The influence of carbamazepine on the heme biosynthetic pathway

The effect of therapeutic drugs and other pharmacologic agents on activity of porphobilinogen deaminase, the enzyme that is deficient in intermittent acute porphyria

Drugs and toxins precipitate life-threatening acute attacks in patients with intermittent acute porphyria. These materials may act by directly inhibiting enzyme activity, thus further reducing porphobilinogen (PBG) deaminase activity below the ca. 50% level that results from the gene defect. To test this, we studied the effects of drugs that precipitate acute attacks (lead, phenobarbital, griseofulvin, phenytoin, sulfanilamide, sulfisoxazole, 17alpha-ethinyl estradiol, 5beta-pregnan-3alpha-ol-20-one), drugs that are safe (lithium, magnesium, chlorpromazine, promethazine), and those with uncertain effects (ethyl alcohol, imipramine, diazepam, haloperidol) on activity of PBG deaminase in vitro and in vivo. In the in vitro studies, of PBG deaminase from human erythrocytes from normals and individuals with IAP, only lead (> or = .01 mM) inhibited enzyme activity. Chlorpromazine (> or = .01 mM), promethazine (> or = .01 mM) and imipramine (1 mM) seemed to increase enzyme activity. In most in vivo experiments, male rats were injected intraperitoneally with test material twice daily for 3 days and once on day four; and erythrocyte and hepatic PBG deaminase activity was assayed thereafter. Effects on enzyme activity were observed only with 17alpha-ethinyl estradiol (0.05 microg/kg/day; reduction of 11% in erythrocyte enzyme [NS], and of 20% in liver enzyme [P=.02]), and imipramine (12.5 mg/kg/day; reduction in erythrocyte enzyme activity of 13% [P<.001]). Rats given lead acetate in their drinking water (10 mg/ml) for the first 60 days of life, resulting in high blood and liver lead levels, had increased erythrocyte PBG deaminase (167% of control; P=.004). Thus, enzyme inhibition by lead in vitro was not reflected in a similar in vivo inhibition. The only inhibitory effects in vivo, with ethinyl estradiol and imipramine, appear to be mild and biologically inconsequential. We conclude that inhibition of PBG deaminase activity by materials that precipitate acute attacks is an unlikely mechanism by which these materials exert their harmful effects in patients with IAP.

Effects of acute carbamazepine administration on haem metabolism in rat liver

The effects of acute carbamazepine (CBZ) administration on haem metabolism in rat liver were examined in relation to the mechanism by which it exacerbates hepatic porphyrias. In a screening test for drug exacerbation of porphyria developed in this laboratory, CBZ at a very small dose (1.5 mg/kg, p.o.) behaved as an exacerbator, potentiating the loss of haem utilized by tryptophan pyrrolase (TP; tryptophan 2,3-dioxygenase; L-tryptophan-O2 oxido-reductase, decyclizing; EC 1.13.11.11) and the associated induction of activity of the rate-limiting enzyme of haem biosynthesis, 5-aminolaevulinate synthase (5-ALA-S) caused by the experimental porphyrogen 3,5-diethoxycarbonyl-1,4-dihydrocollidine. A larger dose of CBZ (50 mg/kg, i.p.) induced 5-ALA-S activity by 40-100% at 3 hr. This induction was preceded by an increase in the haem saturation of TP, and was abolished when such an increase was prevented by allopurinol. 5-ALA-S induction by CBZ was not associated with decreased turnover of the enzyme, nor with any significant changes in concentration of the major hepatic haemoprotein, cytochrome P450. It is suggested that CBZ may exacerbate the hepatic porphyrias by inducing 5-ALA-S activity secondarily to an increased utilization of haem by TP.

Protoporphyrinaemia and decreased activities of 5-aminolevulinic acid dehydrase and uroporphyrinogen I synthetase in erythrocytes of a Vitamin B6-deficient epileptic boy given valproic acid and carbamazepine

Carbamazepine (CBMZP) has been implicated as an inhibitor of the activities of 5-aminolaevulinic acid dehydratase (ALA-D) and uroporphyrinogen I synthetase (URO-S). In an epileptic boy undergoing long-term treatment with valproic acid (VPA), 1.3 g/d, CBMZP, 0.9 g/d and folic acid, 7.5 mg/d, decreased activities of ALA-D and URO-S coincided with increased levels of erythrocyte protoporphyrin (EP) in the absence of Pb poisoning, iron depletion and erythropoietic protoporphyria. A progressive fall in plasma pyridoxal 5'-phosphate (B6-P) to 7.7 nmol/L (lower reference limit, 14.6 nmol/L) prompted implementation of pyridoxine HCl (B6-HCl), 87.5 mg/d followed by administration of both B6-HCl and preformed B6-P (50 mg/d each). This permitted the eventual withdrawal of VPA and a net reduction of CBMZP to 450 mg/d. During these manipulations, ALA-D and URO-S activities, EP and urinary porphyrins and their precursors were measured serially. An assay system utilizing red cell ALA-D for generation of porphobilinogen (PBG) from added ALA at pH 7.4 was used for determination of ALA-D and URO-S activities in separate aliquots of the same assay mixture both in the absence and presence of Zn and dithiothreitol (DTT). One unit (U) for ALA-D = 1 nmol PBG/L RBC/s; for URO-S = 1 nmol porphyrin/L/s; minimum normal level for ALA-D = 135 U; for URO-S = 6 U. B6-HCl alone entailed increases in ALA-D and URO-S prior to any reduction of CBMZP. After administration of both B6-HCl and B6-P and withdrawal of VPA, the overall increase in ALA-D was from 54.59 to 197.2 U (-Zn; -DTT) and from 50.76 to 217.3 U (+Zn; +DTT). The overall increase in URO-S was from 2.67 to 8.90 U (-Zn; -DTT) and from 3.02 to 8.66 U (+Zn; +DTT). During stepwise reduction of VPA, EP remained elevated to values as high as 2.48 mumol/L (upper reference limit, 1.33 mumol/L). Only after permanent withdrawal of VPA did concentrations of EP fall to normal levels. Values for porphyrins and their precursors in urine were normal throughout. Since both VPA and B6-P are strongly protein-bound, it is suggested that VPA displaced B6-P from protective protein binding sites and that the resulting deficit in B6-P (rather than CBMZP) reduced ALA-D and URO-S activities via primary reduction of ALA-synthetase activity. Increases in EP emerge as a hitherto unappreciated effect of VPA warranting further investigation.

Hepatic uroporphyrin accumulation and uroporphyrinogen decarboxylase activity in cultured chick-embryo hepatocytes and in Japanese quail (Coturnix coturnix japonica) and mice treated with polyhalogenated aromatic compounds

The relationship between hepatic uroporphyrin accumulation and uroporphyrinogen decarboxylase (EC 4.1.1.37) activity was investigated in cultured chick-embryo hepatocytes, Japanese quail (Coturnix coturnix japonica) and mice that had been treated with polyhalogenated aromatic compounds. Chick-embryo hepatocytes treated with 3,3',4,4'-tetrachlorobiphenyl accumulated uroporphyrin in a dose-dependent fashion without a detectable decrease in uroporphyrinogen decarboxylase activity when either pentacarboxyporphyrinogen III or uroporphyrinogen III were used as substrates in the assay. Other compounds, such as hexachlorobenzene, parathion, carbamazepine and nifedipine, which have been shown previously to cause uroporphyrin accumulation in these cells, did not decrease uroporphyrinogen decarboxylase activity. Japanese quail treated with hexachlorobenzene for 7-10 days also accumulated hepatic uroporphyrin without any decrease in uroporphyrinogen decarboxylase activity. In contrast, hepatic uroporphyrin accumulation in male C57BL/6 mice treated with iron and hexachlorobenzene was accompanied by a 20-80% decrease in uroporphyrinogen decarboxylase activity, demonstrating that the assay used for uroporphyrinogen decarboxylase, using pentacarboxyporphyrinogen III as substrate, could detect decreased enzyme activity. Our results with chick hepatocytes and quail, showing uroporphyrin accumulation without a decrease in uroporphyrinogen decarboxylase activity, are consistent with a new two-stage model of the uroporphyria: initially uroporphyrinogen is oxidized by a cytochrome P-450-mediated reaction, followed in rodents by a progressive decrease in uroporphyrinogen decarboxylase activity.