Synergistic induction of delta-aminolevulinate synthase by glutethimide and iron: relationship to the synergistic induction of heme oxygenase

Effects of modulators of protein phosphorylation on heme metabolism in human hepatic cells: induction of delta-aminolevulinic synthase mRNA and protein by okadaic acid

Effects of modulators of protein phosphorylation on delta-aminolevulinic acid (ALA) synthase and heme oxygenase-1 mRNA were analyzed in the human hepatic cell lines Huh-7 and HepG2 using a quantitative RNase protection assay. Okadaic acid was found to induce ALA synthase mRNA in a concentration-dependent fashion in both Huh-7 and HepG2 cells. The EC(50) for induction of ALA synthase mRNA in Huh-7 cells was 13.5 nM, with maximum increases occurring at okadaic acid concentrations of 25-50 nM. The EC(50) for induction of ALA synthase mRNA in HepG2 cells was 35.5 nM, with maximum increases occurring at okadaic acid concentrations of 50 nM. Concentration-dependent induction of ALA synthase mRNA paralleled the increase in ALA synthase protein. Maximum induction of ALA synthase was observed between 5 and 10 h post-treatment in both cell lines. Induction of ALA synthase mRNA in Huh-7 cells, but not HepG2 cells, was associated with an increase in ALA synthase mRNA stability. Okadaic acid also induced heme oxygenase-1 mRNA in both cell lines, but the magnitude of induction was only twofold, and was rapid and transient. Okadaic acid and phorbol 12-myristate 13-acetate significantly decreased heme-mediated induction of heme oxygenase-1 mRNA in both Huh-7 and HepG2 cells. Wortmannin diminished the heme-mediated induction of heme oxygenase-1 mRNA in HepG2 cells, but not Huh-7 cells. These results report a novel property of okadaic acid to affect heme metabolism in human cell lines.

Regulation of heme metabolism in rat hepatocytes and hepatocyte cell lines: delta-aminolevulinic acid synthase and heme oxygenase are regulated by different heme-dependent mechanisms

Regulation of delta-aminolevulinic acid (ALA) synthase and heme oxygenase was analyzed in primary rat hepatocytes and in two immortalized cell lines, CWSV16 and CWSV17 cells. ALA synthase was induced by 4,6-dioxohepatnoic acid (4,6-DHA), a specific inhibitor of ALA dehydratase, in all three systems; however, the induction in CWSV17 cells was greater than in either of the other two systems. Therefore, CWSV17 cells were used to explore the regulation of both enzymes by heme and 4,6-DHA. Data obtained from detailed concentration curves demonstrated that 4,6-DHA induced the activity of ALA synthase once ALA dehydratase activity became rate-limiting for heme biosynthesis. Heme induced heme oxygenase activity with increases occurring at concentrations of 10 microM or greater. Heme blocked the 4,6-DHA-dependent induction of ALA synthase with an EC50 of 1.25 microM. Heme-dependent decreases of ALA synthase mRNA levels occurred more quickly and at lower concentrations than heme-dependent increases of heme oxygenase mRNA levels. ALA synthase mRNA remained at reduced levels for extended periods of time, while the increases in heme oxygenase mRNA were much more transient. The drastic differences in concentrations and times at which heme-dependent effects were observed strongly suggest that two-different heme-dependent mechanisms control the ALA synthase and heme oxygenase mRNAs. In CWSV17 cells, heme decreased the stability of ALA synthase mRNA from 2.5 to 1.3 h, while 4,6-DHA increased the stability of the mRNA to 5.2 h. These studies demonstrate that regulation of ALA synthase mRNA levels by heme in a mammalian system is mediated by a change in ALA synthase mRNA stability. The results reported here demonstrate the function of the regulatory heme pool on both ALA synthase and heme oxygenase in a mammalian hepatocyte system.

Heme biosynthesis in a chicken hepatoma cell line (LMH): comparison with primary chick embryo liver cells (CELC)

5-Aminolevulinic acid synthase (ALA synthase), the rate-controlling enzyme of hepatic heme biosynthesis, is feed-back repressed by heme. In the liver, chemicals such as barbiturates markedly induce ALA synthase, especially in the presence of partial defects of heme biosynthesis. The inducibility and regulation of ALA synthase have been investigated using a variety of models, including intact animals and liver cell culture systems. A widely used model that closely approximates what occurs in vivo and in humans is that of primary cultures of chick embryo liver cells (CELCs). However, CELCs have some limitations: the cells obtained are somewhat heterogeneous; isolation and culture must be repeated every week resulting in weekly variations; and cells are short-lived limiting the feasibility of time-course and transfection studies. The aim of this study was to determine if LMH cells, a chick hepatoma cell line, are a good model comparable to that of CELCs. In both cells similar patterns of response of, ALA synthase activities and mRNA levels, and of porphyrin accumulation were obtained following treatments known to affect heme biosynthesis. Similarly, heme repressed ALA synthase mRNA levels in both cell types and ALA synthase activities in LMH cells. We conclude that LMH cells are a useful model for the study of hepatic heme biosynthesis and regulation of ALA synthase.

Effect of sodium arsenite on heme metabolism in cultured chick embryo hepatocytes

We had previously reported that low concentrations of sodium arsenite (1-5 microM) decreased the induction of cytochrome P450 CYP1A and CYP2H in cultured chick embryo hepatocytes in parallel with increases in heme oxygenase. However, in those studies exogenous heme did not prevent the decrease in CYPs. In this study, we investigated the effect of arsenite on the synthesis and degradation of heme. Arsenite had no effect on induction of 5-aminolevulinic acid synthase mRNA or activity. Arsenite, at concentrations from 1 to 25 microM, had no effect on protoporphyrin synthesis from 5-aminolevulinic acid and did not increase the accumulation of other porphyrins, indicating that the enzymes in the pathway between 5-aminolevulinic acid synthase and ferrochelatase were unaffected by arsenite. Synthesis of heme from radioactive 5-aminolevulinic acid was slightly decreased (less than 20%) by 2.5 microM arsenite, a concentration that decreased induction of CYP1A and CYP2H by greater than 50%. Rates of biliverdin formation and degradation of exogenous heme were not different in cultures treated simultaneously with arsenite and heme or with heme alone. However, arsenite treatment increased biliverdin formation from heme synthesized from added 5-aminolevulinic acid by 60% and decreased the endogenous heme content of the cells by 30%. Our results suggest that although 2.5 microM arsenite induced heme oxygenase four- to sixfold, this had no effect on degradation of exogenous heme. Degradation of heme synthesized from 5-aminolevulinic acid was increased but this did not affect the regulatory heme pool.

Effects of selected antihypertensives and analgesics on hepatic porphyrin accumulation: implications for clinical porphyria

When patients with acute porphyrias are treated with antihypertensives and analgesics, they could be placed at increased risk of developing porphyric attacks, since little is known about the potential for many of these drugs to induce these attacks. We used primary chick embryo liver cells, which maintain intact heme synthesis and regulation, to study the effects of antihypertensives and analgesics on porphyrin accumulation. Cells were treated with desferrioxamine to block heme synthesis partially, simulating conditions encountered in porphyric patients. Typically, cells were treated for 20 hr with the test drugs (3.16 to 1000 microM), along with desferrioxamine. Porphyrins were measured spectrofluorometrically, as uro-, copro,- and protoporphyrin. The evaluated drugs included six antihypertensives (two calcium channel blockers, an angiotensin receptor antagonist, and three inhibitors of angiotensin converting enzyme) and eight analgesics. Of the calcium channel blockers tested, nifedipine greatly increased porphyrin accumulation, whereas diltiazem caused only a slight increase. Losartan (an angiotensin receptor antagonist), captopril, or lisinopril (two angiotensin converting enzyme inhibitors) produced only small increases in porphyrin accumulation. In contrast, enalapril (another angiotensin converting enzyme inhibitor) substantially increased porphyrin accumulation when given in high concentrations. Among the analgesics tested, fentanyl and tramadol produced the highest porphyrin accumulations. Nalbuphine, hydrocodone, oxycodone, and dezocine were moderately or weakly porphyrogenic, whereas buprenorphine and morphine did not increase porphyrin accumulation. These studies suggest that patients with acute porphyrias may be at greater risk for developing porphyric attacks when treated with nifedipine (compared with diltiazem), enalapril (compared with captopril or lisinopril), and tramadol (compared with the other analgesics).

Mechanism of the synergistic induction of CYP2H by isopentanol plus ethanol: comparison to glutethimide and relation to induction of 5-aminolevulinate synthase

We had previously found that combined treatment with isopentanol and ethanol synergistically induced CYP2H protein and activity in cultured chick nepatoytes. Here we investigated the mechanism of induction of CYP2H by the alcohols and whether they caused a coordinate induction of 5-aminolevulinate synthase (ALAS) mRNA. Treatment with isopentanol alone or in combination with ethanol resulted in coordinate increases in CYP2H1 and ALAS mRNAs. With isopentanol alone, the amounts of CYP2H1 and ALAS mRNAs at 4 to 6 h were similar to those observed after treatment with the alcohol combination, but declined by 11 h. Readdition of isopentanol at 11 h again increased the expression of both mRNAs, indicating that the decreases at 11 h were due to limiting amounts of inducer. Similar results were observed in cells exposed to low concentrations of glutethimide. In the combined alcohol treatment, increases in CYP2H1 and ALAS mRNAs were sustained from 4 h to 11 h after addition of the alcohols, but decreased to control levels by 24 h. Using pulse labeling to measure de novo synthesis of CYP2H1/2 protein, we found that the increases in CYP2H1/2 protein reflected the increases in CYP2H1 mRNA. The half-life of CYP2H1/2 protein, measured from pulse-chase experiments, was approximately twofold greater than the half-life of CYP2H1 mRNA. Our results indicate that the alcohols and glutethimide coordinately increase ALAS and CYP2H1 mRNA, and that increases in CYP2H1/2 protein arise from increases in its mRNA.

Mechanism of sodium arsenite-mediated induction of heme oxygenase-1 in hepatoma cells. Role of mitogen-activated protein kinases

Heme oxygenase-1 is an inducible enzyme that catalyzes heme degradation and has been proposed to play a role in protecting cells against oxidative stress-related injury. We investigated the induction of heme oxygenase-1 by the tumor promoter arsenite in a chicken hepatoma cell line, LMH. We identified a heme oxygenase-1 promoter-driven luciferase reporter construct that was highly and reproducibly expressed in response to sodium arsenite treatment. This construct was used to investigate the role of mitogen-activated protein (MAP) kinases in arsenite-mediated heme oxygenase-1 gene expression. In LMH cells, sodium arsenite, cadmium, and heat shock, but not heme, induced activity of the MAP kinases extracellular-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. To examine whether these MAP kinases were involved in mediating heme oxygenase-1 gene expression, we utilized constitutively activated and dominant negative components of the ERK, JNK, and p38 MAP kinase signaling pathways. Involvement of an AP-1 site in arsenite induction of heme oxygenase-1 gene expression was studied. We conclude that the MAP kinases ERK and p38 are involved in the induction of heme oxygenase-1, and that at least one AP-1 element (located -1576 base pairs upstream of the transcription start site) is involved in this response.

Regulation of expression of the human heme oxygenase-1 gene in transfected chick embryo liver cell cultures

Induction of heme oxygenase (HO) has been proposed as a protective cellular mechanism against oxidative damage. In previous work (Tyrrell et al., Carcinogenesis [1993] 14, 761-765), portions of the 5' promoter region of the human HO-1 gene linked to the reporter gene chloramphenicol acetyl transferase (CAT), had been transiently expressed in HeLa cells. To extend the study of human HO gene expression into primary liver cells, these reporter gene fusion constructs, containing 121 or 1416 base pairs of the untranscribed 5'-upstream sequences of the human HO-1 gene, were used along with pSV beta-Gal plasmid to dually transfect primary cultures of chick embryo liver cells (CELC). The transfected cells were treated with selected metals, heme, phorbol ester, and chemical agents that produce oxidative stress (H2O2 or sodium arsenite). Reporter gene activities were measured 18-20 h later. Our major findings are: (1) these HO-CAT constructs were expressed in CELC; (2) unlike HeLa cells, the expression of CAT was detected in CELC without the need for the SV40 enhancer; (3) sodium arsenite and cobalt chloride induced the expression of the HO-CAT constructs whereas heme had no effect on or decreased CAT expression for all of the transfected constructs; (4) study of endogenous chick HO-1 gene expression in CELC showed that HO-1 responded to sodium arsenite treatment in a dose-dependent fashion, and the response was rapid and transient. We conclude that, in chick liver cell cultures, induction of the HO-1 gene by heme is fundamentally different from that produced by transition metals or sodium arsenite. Furthermore, the results suggest that expression of the HO-1 gene is highly conserved across species.

Mechanism of induction of heme oxygenase by metalloporphyrins in primary chick embryo liver cells: evidence against a stress-mediated response

Heme oxygenase catalyzes the first and rate-controlling step in heme catabolism. One of the two forms of heme oxygenase (heme oxygenase-1) has been shown to be increased by heme, metals, and in some systems, by certain environmental stresses. However, it remains uncertain whether heme induces hepatic heme oxygenase-1 by a general stress response, or a specific heme-dependent cellular response. The work communicated here explores this issue by examining possible mechanisms whereby heme and other metalloporphyrins induce heme oxygenase-1 in normal liver cells. Primary cultures of chick embryo liver cells were tested for their ability to increase heme oxygenase mRNA after exposure to selected metalloporphyrins (heme, chromium mesoporphyrin, cobalt protoporphyrin and manganese protoporphyrin). The ability of antioxidants to decrease metalloporphyrin-mediated induction of heme oxygenase-1 mRNA was also tested. Our results indicate that: 1) the increase in heme oxygenase-1 mRNA mediated by heme or other metalloporphyrins may involve a short-lived protein(s) since the increase was prevented by several inhibitors of protein synthesis; and 2) in normal liver cells, heme-dependent oxidative stress does not play a key role in the heme-mediated induction of heme oxygenase-1. We conclude that heme and other non-heme metalloporphyrins induce heme oxygenase-1 through a mechanism requiring protein synthesis, not because metalloporphyrins increase cellular oxidative or other stress.

Hepatic 5-aminolevulinic acid synthase mRNA stability is modulated by inhibitors of heme biosynthesis and by metalloporphyrins

Hepatic 5-aminolevulinic acid synthase, the first and normally rate-controlling enzyme of heme biosynthesis, is regulated by heme. One of the known mechanisms whereby increased cellular heme regulates 5-aminolevulinic acid synthase is by decreasing the stability of its mRNA. In primary cultures of chick embryo liver cells, we tested whether a decrease in cellular heme might increase 5-aminolevulinic acid synthase mRNA stability and whether heme or other metalloporphyrins could reverse this stabilization. We found that: (a) The stability of 5-aminolevulinic acid synthase mRNA was markedly increased by inhibitors of heme biosynthesis, namely, 4,6-dioxoheptanoic acid or deferoxamine; (b) This increased stability of 5-aminolevulinic acid synthase mRNA was reversed by the addition of heme (10 microM) or by the combination of zinc mesoporphyrin (50 nM), an inhibitor of heme oxygenase, and heme (200 nM); (c) Repression of 5-aminolevulinic acid synthase mRNA levels by zinc mesoporphyrin (10 microM) was due to inhibition of heme oxygenase, rather than a direct, heme-like, effect of zinc mesoporphyrin on 5-aminolevulinic acid synthase mRNA; (d) Among the several non-heme metalloporphyrins tested, only zinc mesoporphyrin and chromium mesoporphyrin significantly decreased 5-aminolevulinic acid synthase mRNA without increasing heme oxygenase mRNA.

Loss and degradation of enzyme-bound heme induced by cellular nitric oxide synthesis

We report here that, like nonheme iron, protein-bound intracellular heme iron is also a target for destruction by endogenously produced nitric oxide (NO). In isolated rat hepatocytes NO synthesis results in substantial (approximately 60%) and comparable loss of catalase and cytochrome P450 as well as total microsomal heme, and decreased heme synthetic (delta-aminolevulinate synthetase and ferrochelatase) and increased degradative (heme oxygenase) enzymatic activities. The effect is reversible, and intact cytochrome P450 apoproteins are still present, as judged by heme reconstitution of isolated microsomes. The effects on delta-aminolevulinate synthetase and heme oxygenase are likely to be secondary to heme liberation, while the effects on ferrochelatase appear to be a direct effect of NO, perhaps destruction of its nonheme iron-sulfur center.

Effects of mifepristone (RU-486) on heme metabolism and cytochromes P-450 in cultured chick embryo liver cells, possible implications for acute porphyria

Mifepristone (RU-486), a potent progesterone receptor antagonist and inducer of cytochromes P-450, is currently in use in Europe, particularly as a post-coital oral contraceptive. Soon it will be available in the United States, as well. Since progesterone has been implicated in the pathogenesis of acute attacks of porphyria, the use of RU-486 or related compounds might be considered in porphyric patients. However, as with other cytochrome P-450 inducers, RU-486 may have the ability to precipitate or exacerbate attacks of acute porphyria. The acute porphyrias in relapse are associated with an increase in activity of delta-aminolevulinic acid synthase, the first and normally rate-controlling enzyme in heme biosynthesis. We have used primary cultures of chick embryo liver cells to test the ability of RU-486 to induce delta-aminolevulinic acid synthase activity and mRNA, cytochromes P-450, porphyrin accumulation, and heme oxygenase. We found that RU-486, at concentrations observed in human plasma after a single oral dose, induced the mRNA and activity of delta-aminolevulinic acid synthase, both by itself and in the presence of deferoxamine, a potent iron chelator that inhibits ferrochelatase. RU-486 and deferoxamine together also produced significant accumulations of protoporphyrin. These results indicate that RU-486 may pose a risk in patients with known acute porphyria and should be used with caution. RU-486 increased the concentration of total cytochrome P-450, and the activity of erythromycin demethylase, an activity specifically catalyzed by cytochrome P-450 3A. Unlike several other porphyrogens (e.g. hydantoins, barbiturates), RU-486 does not produce accumulation of uroporphyrin or induction of heme oxygenase in chick embryo liver cells.

Differential effects of metalloporphyrins on messenger RNA levels of delta-aminolevulinate synthase and heme oxygenase. Studies in cultured chick embryo liver cells

The acute porphyrias in relapse are commonly treated with intravenous heme infusion to decrease the activity of delta-aminolevulinic acid synthase, normally the rate-controlling enzyme in heme biosynthesis. The biochemical effects of heme treatment are short-lived, probably due in part to heme-mediated induction of heme oxygenase, the rate-controlling enzyme for heme degradation. In this work, selected nonheme metalloporphyrins were screened for their ability to reduce delta-aminolevulinic acid synthase mRNA and induce heme oxygenase mRNA in chick embryo liver cell cultures. Of the metalloporphyrins tested, only zinc-mesoporphyrin reduced delta-aminolevulinic acid synthase mRNA without increasing heme oxygenase mRNA. The combination of zinc-mesoporphyrin and heme, at nanomolar concentrations, decreased delta-aminolevulinic acid synthase mRNA in a dose-dependent manner. The combination of zinc-mesoporphyrin (50 nM) and heme (200 nM) decreased the half-life of the mRNA for delta-aminolevulinic acid synthase from 5.2 to 2.5 h, while a similar decrease was produced by heme (10 microM) alone (2.2 h). The ability of zinc-mesoporphyrin to supplement the reduction of delta-aminolevulinic acid synthase mRNA by heme, in a process similar to that observed with heme alone, provides a rationale for further investigation of this compound for eventual use as a supplement to heme therapy of the acute porphyrias and perhaps other conditions in which heme may be of benefit.

Repression of ALA synthase by heme and zinc-mesoporphyrin in a chick embryo liver cell culture model of acute porphyria

We characterize a liver cell culture model for acute hepatic porphyrias that recapitulates the biochemical features of the human syndrome. In chick embryo liver cells in primary culture exposed to glutethimide and 4,6-dioxoheptanoic acid, heme alone produced a transient dose-dependent decrease in delta-aminolevulinate synthase and a concomitant increase in heme oxygenase. The addition of low concentrations of zinc-mesoporphyrin (50-200 nM), an inhibitor of heme oxygenase, led to more prolonged decreases in activity of the synthase and to an additive effect with heme. These effects of zinc-mesoporphyrin were associated with prolonged inhibition of heme oxygenase. These results suggest that the treatment of choice of acute porphyric syndromes may be the combination of low doses of heme and zinc-mesoporphyrin or another similarly non-toxic inhibitor of heme oxygenase.

3,5,5-Trimethylhexanoylferrocene induction of heme oxygenase activity in normal hepatocytes

Recent work showed that the combination of 50 microM glutethimide plus 50 microM ferric nitrilotriacetate (FeNTA) synergistically induces heme oxygenase (HO) activity in cultured chick embryo liver cells (Cable et al., Biochem Biophys Res Commun 168: 176-181, 1990). This synergistic induction is due to increased heme synthesis, which then acts to increase HO gene transcription. The aim of the current studies was to characterize the effects on hepatic heme metabolism of (3,5,5-trimethylhexanoyl)ferrocene (TMH-ferrocene), which causes hepatic iron-loading in rats. Unlike FeNTA, TMH-ferrocene alone maximally induced HO activity at 5-10 microM TMH-ferrocene. At higher concentrations, HO activities declined, as did total cellular protein synthesis. Induction of HO was maximal after a 12-hr exposure to TMH-ferrocene, similar to induction by glutethimide plus FeNTA. The effect of TMH-ferrocene on HO could not be ascribed to greater cellular uptake of iron, since cell-associated iron levels were higher after FeNTA than after TMH-ferrocene treatment. TMH-ferrocene (up to 20 microM) did not induce delta-aminolevulinic acid synthase activity. Uroporphyrin accumulation in cells treated with TMH-ferrocene was minimal, but the combination of TMH-ferrocene and glutethimide caused a synergistic increase in uroporphyrin accumulation, similar to treatment with glutethimide plus FeNTA. 4,6-Dioxoheptanoic acid, an inhibitor of heme synthesis, blocked the induction of HO caused by glutethimide and FeNTA, but did not decrease the induction of HO by TMH-ferrocene. TMH-ferrocene-mediated induction of HO does not appear to be due to lipid peroxidation, since malondialdehyde formation was greater for ferrocene (a structural analog of TMH-ferrocene that does not induce HO) than for TMH-ferrocene. Furthermore, the anti-oxidant, butylated hydroxyanisole, which prevented lipid peroxidation, decreased HO induced by glutethimide plus FeNTA, but butylated hydroxyanisole did not affect HO induced by TMH-ferrocene. We conclude that, unlike the combination of glutethimide plus FeNTA, TMH-ferrocene induces HO activity by a mechanism that is independent of cellular heme synthesis.

Molecular basis for heme-dependent induction of heme oxygenase in primary cultures of chick embryo hepatocytes. Demonstration of acquired refractoriness to heme

The effects of heme on the induction of mRNA and protein synthesis for heme oxygenase-1 have been studied in primary cultures of chick embryo liver cells. Heme increased the amount of mRNA and the rate of heme oxygenase-1-gene transcription in a dose-dependent fashion, with a maximal 20-fold increase occurring at 20 microM heme. The largest increase in the rate of transcription, measured by nuclear run-on assays, occurred at 5 h, 2 h earlier than the maximum increase in the amount of mRNA, measured by densitometry of Northern blots. 7-15 h after heme addition, the half-life of heme-oxygenase-1 mRNA was 3.5 h in the presence or absence of actinomycin D. In contrast, addition of cycloheximide markedly increased the stability of the message (half-life = 18 h), suggesting that a short-lived protein plays a key role in modulating heme oxygenase-1 mRNA levels. The half-life of heme-induced heme-oxygenase-1 protein, measured by [35S]methionine labelling and immunoprecipitation, was 15 h. This long half-life of the protein can largely account for the additional finding that, following addition of heme, the amount of enzyme protein in the cells increased for 10 h, after which it remained essentially constant for 15 h. A striking finding was that, after an initial burst of heme-stimulated gene transcription, the cells became refractory to further heme-mediated induction. This acquired resistance could not be attributed to the following: a longer duration of culture time; cellular toxicity caused by heme; a lack of heme in the medium or the cells; secretion of heme-binding proteins into the medium, preventing further heme uptake; the induction of cellular heme catabolism sufficient to deplete cellular heme. Instead, the results suggest a down-regulation of the intracellular machinery required for heme-dependent induction of heme oxygenase-1.

Porphyrogenic properties of the terpenes camphor, pinene, and thujone (with a note on historic implications for absinthe and the illness of Vincent van Gogh)

Camphor, alpha-pinene (the major component of turpentine), and thujone (a constituent in the liqueur called absinthe) produced an increase in porphyrin production in primary cultures of chick embryo liver cells. In the presence of desferrioxamine (an iron chelator which inhibits heme synthesis and thereby mimics the effect of the block associated with acute porphyria), the terpenes enhanced porphyrin accumulation 5- to 20-fold. They also induced synthesis of the rate-controlling enzyme for the pathway, 5-aminolevulinic acid synthase, which was monitored both spectrophotometrically and immunochemically. These effects are shared by well-known porphyrogenic chemicals such as phenobarbital and glutethimide. Camphor and glutethimide alone led to the accumulation of mostly uro- and heptacarboxylporphyrins, whereas alpha-pinene and thujone resulted in lesser accumulations of porphyrins which were predominantly copro- and protoporphyrins. In the presence of desferrioxamine, plus any of the three terpenes, the major product that accumulated was protoporphyrin. The present results indicate that the terpenes tested are porphyrogenic and hazardous to patients with underlying defects in hepatic heme synthesis. There are also implications for the illness of Vincent van Gogh and the once popular, but now banned liqueur, called absinthe.