Protoporphyrinogen oxidase and ferrochelatase in porphyria variegata

Nine novel mutations in the protoporphyrinogen oxidase gene in Swedish families with variegate porphyria

Variegate porphyria (VP) is an autosomal-dominant disorder that is caused by inheritance of a partial deficiency of the enzyme protoporphyrinogen oxidase (EC 1.3.3.4). It is characterized by cutaneous photosensitivity and/or various neurological manifestations. Protoporphyrinogen oxidase catalyses the penultimate step of haem biosynthesis, and mutations in the PPOX gene have been coupled to VP. In the present study, sequencing analysis revealed 10 different mutations in the PPOX gene in 14 out of 17 apparently unrelated Swedish VP families. Six of the identified mutations, 3G > A (exon 2), 454C > T (exon 5), 472G > C (exon 6), 614C > T (exon 6), 988G > C (exon 10) and IVS12 + 2T > G (intron 12), are single nucleotide substitutions, while 604delC (exon 6), 916-17delCT (exon 9) and 1330-31delCT (exon 13) are small deletions, and IVS12 + 2-3insT (intron 12) is a small insertion. Only one of these 10 mutations has been reported previously. Three of the mutations were each identified in two or more families, while the remaining mutations were specific for an individual family. In addition to the 10 mutations, one previously unreported single nucleotide polymorphism was identified. Mutation analysis of family members revealed two adults and four children who were silent carriers of the VP trait. Genetic analysis can now be added to the conventional biochemical analyses and used in investigation of putative carriers of a VP trait in these families.

Allosteric inhibition of human lymphoblast and purified porphobilinogen deaminase by protoporphyrinogen and coproporphyrinogen. A possible mechanism for the acute attack of variegate porphyria

Variegate porphyria (VP) is characterized by photocutaneous lesions and acute neuropsychiatric attacks. Decreased protoporphyrinogen oxidase activity results in accumulation of protoporphyrin (ogen) IX and coproporphyrin (ogen) III. During acute attacks delta-aminolevulinic acid and porphobilinogen also increase, suggesting that porphobilinogen deaminase (PBG-D) may be rate limiting. We have examined the effects of porphyrinogens accumulating in VP on PBG-D activity in Epstein-Barr virus-transformed lymphoblast sonicates from 12 VP and 12 control subjects. Protoporphyrinogen oxidase activity was decreased and protoporphyrin increased in VP lymphoblasts. PBG-D in control lymphoblasts obeyed Michaelis-Menten kinetics (Vmax 28.7 +/- 1.8 pmol/mg per h, Hill coefficient 0.83 +/- 0.07). VP sonicates yielded sigmoidal substrate-velocity curves that did not obey Michaelis-Menten kinetics. Vmax was decreased (21.2 +/- 2.0 pmol/mg per h) and the Hill coefficient was 1.78 +/- 0.17. Addition of protoporphyrinogen IX and coproporphyrinogen III to control sonicates yielded sigmoidal PBG-D substrate-velocity curves and decreased PBG-D Vmax. Addition of porphyrins or uroporphyrinogen III did not affect PBG-D activity. Removal of endogenous porphyrin (ogens) from VP sonicates restored normal PBG-D kinetics. Purified human erythrocyte PBG-D obeyed Michaelis-Menten kinetics (Vmax 249 +/- 36 nmol/mg per h, Km 8.9 +/- 1.5 microM, Hill coefficient 0.93 +/- 0.14). Addition of protoporphyrinogen yielded a sigmoidal curve with decreased Vmax. The Hill coefficient approached 4. These findings provide a rational explanation for the increased delta-aminolevulinic acid and porphobilinogen during acute attacks of VP.

High-performance liquid chromatographic assays for protoporphyrinogen oxidase and ferrochelatase in human leucocytes

Rapid, sensitive and specific high-performance liquid chromatographic assays are described for protoporphyrinogen oxidase and ferrochelatase in human leucocytes. The enzyme reaction products were separated and quantitated by reversed-phase high-performance liquid chromatography with fluorescence detection. The optimal pH for the protoporphyrinogen oxidase assay was 8.6 and the Michaelis constant for protoporphyrinogen IX was 9.78 +/- 0.96 microM (mean +/- S.D.). The mean (+/- S.D.) activity of protoporphyrinogen oxidase in fourteen apparently healthy subjects was 0.146 +/- 0.023 nmol protoporphyrin IX per min per mg protein. In one patient with variegate porphyria, the activity was 0.028 nmol protoporphyrin IX per min per mg protein. The optimal pH for ferrochelatase was 7.4 and with protoporphyrin and Zn2+ as substrates, the Michaelis constants were 1.49 and 8.33 microM, respectively. The mean activity of ferrochelatase in ten control subjects was 0.24 nM Zn-protoporphyrin or 2.05 nM Zn-mesoporphyrin formed per h per mg protein.

Bile porphyrin analysis in the evaluation of variegate porphyria

BACKGROUND

Variegate porphyria is a genetic disorder of porphyrin metabolism in which patients may have both neurologic dysfunction and photocutaneous lesions. Biochemical confirmation of the diagnosis can be difficult, particularly in patients without neurologic dysfunction at the time of testing. The demonstration of increased fecal excretion of porphyrin is frequently used for this purpose, but levels may be normal. Since elevated fecal porphyrin levels in variegate porphyria are presumably a consequence of increased biliary excretion, we evaluated whether analysis of porphyrins in bile distinguishes better between patients with variegate porphyria and controls.

METHODS

Bile samples were collected by duodenal aspiration from 10 patients with proved variegate porphyria who had no neurologic symptoms when they were studied and 17 control subjects. Bile and fecal porphyrin levels were measured fluorometrically.

RESULTS

The mean total porphyrin concentration in bile in the patients with variegate porphyria was significantly higher than that in the controls (67.8 vs. 0.71 mumol per liter; P less than 0.00002). There was more than a ninefold difference between the highest level in any control subject and the lowest level in any patient with variegate porphyria. The mean fecal porphyrin level in the patients with variegate porphyria also differed significantly from that in the controls (0.79 vs. 0.14 mumol per gram of dry weight; P less than 0.007), but four patients had levels within the control range.

CONCLUSIONS

The concentration of porphyrin in bile is higher in patients with variegate porphyria than in controls, and the difference is greater than that for fecal porphyrin. Bile porphyrin measurements may be helpful in the evaluation of asymptomatic patients suspected of having variegate porphyria.

Coproporphyrinogen oxidase, protoporphyrinogen oxidase and ferrochelatase activities in human liver biopsies with special reference to alcoholic liver disease

The activities of coproporphyrinogen oxidase, protoporphyrinogen oxidase and ferrochelatase have been assayed in human liver biopsies using recently developed highly sensitive specific enzyme assays. The specific activities (nmol/min/mg protein) in controls were 0.010 +/- 0.003 (mean +/- S.D., n = 11), 0.18 +/- 0.07 (n = 9) and 0.062 +/- 0.022 (n = 8), respectively. The total activities (mumol/min/liver) were determined, using ultrasound to determine liver volumes, and were 2.6 +/- 0.6 (n = 5), 36.6 +/- 13.9 (n = 6) and 14.2 +/- 5.4 (n = 3), respectively. Both specific and total enzyme activities in alcoholics with fatty liver were not significantly different from normal controls. Decreased protoporphyrinogen oxidase activity (0.08 nmol/min/mg protein or 20.2 mumol/min/liver) was found in two patients with variegate porphyria. In a patient with erythrohepatic protoporphyria a reduction of the ferrochelatase activity (0.01 nmol/min/mg protein) was demonstrated.

Mouse protoporphyrinogen oxidase. Kinetic parameters and demonstration of inhibition by bilirubin

The penultimate step of haem biosynthesis, the oxidation of protoporphyrinogen to protoporphyrin, was examined with purified murine hepatic protoporphyrinogen oxidase (EC 1.3.3.4) in detergent solution. The kinetic parameters for the two-substrate (protoporphyrinogen and oxygen) reaction were determined. The limiting Km for protoporphyrinogen when oxygen is saturating is 6.6 microM, whereas the Km for oxygen with saturating concentrations of protoporphyrinogen is 125 microM. The kcat. for the overall reaction is 447 h-1. The ratio of kcat. to the Km for protoporphyrinogen is approx. 20-fold greater than the kcat./Km,O2 ratio. The ratio of protoporphyrin formed to dioxygen consumed is 1:3. Ubiquinone-6, ubiquinone-10 and dicoumarol stimulate protoporphyrinogen oxidase activity at low concentrations (less than 15 microM), whereas coenzyme Q0 and menadione show no activation at these concentrations. Above 30 microM, all five quinones inhibit the enzyme activity. FAD does not significantly affect the activity of the enzyme. Bilirubin, a product of haem catabolism, is shown to be a competitive inhibitor of the penultimate enzyme of the haem-biosynthetic pathway, protoporphyrinogen oxidase, with a calculated Ki of 25 microM. The terminal enzyme of haem-biosynthetic pathway, namely ferrochelatase, is not inhibited by bilirubin at concentrations over double the Ki value for the oxidase. In contrast with other enzymic systems, the toxicity of bilirubin is not reversed by binding to albumin.

Purification of bovine protoporphyrinogen oxidase: immunological cross-reactivity and structural relationship to ferrochelatase

Protoporphyrinogen oxidase, the penultimate enzyme in the haem biosynthetic pathway has been purified to apparent homogeneity from bovine liver mitochondria, by a published method (Dailey, H.A. and Fleming, J.E., (1983)), with an additional ion-exchange chromatography step, using a Mono Q column on an FPLC-system. This gave a product with a 68% yield and 870-fold purification. Protoporphyrinogen oxidase (EC 1.3.3.4) has an apparent Mr of 57,000 and the Km for protoporphyrinogen IX was 16.6 microM. Activity of the isolated enzyme was increased by 66% in the presence of oleic acid, and evidence was obtained for a FAD prosthetic group. Ferrochelatase (EC 4.99.1.1) was purified and antibodies were raised in rabbits against ferrochelatase and protoporphyrinogen oxidase, respectively. Anti-protoporphyrinogen oxidase IgG showed marked cross-reactivity with ferrochelatase and anti-ferrochelatase IgG cross-reacted with protoporphyrinogen oxidase. In addition, radiolabelled peptides of both enzymes, generated by chymotrypsin, demonstrated common peptides when analysed by two-dimensional chromatography.

Protoporphyrinogen oxidase and porphobilinogen deaminase in variegate porphyria

Two enzymes of the haem biosynthetic pathway were investigated in patients with variegate porphyria. Protoporphyrinogen oxidase in cultures of Epstein-Barr virus transformed lymphoblasts from twenty-seven patients showed a mean maximal velocity (Vmax) of 0.39 +/- 0.08+ nmol of protoporphyrin mg protein-1 h-1, a 52% reduction (P less than 0.001) from a non-porphyric control group (0.82 +/- 0.10). Km values (1.00 +/- 0.27 microM) did not differ significantly (P greater than 0.05) from control values in any of the patients. The mean Vmax of porphobilinogen deaminase in the cultures was 1.50 +/- 0.18 nmol of uroporphyrin mg protein-1 min-1, a 24% reduction (P less than 0.001) from controls (1.94 +/- 0.14). Mean porphobilinogen deaminase activity in the erythrocytes of twenty-one patients with variegate porphyria was 8.37 +/- 1.99 nmol of uroporphyrin 1 erythrocytes-1 s-1, a 28% reduction (P less than 0.001) from normal (11.98 +/- 2.11). The reduced activities of these two enzymes comply with the expression of variegate porphyria during its quiescent and acute phases.

Chester porphyria: biochemical studies of a new form of acute porphyria

A previously unrecognised form of acute porphyria has been identified in a large family in Chester, UK. Patients presented with attacks of neurovisceral dysfunction and none had experienced cutaneous photosensitivity. Biochemically, the excretion pattern of haem precursors varied between individuals; some had a pattern typical of acute intermittent porphyria, others showed that of variegate porphyria, and some had an intermediate pattern. Studies of the enzymes of haem biosynthesis in peripheral blood cells showed a dual enzyme deficiency, with reduced activity of both porphobilinogen deaminase, as seen in acute intermittent porphyria, and protoporphyrinogen oxidase, as seen in variegate porphyria. The genetic basis of this dual form of acute porphyria and its relation to the other acute porphyrias are not clear.

Kinetic properties of the membrane-bound human liver mitochondrial protoporphyrinogen oxidase

We studied the kinetic properties of the membrane-bound human liver mitochondrial protoporphyrinogen oxidase. The activity was monitored by direct recording of protoporphyrin fluorescence appearance in the incubation medium without extraction or dilution. The human liver enzyme shows some different catalytic properties than the rat enzyme since its optimum pH was found at 7.2. We also measured the optimum pH on partially purified protoporphyrinogen oxidase from solubilized human mitochondrial membranes. Like the rat enzyme, the human enzyme had a molecular weight of congruent to 32,000 as determined by gel filtration, but its optimum pH was the same as that of the membrane-bound enzyme. The apparent Km for protoporphyrinogen IX of the membrane-bound enzyme was a function of the pH; Km = 0.16 microM at pH 7.2, 0.33 microM at pH 8.0, and 0.55 microM at pH 8.5. Moreover, there was inhibition by excess protoporphyrinogen IX (KI = 5 microM at pH 7.2). The human enzyme was able to catalyze the oxidation of mesoporphyrinogen IX to mesoporphyrin IX. In this case, the Lineweaver-Burk plot of the data showed a biphasic curve with two different apparent Km's for mesoporphyrinogen IX of 0.5 microM (Vmax = 2.40 nmol h-1 mg-1) and 4 microM (Vmax = 5.7 nmol h-1 mg-1). Human liver protoporphyrinogen oxidase was sensitive to inhibition by some metalloporphyrins such as Mn- and Co-protoporphyrin, and to a lesser degree by Cd-, Ni- and Fe-protoporphyrin (heme). Cu-, Mg-, Sn-, and Zn-protoporphyrins were not inhibitors of the activity. A detailed study of the inhibition of protoporphyrinogen oxidase by cobalt-protoporphyrin shows a noncompetitive mechanism of inhibition with respect to protoporphyrinogen IX (KIapp = 0.8 microM).

Protoporphyrin accumulation by mitogen stimulated lymphocytes and protoporphyrinogen oxidase activity in patients with porphyria variegata and erythropoietic protoporphyria: evidence for deficiency of protoporphyrinogen oxidase and ferrochelatase in both diseases

In erythropoietic protoporphyria (EPP) and porphyria variegata (PV) excess protoporphyrin is excreted in the stool, suggesting one or more enzyme defects in the terminal steps of the haem biosynthetic pathway. We measured protoporphyrinogen oxidase (PPO), which catalyses the oxidation of protoporphyrinogen to protoporphyrin, in both EPP and PV patients and in the offspring of PV patients. In the same subjects we measured protoporphyrin formation by mitogen stimulated lymphocytes, with delta aminolaevulinic acid (ALA) as substrate and with the addition of chelators or iron, an indirect measure of ferrochelatase activity. PPO activity was reduced by 41% (P less than 0.001) in PV patients and in 50% of their offspring, and by 36% (P less than 0.001) in EPP patients. Protoporphyrin accumulation in stimulated lymphocytes was increased by 1.3-fold (P less than 0.001) in EPP and 1.5-fold (P less than 0.001) in PV patients compared to normal subjects. There was a significant difference in protoporphyrin accumulation between iron deficient and iron replete cells from PV patients as compared to normals but not as marked as for EPP cells treated similarly. Stimulated lymphocytes from prepubertal PV offspring with reduced PPO activity accumulated normal amounts of protoporphyrin. We have interpreted our findings as follows: PPO is significantly reduced in both diseases. Ferrochelatase becomes defective in PV patients after puberty. This could explain why PV is clinically and biochemically manifest only after puberty. As it has been repeatedly shown that ferrochelatase is markedly reduced in EPP, it would appear that both enzymes are deficient in these two porphyrias.