Enzym e D efects in Hereditary Porphyria

Transcription

1 ANNALS OF CLINICAL AND LABORATORY SCIENCE, Vol. 10, No. 5 Copyright 1980, Institute for Clinical Science, Inc. Enzym e D efects in Hereditary Porphyria W. HAROLD CIVIN, M.D. and EM A N U EL E PST E IN, Ph.D. Department o f Clinical Pathology, William Beaumont Hospital, Royal Oak, MI ABSTRACT H em e is an im portant prosthetic group for proteins concerned w ith energy m etabolism. All cells in the body probably m ake hem e, b u t nucleated erythroid and hepatic cells have b een studied the most. F eedback control of hem e form ation differs in the red cells and in the liver. A bout eight enzym es have a place in the form ation of h em e. D efects in the enzym e pathw ays may be the result of genetic abnorm alities and phenotypically occur as hereditary porphyrias. If the major defect occurs in the red cell line, erythropoietic porphyrias occur; if the liver has the m ajor defect, th en hepatic porphyrias are present. T here are probably th ree erythropoietic porphyrias and four hepatic porphyrias w hich are genetically determ ined. H ow ever, som e are not clearly classified, w ith erythropoietic protoporphyria involving hepatic and erythroid cells and porphyria cutanea tarda not b ein g a clear cut genetic abnorm ality, at least som e of the tim e. E lucidation of the genetic enzym atic defects introduces new diagnostic tools and also has led to at least one revolutionary new treatm ent for some hepatic porphyrias. Introduction In recent years, great strides have been m ade in elucidating the enzym es and substrates involved in the synthesis of hem e, the prosthetic group of hem oproteins such as hem oglobin, m yoglobin, cytochrom e o x id ases, cata la se s, p e ro x id a se s, etc. H e m o p ro te in s are in v o lv e d in b asic energy reactions, such as tran sp o rt of m olecular oxygen (hem oglobin), activation of oxygen (cytochrom e oxidases, tryptophan oxygenase, and m ixed function oxygenases su ch as m icrosom al cy to chrom es), activation of peroxide (peroxidases), decom p osition of hydro gen peroxide (catalase), and transport of electrons (m itochondrial cytochrom es).15,16,31 H em e form ation has been detected in a num ber of tissues, b u t it has especially b een stu d ied in hepatic and n u cleated erythroid cells (figure 1). A lthough the pathw ay seem s the same in both of these tissues, its regulation differs in each. D elta a m in o le v u lin ic acid sy n th e ta se (ALA synthetase) represents the rate lim iting enzym e norm ally in both ery th ro id an d /80/ $01.20 Institute for Clinical Science, Inc.

2 396 CIVIN AND EPSTEIN Glycine - Succinyl CoA ALA-S (in mitochondrion) r ^ Induced by: \ In RBC Hypoxia Erythropoietin ALA-D (in cytosol) 5f5H steroids* URO I-S URO III-COS TInhibited n V iik i by: In RBC Heme In Hepatic Cell Lipophilic Drugs & Chemicals Fasting 56H steroids In Hepatic Cell Heme Glucose Copro'gen Copro'gen III ^ Proto*gen IX COPRO-O (in mitochondrion) PROTO-O {in mitochondrion) F ig u r e 1. H e m e fo rm a tio n in h e p a tic a n d n u c le a te d e ry th ro id cells. Protoporphyrin IX Heme FERRO-C (in mitochonrion) *56H steroids have a hydrocarbon nucleus of the 56 andiostane or 56 pregnane type. All are metabolites of intermediates in synthesis of testosterone or progesterone. Examples are 11 ketopregnanolone, etiocholoanolene, and pregnandiol. Copro1gen I and III = Coproporphyrinogen I and III PBG = Porphobilinogen Uro'gen I and III = Uroporphyrinogen I and III ALA = Delta aminolevulinic acid Proto'gen IX = Protoporphyrinogen IX ALA-S = ALA synthetase URO III = Uroporphyrinogen III cosynthetase AliA-D = ALA dehydratase URO I-S - Uroporphyrinogen I synthetase FERRO-C = Ferrochelatase URO-D = Uroporphyrinogen decarboxylase COPRO-O = Coproporphyrinogen oxidase PR0T0=0 = Protoporphyrinogen oxidase hepatic pathw ays. Feedback inhibition of this by hem e as a corepressor is the major controlling factor in th e hepatic p ath. Additionally in th e liver, am inolevulinic acid (ALA) synthetase is induced by lipophilic drugs and chem icals,9,14>2i>22>32 and fasting. O n the other hand, glucose inhibits induction.33 D rugs inducing ALA synthetase activity have tw o properties in common. T he first is they decrease intracellular hem e and hem oproteins (e.g., cytochrom e p450, catalase), eith er by inhibition of synthesis or by increasing breakdow n. Second, they are lipid soluble and as such as potential inducers of hepatic m icrosom al enzymes. E rythroid hem e form ation is controlled differently. T he feedback inhibition of ALA synthetase by hem e is not of prim ary im portance. Rather, hem e produces feedback inhibition by preventing dissociation of iron from transferrin.27 This results in prevention of iron uptake by the reticulocyte. E rythroid ALA synthetase is induced by hypoxia and erythropoietin w hich are not factors in induction in the liver.6 O nly hem e and possibly 5/3-H steroids influence synthetase in both erythroid and hepatic tissues. This explains why erythropoietic and hepatic genetic porp h y ria s d iffe r in th e ir c lin ic a l p r e sen tatio n s.16,18,34 Enzym es in th e Form ation of H em e ALA synthetase is the first enzym e in the system. It brings about a condensation of glycine w ith succinyl CoA form ed particularly in the tricarboxylic acid cycle. T his is the only reaction in the formation of hem e requiring both a vitam in cofactor a n d th e in tro d u c tio n o f en e rg y. T h e form er is pyridoxal p h osphate and the re a c tio n is oxygen d e p e n d e n t. D e lta am in o lev u lin ic acid (ALA) is form ed. 13,18,24,30,33 T h e form ation involves a re action b etw een a sulfhydryl group in the active site of the enzym e and the vitam in cofactor. T he resulting com pound forms a S chiff b ase w ith glycine an d th is condenses w ith succinyl CoA. T here follows

3 HEREDITARY PORPHYRIA 397 a decarboxylation of the glycylcarbonyl group. T he entire process takes place in the m itochondrion. T h e ALA fo rm e d m oves in to th e cytosol, an d tw o m o lecu les co n d ense u n d e r th e in flu e n c e of d e lta a m in o levulinic acid dehydratase [(dehydrase) (ALA d e h y d ra ta s e )] to form p o r p h o b ilin o g e n (PB G ).17 F irst, an in te r m ediate, a Schiff base, is form ed by condensation of one m olecule of ALA and one subunit of ALA dehydratase.26 Next, in the cytoplasm, uroporphyrinogen is fo rm ed by th e actio n o f u ro p o rp h y rin o g e n I s y n th e ta s e (U R O I synthetase), acting on four m olecules of PBG. T his leads to the formation of one m olecule of uroporphyrinogen I and four m olecules of am m onia.26 T he action of URO (I) s y n th e ta s e an d u ro p o rp h y rin o g e n (III) c o sy n th e ta se (U RO III cosynthetase) forms u ro p o rp h y rin o g en III from PB G.3,4 By itself, th e cosynth e ta s e c a n n o t u tiliz e e ith e r PB G or uroporphyrinogen I as substrate. T here is a d isag re em en t as to w h e th e r URO I synthetase produces an interm ediate on w h ich U RO I I I c o sy n th e ta se acts or w hether uroporphyrinogen I and III are form ed w ith different dipyrrylm ethanes (co n ju g ates o f tw o m o lecu les o f p o r phobilinogen differently linked) and th en the cosynthetase alters the pattern of PBG co ndensation.12,20 If the latter putative m echanism is correct, as favored by the extensive work of Frydm an,12 the synthetase w ill produce different isom ers and the cosynthetase is a m odifier protein and not an enzym e. U ro p o rp h y rin o g e n s I an d III are oxidized to uroporphyrin I and III, respectively. T he reduced substances are colorless and the oxidized products are colored b ut are found in m inor am ounts in the body norm ally.26 Next, uroporphyrinogen I and III are decarboxylated stepw ise enzym atically by u ro p o rp h y rin o g e n d e c a rb o x y la se (URO decarboxylase) u n til four acetic acid side chains are rem oved and coproporphyrinogens I and III, respectively, are formed. T hese steps produce porphyrinogens w ith seven, six, and five carboxyl groups (hepta, hexa, and pentaporphyrinogens).26 T he m ore carboxyl groups a porphyrin has, the more w ater soluble it is, accounting for the predom inantly urinary or fecal distribution of various porphyrins. At any one tim e, the co ncentration of these interm ediates are small, since the Km of the decarboxylating enzym e is low and a high turnover of products w ith m ore than four carboxyl groups occurs. Copropoiphyrinogen I and uroprophyrinogen I and their interm ediates are not able to be utilized in the hem e m etabolic pathw ays and, w ith th e ir in term ed iates, are excreted as I isom ers.13 24,25,30 C oproporphyrinogen III m oves back into the m itochondrion from the cytosol, and it is converted into protoporphyrinogen IX by the action of the m itochondrial en z y m e c o p ro p o rp h y rin o g e n o x id ase (COPRO oxidase). This enzym e requires o x y g en to d e c a rb o x y la te o x id a tiv e ly tw o propionyl groups of coproporphyrinogen Protoporphyrinogen IX is converted to p ro to p o rp h y rin IX by p ro to p o rp h y rin ogen oxidase (PROTO oxidase) in the m itochondrion.26 T he rapid introduction of iron into protoporphyrin IX is catalyzed by ferrochelata se (h em e sy n th e ta se ) w h ic h is associated w ith th e in n e r m ito ch o n d rial m em brane. Pyridoxal phosphate and copp er are n eeded for full activity. Once iron is introduced, hem e is form ed.5 13, In the past, genetic defects of porphyrin m etabolism have been classified on the basis of sym ptoms and family studies.26,31 A fu rth e r d e lin e a tio n h as b e e n acc o m p lis h e d by th e a lm o st c o m p le te elucidation of the enzym atic pathw ay for hem e form ation.10,26,31 T he pathogenesis of the sym ptom atology and the clinical Km = Michaelis constant.

4 398 CIVIN AND EPSTEIN course has becom e m uch clearer.8,26,31 It is now easier to understand the different excretory patterns of the h ered itary p o r phyrias. E nzym atic d efects lead to accum ulation of substances in the hem e synthetic pathw ay upstream from th e d e ficiency. Also, new know ledge has given m ore specific direction to treatm ent.36 Tests have b een developed for m easu rem ent of m ost of the enzym es in the hem e cycle.7,8 As yet no procedures for m easuring PRO TO oxidase is available. T he test for URO III cosynthetase is cum bersom e and not g en erally p erform ed even in research laboratories.23 T hose enzym es located in the m itoch o n d rio n can be m easu red in leu k o cytes.8 T hese are ALA synthetase (EC Type C.E.P. E.P.P. E.C.P. A.I.P. V.P. TABLE I Classification of Hereditary Prophyrias Symptoms Cutaneous Mild cutaneous Mild cutaneous GI & neurologic symptoms GI & neurologic symptoms Enzyme Abnormality Most Helpful Diagnostic Studies ERYTHROPOIETIC PORPHYRIAS ALA-S f URO I-S + URO III COS FERRO-C + + Red cell uroporphyrin Red cell protoporphyrin? Red cell coproporphyrin increased HEPATIC PORPHYRIAS URO I-S + FERRO-C 4- or PROTO-O + H.C.P. GI & neurologic symptoms COPRO-C 4- P.C.T. Cutaneous URO-D & hepatic abnormalities URO I-S decreased in red cells Fecal coproporphyrin & uroporphyrin III increased (equally) Fecal coproporphyrin III increased URO-D decreased in red cells Urinary uroporphyrin I increased AR = Autosomal recessive = Autosomal dominant C.E.P. = Congenital erythropoietic porphyria E.P.P. = Erythropoietic protoporphyria B.C.P..= Erythropoietic coproporphyria A.I.P. = Acute intermittent prophyria V.P. = Variegate porphyria * Hereditary coproporphyria P.C.T. = Porphyria cutanea tarda Refer to figure 1 for additional legends. Inheritance AR? or acquired ), COPRO oxidase (EC ), and ferro ch elatase (EC ). T he cytosolic enzym es are m easured in the m ature red cell and com prise ALA dehydratase (EC ), URO I synthetase (EC ), and U RO decarboxylase (EC ). C lassification O ne of the com m on classifications of the h ered itary porphyrias is show n in tab le I.7,26,31 T h e classification is by no m eans u n i versally accep ted. M eyer and Schm id consider E.C.P. as a variant of C.E.P. and P.C.T. as a third subgroup along w ith the erythropoietic and hepatic sub-groups.26 Eales considers C.E.P. and P.C.T. as erythrohepatic porphyrias.10 Before the enzym atic pathw ays becam e clearer, th e e ry th ro p o ie tic p o rp h y rias w ere recognized by th eir characteristics of cutaneous and intense fluorescence evoked m ost effectively by long w ave ultraviolet radiation (around 400 nm) acting on uro, copro, or protoporphy rin w hich accum ulate in excess u p stream from the specific block in each defect. In C.E.P., an autosom al recessively inherited condition, the abnorm ality is an im b a la n c e b e tw e e n ALA sy n th e ta se and/or URO I synthetase and URO III cosynthetase w ith an increase in one or both of the first two or/and a decrease of th e th ird.4,10,26,31,35 W atson favors the increase th eo ry w h ile L ev in claim s an etiologic role for cosynthetase decrease.23 The im balance has b een dem onstrated in cultured fibroblasts as w ell as in m ature red cells.7 T here is an accum ulation of uroporphyrinogens. Since norm ally there is great excess of cosynthetase, hem e for hem oglobin form ation is still synthesized in adequate am ounts despite som e quantitative decrease. Thus, anem ia from lack of hem oglobin form ation does not occur. In E.P.P., an autosom al d om inant condition, th ere is a partial deficiency of fe rro c h e la ta s e.7,10,26 S in ce th is is a

5 HEREDITARY PORPHYRIA 399 m itochondrial enzym e, the defect is found in im m ature red cells and has also been dem onstrated in leukocytes, in cultured fibroblasts, and in hepatocytes.7 T his latter finding is the reason E ales considers th e c o n d itio n an e ry th ro h e p a tic p o r phyria. T he lack of anem ia in this condition is said to be due to secondary regulatin g m echanism s lead in g to ad e q u a te hem e formation. L ittle is know n of E.C.P., and no extensive enzym e studies have b een done on this.26,31 It is said to be in h erited as an autosom al dom inant, b u t only a few cases have b een reported. T h e h e p a tic p o rp h y rias are ch aracterized by bouts of abdom inal pain and neurologic sym ptoms of acute onset.26,31,36 Increased ALA, PBG, and other m onopyrroles form ed upstream from the enzym e d efe c t accu m u late in th e cen tral and p e rip h e ra l n erv o u s system s o w in g to A LA -synthetase increase. T h ey are concentrated in the hypothalam us and inhibit brain and blood N a/l ATPase affecting transport across cellular m em branes and altering n euro m u scular functions. M onop y rro le s can cau se spasm o f in te s tinal tract through their effect on the nervous system.7 T he syndrom e of inappropriate H secretions (S.I.A.D.H.) is also found often.7 C haracteristically, the hepatic porphyrias are in d u cib le, th e term applied by W atson to those cases w here ALA synthetase is increased because the hem e form ed is inadequate to sh u n t it off (feedback control), and acute neruological sym ptom s are produced.7,36 Since hem e pro bably does not exert im portant feed back control over the red cell line, the erythropo ietic porphyrias are not in d u cible in this sense. Symptoms in hepatic prophyrias are often set off by substances inducing ALA synthetase in the face of deficient hem e formation. A.I.P. is in h erited as an autosom al dom in an t disease and is a m anifestation of 50 percent deficiency of URO I synthetase. T he defect has b een dem onstrated in red cells, cultured skin fibroblasts, and amniotic cells as w ell as in hepatocytes.7,36 V.P. is also in h erited as an autosom al d o m in a n t a n d is p ro b a b ly d u e to ferrochelatase deficiency.7,11,36 T he d e fect is said to occur in erythroid precessors and in leukocytes, b u t G oldberg e t a l7 have b een unable to verify this and they feel that the defect may be one of PROTO oxidase. H.C.P. is in h e rite d as an autosom al d o m in an t also.36 T h e d efic ien cy is in COPRO oxidase and can be dem onstrated in cultured fibroblasts and leukocytes.7,8 No significant studies of the hepatocyte c o n te n t of th e e n z y m e h a v e b e e n conducted. P.C.T. is not an inducible condition by the W atson definition and the status of ALA synthetase here is unclear.1,2 T he d e fect is one of uroporphyrinogen decarboxylase. Excess excretion of uroporphyrinogens especially is noted along w ith large am ounts of heptacarboxylic porphyrins. T he enzym atic defect has b een found in the hepatocytes and in red cells. For this reason and b ecau se o f cu tan eous sym ptoms, the exact place of this in the overall classification is in dispute. T he red cell defect has not b een confirm ed in all cases and at p resen t there is debate as to the exact status of P.C.T. Some feel the disease is in h erited 1 and some say it is acquired.2,14 Eales and coworkers feel that som e cases are in h erited as autosom al dom inant and others, the vast majority, are acquired.2 For the defect to b e m anifested clinically, there m ust be an added abnorm ality in th e liv er such as alcohol associated hepatic disease or iron overload. Studies of the enzym e pathw ay of hem e formation have given m ore insight into the genesis of hereditary porphyrias, have pointed ways to greater assurance in diagnosis, and have lead to the developm ent of a better form of treatm ent. F or the inducible porphyrias, nam ely acute interm ittent p o rp h y ria, v a rie g a te p o rp h y ria, an d h ered itary coproporphyria, acute attacks

6 400 CIVIN AND EPSTEIN of neurological sym ptoms and abdom inal pain w hich at tim es may b e fatal have b een aborted or rapidly term inated by adm inistration of hem atin.36 T his is based on the prem ise that hem e (and its derivative hem atin) acts as a feedback m ech a nism to shut off 8ALA synthetase; in practice this has proved to be so. In tim e, perhaps prevention by enzym e replacem e n t m ay b e d e v e lo p e d. C e rta in ly, studies in the last ten years have resulted in b etter understanding of this com plex group of hereditary m etabolic diseases. R eferences 1. Be n d e t t o, A. V., Ku s h n e r, J. P., and T a y l o r, J. S.: Porphyria cutanea tarda in three generations of a single family. New Eng. J. Med., 298: , Bl e k k e n h o r s t G. H., D a y, R. S., a n d E a l e s, L.: T w o form s o f p o rp h y ria c u ta n e a ta rd a. N ew E ng. J. M ed. 300:93, BOGOR, L.: The enzymatic synthesis of porphyrias from porphobilinogen. I. U roporphyrinogen I. J. Biol. Chem. 233: , Bo g o r a d, L.: T h e en z y m a tic sy n th e sis o f p o r p h y r ia s fro m p o r p h o b ilin o g e n. I I.U r o p o r p h y rin o g e n I II. J. B iol. C h e m : , BOTTOMLEY, S. S.: Characterization and measurem ent of heme synthetase in normal human bone marrow. Blood 31: , Bo t t o m l e y, S. S. and S m it h e e, G. A.: Effect of ery th ro p o ie tin on bone m arrow delta am inolevulinic acid synthetase and hem e synthetase. J. Lab. Clin. Med. 74: , B r o d ie, M. J., M o o r e, M. R., and G o l d b e r g, A.: Enzyme abnormalities in the porphyrias. Lancet 2: , Br o d ie, M. J., T h o m p s o n, G. G., M o o r e, M. R., B e a t t i e, A. D., and G o l d b e r g, A.: Hereditary coproporphyrin. Demonstration of the abnorm alities in hem e biosynthesis in peripheral blood. Amer. J. Med. 46: , D e M a t t e is, F.: D isturbance of liver porphyrin metabolism caused by drugs. Pharmacol. Rev. 19: , E a l e s, L., G r o s s e r, Y., and S e a r s, W. G.: H epatocutaneous porphyrias. T he clinical biochemistry of the human hepatocutaneous porphyrias in the light of recent studies of newly identified intermediates and porphyrin derivatives. Ann. N. Y. Acad. Sci. 244: , F r o m k e, V.L., Bo s s e n m a ie r, J., C a r d in a l, K., and WATSON, C. J.: Porphyria variegata. Study o f a large kindred in the U nited States. A m er. J. Med. 65:80-88, F r y d m a n, B., F r y d m a n, R., V o l a s i n a s, A., L e v y, S., and F e in s t e in, G.: T he m etabolism of uroporphyrinogen biosynthesis. Ann. N. Y. Acad. Sci. 244: , G ib s o n, K. D., M a t t h e w, M., N e w b e r g e r, a., a n d T A lt, G. H.: B io sy n th e sis o f p o r p h y ria s a n d chloro p h yll. N atu re 292: , G r a n ic k, S.: The induction in vitro o f the snythesis of delta aminolevulinic acid synthesis in chemical porphyria. A response to certain drugs, sex hormones, and foreign chem icals. J. Biol. Chem. 242: , G r a n ic k, S. and G i l d e r, H.: D istribution, structure, and properties of the tetrapyrroles. Adv. Enzymology 7: , G r a n ic k, S. and Ka p p a, A.: Steroid induction of porphyrin synthesis in liver cell culture. I. Structural basis and possible physiological role in control of heme formation. J. Biol. C hem. 242: , G r a n ic k, S., and M a u z e r a l l, D.: Porphyrin biosynthesis in erythrocytes. II. Enzym es converting delta aminolevulinic acid to coproporphyrinogen. J. Biol. Chem. 232: , G r a n ic k, S. and U r a t a, G.: Increase in activity of delta aminolevulinic acid synthetase in liver mitochondria induced by feeding of 3,5 decarbethoxy-1,4 dihydrocollidine. J. Biol. C hem. 238: , G r o s s m a n, M. E., B i c k e r s, D. R., P o t - F i t z p a t r i c k, M. B., D e L e o, V. A., and H a r b e r, L. C.: Porphyria cutanea tarda. C linical features and laboratory findings in forty patients. Amer. J. Med. 67: , HlGUCHl, M. and B o g o r a d, L.: The purification and properties of uroporphyrinogen I synthetases and uroporphyrinogen III cosynthetase. Interaction betw een the enzymes. Ann. N. Y. Acad. Sci. 244: , Ka p p a s, A. and G r a n ic k, S.: Steroid induction of porphyrin synthesis in liver cell culture. II. T he effect of hem e, u rid in e d ip h o sp h ate glucuronic acid, and inhibitors of nucleic acid and protein synthesis on the induction process. J. Biol. Chem. 243: , K a p p a s, A. and G r a n ic k, S.: Experim ental hepatic porphyria. Studies w ith steroids of physiological origin in man. Ann. N. Y. Acad. Sci. 252: , L e v in, E. Y.: Comparative aspects of porphyria in man and animals. Ann. N. Y. Acad. Sci. 244: , M a u z e r a l l, D.: Normal porphyrin m etabolism. J. Pediat. 64 :5-16, M a u z e r a l l, D. and G r a n ic k, S.: Porphyrin biosynthesis in erythrocytes. III. Uroporphyrinogen and its decarboxylase. J. Biol. Chem. 232: , M e y e r, U. J. and S c h m id, R.: The porphyrias. The Metabolic Basis of Inherited Disease, 4th ed. Stanbury, J. B., Wyngaarden, J. B., and

7 HEREDITARY PORPHYRIA Fredrickson, D. S., eds. New York, McGraw- Hill Book Co., 1978, pp P o n k a, P., N e u w ir t, J., and B o r o v a, J.: The role of heme in release of iron from transferrin to reticulocytes. Enzyme 27:91-99, P o r r a, R. J., and JONES, O. T. G.: Studies on ferrochelatase. 2. An investigation of the role of ferrochelatase in the biosynthesis of various haem prosthetic groups. Biochem. J. 87: , S a n o, S. and G r a n c ik, S.: Mitochondrial coproporphyrinogen oxidase and protoporphyrin formation. J. Biol. Chem. 236: , S h e m in, D.: The biosynthesis of porphyrin. Harvey Lecture 50: , I s c h u d y, D. P.: Prophyrin metabolism and the porphyrias. D uncan s Disease of Metabolism. Genetics and Metabolism, 7th ed. Bondy, P. L. and Rosenberg, L. E., eds. Philadelphia, W. B. Saunders Co., 1974, pp A. 32. I s c h u d y, D. P. and B o n k o w s k y, H.: Experimental porphyria. Fed. Proc. 31: , I s c h u d y, D. P., W e l l a n d, P. H., C o l l in s, A., and H u n t e r, G. W., J r.: The effect of carbohydrate feeding on delta aminolevulinic acid synthetase. Metabolism 23: , W a d a, D., Sa s s a, S., T a k a k e, F., Ya n o, Y., U r a t a, G., and M a h a n, K.: D ifferent responses of hepatic and erythropoietic delta am in o lev u lin ic acid sy n th etase o f m ice. Biochem. Biophys. Acta 248: , W a t s o n, C.J., R u n g e, W., T a d d e i n i, L., Bo s s m a ie r, I., and C a r d in a l, R. A.: Suggested control gene mechanism for excessive production of types I and I II porphyrins in congenital erythropoietic porphyria. Proc. Natl. Acad. Sci. 52: , W a t s o n, C. J., T ie r a c h, C. R., B o s s e n m a ie r, J., and C a r d in a l, R. A.: Postulated deficiency of hepatic hem e and repair by hematin infusions in the inducible hepatic porphyrias. Proc. Natl. Acad. Sc. 74: , 1977.