here we report that ascorbic acid (ASC) inhibits UROgen oxidation in primary cultures of chick embryo hepatocytes and hepatic microsomes from chickens and mice. In hepatocyte cultures, 0.15 mM ASC totally prevented the accumulation of uroporphyrin (URO) induced by treatment of cells with the combination of 3,4,3',4'-tetrachlorobiphenyl (TCB) and 2-propyl-2-isopropylacetamide (PIA), but had no effect on the induction of protoporphyrin accumulation by PIA and desferrioxamine. However, addition of 5-aminolevulinic acid (ALA) to cultures treated with PIA plus TCB decreased the ability of ASC to prevent URO accumulation, suggesting that the effectiveness of ASC was dependent on the intracellular concentration of ALA or its metabolites. Similarly, when chick hepatocyte cultures were treated with TCB plus exogenous ALA to produce URO accumulation, the effectiveness of ASC was also less than when ALA was produced endogenously. Under this condition, addition of piperonyl butoxide, a P450 inhibitor, increased ASC inhibition of URO accumulation. ASC competitively inhibited the oxidation of UROgen by hepatic microsomes from chicks or mice treated with 3-methylcholanthrene (MC) with Ki for ASC being about 0.1 mM. ASC prevented formation of a 500-nm absorbing compound, probably tetrahydrouroporphyrin, the first intermediate in UROgen oxidation. These results are consistent with ASC preventing URO accumulation in hepatocytes by competitive inhibition of the first step of UROgen oxidation and suggest a new physiological role of ASC, that of maintaining UROgen in the reduced state.

In vivo fate of hemopexin and heme-hemopexin complexes in the rat

The disposition in the rat of the plasma heme-binding protein hemopexin (Hx), as the native apoprotein and as its heme complex (HHx), has been studied using the residualizing protein label dilactitol-125I-tyramine (*I-DLT). The aim of this work was to identify the tissue sites of Hx uptake and catabolism, independent of heme binding, and to evaluate how heme loading affects Hx catabolism at these sites. *I-DLT-Hx had a circulating half-life of approximately 1.2 days and was recovered in degraded form in comparable amounts in visceral (liver, kidney, spleen) and peripheral (skin, muscle) tissues, indicating a generalized diffuse catabolism of the protein throughout the body. The plasma half-life of *I-DLT-Hx injected as a preformed heme-Hx complex was the same as that of the apoprotein; however, injection of the complex resulted in about a twofold increase in hepatic degradation of Hx. The lack of an effect of heme on overall catabolism of the preformed HHx complex was consistent with the approximately 1-h half-life of heme, injected as 14C-heme-Hx, in the circulation; however, as much as 20-fold more 14C-heme than Hx protein was recovered in liver from 14C-heme-Hx. The absolute amount of *I-DLT-Hx degraded in liver was significantly increased when heme was injected in excess of the heme binding capacity of circulating Hx, while 131I-DLT-albumin catabolism in liver was unaffected. Thus, depending on the physiological conditions studied, the data are consistent with a model in which, following hepatic uptake of heme from HHx, varying proportions of the protein are either returned to the circulation or degraded in the liver.

2-Amino-3,4-dimethylimidazo[4,5-f]quinoline induces and inhibits cytochrome P450 from the IA subfamily in chick and rat hepatocytes

Several heterocyclic amines, found in cooked food, are powerful mutagens in the Ames Salmonella mutagenicity test system. One of these, 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ) is one of the most mutagenic chemicals tested in this assay. In primary cultures of chick and rat hepatocytes, MeIQ, by itself, induced cytochrome P450 from the IA subfamily but was a weak inducer compared to 3-methylcholanthrene. However, in both chick and rat hepatocytes in culture, MeIQ decreased the amount of 3-methylcholanthrene-induced ethoxyresorufin deethylase activity, which is catalyzed by cytochrome P450 IA. The protein moiety of cytochrome P450 IA was decreased at MeIQ concentrations of 2.5 micrograms/ml or greater in chick hepatocytes and 25 micrograms/ml in rat hepatocytes. In hepatic microsomes from methylcholanthrene-treated chicks and rats, MeIQ was a competitive inhibitor of both ethoxyresorufin deethylase activity, a reaction catalyzed mainly by rodent cytochrome P450 IA1, and uroporphyrinogen oxidation, a reaction catalyzed by rodent P450 IA2. In cultured chick hepatocytes, MeIQ also decreased cytochrome P450-mediated oxidation of uroporphyrinogen by intact cells. The ability of MeIQ to inhibit as well as to induce cytochrome P450s of the IA subfamily may be important in assessing the mutagenic and carcinogenic effects of MeIQ in mammals.

Uroporphyrinogen oxidation catalyzed by reconstituted cytochrome P450IA2

Previous work suggested that the oxidation of uroporphyrinogen to uroporphyrin is catalyzed by cytochrome P450IA2. Here we determined whether purified reconstituted mouse P450IA1 and IA2 oxidize uroporphyrinogen. Cytochromes P450IA1 and IA2 were purified from hepatic microsomes from 3-methylcholanthrene (MC)-treated C57BL/6 mice, using a combination of affinity chromatography and high performance liquid chromatography. Reconstituted P450IA1 was more active than P450IA2 in catalyzing ethoxyresorufin-O-deethylase (EROD) activity, whereas P450IA2 was more active than P450IA1 in catalyzing uroporphyrinogen oxidation (UROX). Both reactions required NADPH, NADPH-cytochrome P450 reductase, and either P450IA1 or IA2. Ketoconazole competitively inhibited both EROD and UROX activities, in microsomes from MC-treated mice. Ketoconazole also inhibited UROX catalyzed by reconstituted P450IA2. In contrast, ketoconazole did not inhibit UROX catalyzed by xanthine oxidase in the presence of iron-EDTA. Superoxide dismutase, catalase, and mannitol inhibited UROX catalyzed by xanthine oxidase/iron-EDTA, but did not affect UROX catalyzed by either microsomes or reconstituted P450IA2. These results suggest that UROX catalyzed by P450IA2 in microsomes and reconstituted systems does not involve free reactive oxygen species. Two known substrates of cytochrome P450IA2, 2-amino-3,4-dimethylimidazole[4,5-f]quinoline and phenacetin, were shown to inhibit the microsomal UROX reaction, suggesting that uroporphyrinogen binds to a substrate-binding site on the cytochrome P450.

Effects of diphenyl ether herbicides on porphyrin accumulation by cultured hepatocytes

Several diphenyl ether herbicides, such as acifluorfen methyl, have been previously shown to cause large accumulations of the heme and chlorophyll precursor, protoporphyrin, in plants. Light-induced herbicidal damage is mediated by the photoactive porphyrin. Here we investigate whether diphenyl ether herbicides can affect porphyrin synthesis in rat and chick hepatocytes. In rat hepatocyte cultures, protoporphyrin, as well as coproporphyrin, accumulated after treatment with acifluorfen or acifluorfen methyl. Combination of acifluorfen methyl with an esterase inhibitor to prevent the conversion of acifluorfen methyl to acifluorfen resulted in a greater accumulation of porphyrins than caused by acifluorfen methyl or acifluorfen alone. In vitro enzyme studies of hepatic mitochondria isolated from rat and chick embryos demonstrated that protoporphyrinogen oxidase, the penultimate enzyme of heme biosynthesis, was inhibited by low concentrations of acifluorfen, nitrofen, or acifluorfen methyl with the latter being the most potent inhibitor. These findings indicate that diphenyl ether treatment can cause protoporphyrin accumulation in rat hepatocyte cultures and suggest that this accumulation was associated with the inhibition of protoporphyrinogen oxidase. In cultured chick embryo hepatocytes, treatment with acifluorfen methyl plus an esterase inhibitor caused massive accumulation of uroporphyrin rather than protoporphyrin or coproporphyrin. Specific isozymes of cytochrome P450 were also induced in chick embryo hepatocytes. These effects were not observed in the absence of an esterase inhibitor. These results suggest that diphenyl ether herbicides can cause uroporphyrin accumulation similar to that induced by other cytochrome P450-inducing chemicals such as polyhalogenated aromatic hydrocarbons in the chick hepatocyte system.

Chlorinated biphenyls induce cytochrome P450IA2 and uroporphyrin accumulation in cultures of mouse hepatocytes

Previous enzymatic and immunological studies from this laboratory have indicated a critical role for cytochrome P450IA2-catalyzed uroporphyrinogen oxidation in the development of uroporphyria caused by halogenated aromatic hydrocarbons. To extend these studies, we investigated whether primary cultures of mammalian hepatocytes which are inducible for cytochrome P450IA2 are also inducible for chemically mediated uroporphyria. Hepatocytes were isolated from C57BL/6 mice and maintained on Matrigel, an extracellular matrix isolated from a mouse tumor. When these cultures were treated with 3,4,5,3',4',5'-hexachlorobiphenyl (HCB) and 5-aminolevulinic acid (ALA), they accumulated cytochrome P450IA2 as well as uroporphyrin (URO) and heptacarboxyporphyrin for up to 12 days. Cultures treated with ALA alone accumulated no P450IA2 and very little URO. Neither URO accumulation nor the level of P450IA2 was affected by addition of iron as the nitrilotriacetate complex. Other inducers of P450IA2 in vivo (3,4,5,3',4'-pentachlorobiphenyl, 3,4,3',4'-tetrachlorobiphenyl, and 3-methylcholanthrene) also increased P450IA2 in the cultures and caused URO accumulation in the presence of added ALA. The tetrachlorobiphenyl and methylcholanthrene caused these effects only when given repeatedly. Inducers of other forms of P450 failed to cause URO accumulation in the presence of ALA and iron. Cultures of hepatocytes from DBA mice (which are resistant to the uroporphyria in vivo) accumulated much less P450IA2 or URO when treated with HCB and ALA. These primary cultures of mammalian hepatocytes represent a new experimental model to investigate the role of cytochrome P450IA2 in the mechanism of chemically induced uroporphyria.

Isolation of four forms of acetone-induced cytochrome P-450 in chicken liver by h.p.l.c. and their enzymic characterization

The purpose of this study was to purify and characterize the forms of cytochrome P-450 induced in chicken liver by acetone or ethanol. Using high performance liquid ion-exchange chromatography, we were able to isolate at least four different forms of cytochrome P-450 which were induced by acetone in chicken liver. All four forms of cytochrome P-450 proved to be distinct proteins, as indicated by their N-terminal amino acid sequences and their reconstituted catalytic activities. Two of these forms, also induced by glutethimide in chicken embryo liver, appeared to be cytochromes P450IIH1 and P450IIH2. Both of these cytochromes P-450 have identical catalytic activities towards benzphetamine demethylation. However, they differ in their abilities to hydroxylate p-nitrophenol and to convert acetaminophen into a metabolite that forms a covalent adduct with glutathione at the 3-position. Another form of cytochrome P-450 induced by acetone is highly active in the hydroxylation of p-nitrophenol and in the conversion of acetaminophen to a reactive metabolite, similar to reactions catalysed by mammalian cytochrome P450IIE. Yet the N-terminal amino acid sequence of this form has only 30-33% similarity with cytochrome P450IIE purified from rat, rabbit and human livers. A fourth form of cytochrome P-450 was identified whose N-terminal amino acid sequence and enzymic activities do not correspond to any mammalian cytochromes P-450 reported to be induced by acetone or ethanol.

Effect of serum proteins on haem uptake and metabolism in primary cultures of liver cells

A role of haemopexin in transporting haem to hepatocytes for degradation has been inferred from the high affinity of haemopexin for haem. We have examined this question in primary cultures of chick-embryo and adult rat liver cells. We present here the results of four sets of experiments which indicate that haemopexin retarded haem uptake by hepatocytes in culture. (1) Haem bound to bovine serum albumin is known to repress the activity of delta-aminolaevulinate synthase in chick cultures as indicated by decreased porphyrin accumulation. When haem-albumin was added in the presence of excess purified or freshly secreted chicken haemopexin, no haem-mediated repression of porphyrin production was observed. The haem-mediated repression of porphyrin accumulation was partially prevented when human, but not chicken, albumin was added to cultures. This finding reflected the higher affinity of human albumin for haem compared with that of chicken albumin. (2) Haemopexin inhibited the ability of haem to be incorporated into cytochrome P-450 induced in the chick cultures in the presence of the iron chelator desferrioxamine. (3) The rate of association of [55Fe]haem with cultured rat hepatocytes when [55Fe]haem-haemopexin was added was one-eighth of the rate observed when [55Fe]haem-bovine serum albumin was used as the haem donor. (4) The presence of haemopexin also diminished the catabolism of haem by both rat and chick-embryo liver cell cultures. It is concluded that the uptake and subsequent metabolic effects of haem are inhibited in cultured hepatocytes by proteins such as haemopexin which have a high affinity for haem.

Uroporphyria produced in mice by 20-methylcholanthrene and 5-aminolaevulinic acid

Iron-loaded male C57BL/6 mice allowed free access to an aqueous solution of 5-aminolaevulinic acid (ALA) (2 mg/ml) as their only drink, develop severe uroporphyria within 9 days of a single intraperitoneal dose of 20-methylcholanthrene (MC) (125 mg/kg). At 21 days, uroporphyrinogen decarboxylase (EC 4.1.1.37) activities are less than 10% of control activities. The porphyria is not dependent on pretreatment with iron and persists for at least 21 days after withdrawal of ALA. The same intraperitoneal dose of MC does not produce porphyria within 21 days when given without ALA. Continuous administration of ALA markedly accelerates the onset of porphyria in iron-loaded male C57BL/6 mice after a single intraperitoneal dose of hexachlorobenzene (200 mg/kg); mice given phenobarbitone and ALA do not become porphyric. MC with ALA does not produce porphyria in iron-loaded male DBA/2 mice. At least two separate events are needed to produce uroporphyria in mammals: induction of a specific form of cytochrome P-450 and stimulation of the formation of intermediates of haem biosynthesis in the liver. These results show that severe, persistent porphyria can be produced in mammals by compounds other than polyhalogenated aromatic hydrocarbons and suggest that a similar mechanism underlies the porphyrogenic action of halogenated and non-halogenated compounds.

Mitochondrial reduction of the carcinogen chromate: formation of chromium(V)

Incubation of chromate with isolated rat liver mitochondria in vitro resulted in the uptake and reduction of chromium(VI), as well as the formation of chromium(V) species. Chromate was rapidly taken up and reduced by intact mitochondria. The rate of reduction of chromate by intact mitochondria was increased upon addition of succinate or malate plus glutamate, substrates for the electron-transport chain, but was decreased upon addition of cyanide, an inhibitor of the electron-transport chain. Incubation of chromate with mitochondria in the presence or absence of malate, glutamate, and succinate resulted in a steady increase in the level of chromium(V) over time. The extent of chromium(V) formation was increased upon addition of malate, glutamate, and succinate but was inhibited upon addition of the electron-transport chain inhibitors, antimycin, cyanide, or rotenone, to whole mitochondria. High levels of glutamate plus malate inhibited chromium(V) formation; however, high concentrations of succinate or sulfate had no effect. These studies suggest that the chromate-reductase activity in mitochondria is due to the electron-transport chain as well as other mitochondrial reducing systems which are insensitive to inhibitors of the electron-transport chain. Since chromium(VI) is effectively metabolized by mitochondria in vitro and chromium(V) "reactive intermediates" are formed in the process, mitochondria may play a role in chromium(VI) carcinogenesis.

The effects of caffeine, impulsivity, and sex on memory for word lists

The present study examined the effects of caffeine on memory for supraspan word lists. Twelve groups of male and female college students classified as high or low impulsive received either 0, 2, or 4 mg/kg of caffeine. Female subjects were tested only during the menstrual phase of their cycle and were not taking oral contraceptives. Subjects listened to 12 word lists presented at one of four rates. Caffeine facilitated recall in females after practice with the task, but impaired recall in males only at the medium dose. The observed effects of caffeine were not influenced by subject's verbal ability, typical amount of caffeine consumption, or level of impulsivity. The results suggest that the effects of caffeine on females may vary according to the level of estrogen in the subject's system.

Automated rate-immunonephelometric determination of serum prealbumin (transthyretin)

We describe an automated determination of serum prealbumin (transthyretin), based on the maximum rate of light scatter of an antigen-antibody precipitate. Optimal dilutions of antibody reagent and of 100-microL serum samples are given. Within- and between-assay CVs are less than 5%. Results by this method are nearly identical with those obtained by a radial-immunodiffusion technique. Measuring the rate of light scatter provides a considerably faster test than equilibrium scatter, electrophoretic, or radial-immunodiffusion methods. Treatment before analysis to clear lipemic sera obviates falsely high results.

Automated rate-immunonephelometric determination of serum prealbumin (transthyretin)

We describe an automated determination of serum prealbumin (transthyretin), based on the maximum rate of light scatter of an antigen-antibody precipitate. Optimal dilutions of antibody reagent and of 100-microL serum samples are given. Within- and between-assay CVs are less than 5%. Results by this method are nearly identical with those obtained by a radial-immunodiffusion technique. Measuring the rate of light scatter provides a considerably faster test than equilibrium scatter, electrophoretic, or radial-immunodiffusion methods. Treatment before analysis to clear lipemic sera obviates falsely high results.