Induction of δ-aminolevulinic acid synthetase in the kidney of chicks treated with porphyrinogenic drugs

Quantifying porphobilinogen deaminase mRNA in microdissected nephron segments by a modified RT-PCR

BACKGROUND

Quantifying mRNA levels by reverse transcription-polymerase chain reaction (RT-PCR), although widely exercised, is still difficult.

METHODS

A modified quantitative RT-PCR in which genomic DNA was used as standard was developed. The quantity of mRNA was expressed as the ratio of the PCR product from cDNA and that from genomic DNA (CG ratio). Nephron distribution of porphobilinogen deaminase (PBGD) mRNA was examined in microdissected nephron segments using the method. The enzyme activity and mRNA quantity of PBGD also were measured in tissue homogenates.

RESULTS

Tubular segments expressed substantially more PBGD mRNA than glomeruli (expressed as CG ratios, 1.04 +/- 0.10 in glomeruli, 4.53 +/- 0.32 in PCT, 5.71 +/- 0.25 in PST, 5.13 +/- 0.52 in mTAL, 5.29 +/- 0.20 in cTAL, 4.05 +/- 0.35 in DCT, 2.88 +/- 0.25 in CCD, and 4.90 +/- 0.24 in OMCD). PBGD mRNA level in liver homogenate (3.17 +/- 0.36) was much higher than glomeruli but lower than most of the tubular segments. The enzyme activity in tissue homogenates correlated well with mRNA levels.

CONCLUSION

The method reported here is simple and reliable, and especially suitable for quantitating specific mRNA amounts in minute tissue samples such as microdissected nephron segments.

Administration of the anesthetic isoflurane to mice: a model for acute intermittent porphyria?

The effects of isoflurane, a commonly used volatile anesthetic, on the activity of some haem enzymes in liver, kidney, and blood, and glucose content in liver and blood were studied. Mice were injected with different doses of the drug (0.5-6 mL/kg) and killed at varying intervals after injection (5-240 min). Within this dose range, optimal effects on alteration of haem metabolism were obtained at 2 mL/kg. The time-response profile for each enzyme was different. Blood porphobilinogenase (PBGase) and deaminase showed lower activities 20 min after anesthesia. This diminution coupled with the induction of delta-aminolevulinate synthetase activity observed soon after anesthesia (5 min) would fit well with the expected biochemical changes occurring in acute intermittent porphyria, indicating that this may be a suitable animal model for this disease.

Porphyrinogenic properties of the anesthetic enflurane

1. The effect of enflurane, a volatile anesthetic, on heme metabolism was studied. Different doses (0.5-6.0 ml/kg) of this anesthetic were administered i.p. to mice and animals sacrificed at different times after administration (5-240 min). 2. The dose of 2 ml/kg was chosen as the optimum anesthetic dose producing more alterations in the heme pathway. 3. ALA-S was significantly induced at earlier times of anesthesia. 4. Blood PBGase and deaminase was greatly reduced. 5. This diminution coupled with ALA-S induction are in accordance with the known biochemical changes occurring in acute intermittent porphyria and include enflurane in the list of porphyrinogenic drugs, the use of which is not recommended for the management of anesthesia in porphyric patients.

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.

Control of 5-aminolevulinate synthase in animals

The proposed mechanism by which hepatic ALV-synthase mitochondrial levels are regulated is outlined in Fig. 2. ALV-synthase catalyzes the first and rate-limiting step in the heme pathway and is normally present in low amounts. A cytosolic, regulatory free heme pool tightly controls the amount of ALV-synthase in two ways. In the primary mechanism of regulation, heme is proposed to inhibit the synthesis of ALV-synthase mRNA. Most likely this would be mediated through the action of specific heme-binding protein(s) which recognize regulatory control regions of the ALV-synthase gene. Gene activity therefore is significantly repressed most of the time. When there is an increased demand for heme by newly synthesized cellular hemoproteins, the free heme pool is reduced, leading to a derepression of ALV-synthase mRNA synthesis. Once the need for increased heme synthesis is satisfied, inhibitory heme levels build up again. When drugs such as phenobarbital are administered to animals, there is a rapid induction in the liver of both cytochrome P-450 and ALV-synthase. It is proposed that the heme pool governing ALV-synthase levels is lowered by the increased heme demand due to cytochrome P-450 apoprotein formation. The primary event in the drug induction of ALV-synthase is therefore the increased synthesis of cytochrome P-450 apoprotein. However, the mechanism by which this occurs is unknown, although drugs do increase the synthesis of mRNA for cytochrome P-450 (Fig. 2). (There is evidence that for the aromatic hydrocarbons a specific cytosolic receptor exists.) In the acute hepatic porphyria diseases, uncontrolled synthesis of hepatic ALV-synthase occurs. The various forms are characterized by reduced levels of one of the heme pathway enzymes other than ALV-synthase. Attacks of the disease are commonly precipitated by drugs which induce cytochrome P-450, and the uncontrolled accumulation of ALV-synthase which accompanies these attacks results from the combined action of the block in the heme pathway and the increased cytochrome P-450 levels. A major challenge which now exists is to understand at the molecular level how the genes for ALV-synthase and cytochrome P-450 are regulated in the liver and other tissues.(ABSTRACT TRUNCATED AT 400 WORDS)