Δ-aminolaevulinate Synthase Research Articles

ALAS1 gene expression is down-regulated by Akt-mediated phosphorylation and nuclear exclusion of FOXO1 by vanadate in diabetic mice

Porphyrias are diseases caused by partial deficiencies of haem biosynthesis enzymes. Acute porphyrias are characterized by a neuropsychiatric syndrome with intermittent induction of hepatic ALAS1 (δ-aminolaevulinate synthase 1), the first and rate-limiting enzyme of the haem pathway. Acute porphyria attacks are usually treated by the administration of glucose; its effect is apparently related to its ability to inhibit ALAS1 by modulating insulin plasma levels. It has been shown that insulin blunts hepatocyte ALAS1 induction, by disrupting the interaction of FOXO1 (forkhead box O1) and PGC-1α (peroxisome-proliferator-activated receptor γ co-activator 1α). We evaluated the expression of ALAS1 in a murine model of diabetes and determined the effects of the insulinomimetic vanadate on the enzyme regulation to evaluate its potential for the treatment of acute porphyria attacks. Both ALAS1 mRNA and protein content were induced in diabetic animals, accompanied by decreased Akt phosphorylation and increased nuclear FOXO1, PGC-1α and FOXO1-PGC-1α complex levels. Vanadate reversed ALAS1 induction, with a concomitant PI3K (phosphoinositide 3-kinase)/Akt pathway activation and subsequent reduction of nuclear FOXO1, PGC-1α and FOXO1-PGC-1α complex levels. These findings support the notion that the FOXO1-PGC-1α complex is involved in the control of ALAS1 expression and suggest further that a vanadate-based therapy could be beneficial for the treatment of acute porphyria attacks.

Identification of a Hemizygous R170H Mutation in theALAS2Gene in a Young Male Patient with X-linked Sideroblastic Anemia

X-linked sideroblastic anemia (XLSA) is a rare hereditary disease characterized by microcytic hypo- chromic anemia, ineffective erythropoiesis and the presence of numerous ringed sideroblasts in the bone marrow. The causative gene is the erythroid δ-aminolaevulinate synthase 2 gene (ALAS2) on Xp11.21. We report here a case of XLSA. The patient was a 20-year-old Korean man referred to our hospital under the impression of sideroblastic anemia (SA). Laboratory findings, including a peripheral blood smearand bone marrow study, were compatible with SA. The family history was not remarkable. Based on the early age of onset, we suspected a hereditary form of SA, particularly XLSA. Direct DNA sequenc- ing of ALAS2 detected a hemizygous c.509G >A (R170H) mutation in exon 5 of the gene. The patient showed minimal response to pyridoxine treatment. To the best of our knowledge, this is the first case of genetically confirmed XLSA from a mutation in ALAS2 in Korea. (Korean J Hematol 2008;43:

Involvement of delta-aminolaevulinate synthase encoded by the parasite gene in de novo haem synthesis by Plasmodium falciparum.

The malaria parasite can synthesize haem de novo. In the present study, the expression of the parasite gene for delta-aminolaevulinate synthase (Pf ALAS ) has been studied by reverse transcriptase PCR analysis of the mRNA, protein expression using antibodies to the recombinant protein expressed in Escherichia coli and assay of ALAS enzyme activity in Plasmodium falciparum in culture. The gene is expressed through all stages of intra-erythrocytic parasite growth, with a small increase during the trophozoite stage. Antibodies to the erythrocyte ALAS do not cross-react with the parasite enzyme and vice versa. The recombinant enzyme activity is inhibited by ethanolamine and the latter inhibits haem synthesis in P. falciparum and growth in culture. The parasite ALAS is localized in the mitochondrion and its import into mitochondria in a cell-free import assay has been demonstrated. The import is blocked by haemin. On the basis of these results, the following conclusions are arrived at: PfALAS has distinct immunological identity and inhibitor specificity and is therefore a drug target. The malaria parasite synthesizes haem through the mitochondrion/cytosol partnership, and this assumes significance in light of the presence of apicoplasts in the parasite that may be capable of independent haem synthesis. The Pf ALAS gene is functional and vital for parasite haem synthesis and parasite survival.

Molecular and cellular mechanisms of iron homeostasis and toxicity in mammalian cells

Iron is an essential metal for almost all living organisms due to its involvement in a large number of iron-containing enzymes and proteins, yet it is also toxic. The mechanisms involved in iron absorption across the intestinal tract, its transport in serum and delivery to cells and iron storage within cells is briefly reviewed. Current views on cellular iron homeostasis involving the iron regulatory proteins IRP1 and IRP2 and their interactions with the iron regulatory elements, affecting either mRNA translation (ferritin and erythroid cell δ-aminolaevulinate synthase) or mRNA stability (transferrin receptor) are discussed. The potential of Fe(II) to catalyse hydroxyl radical formation via the Fenton reaction means that iron is potentially toxic. The toxicity of iron in specific tissues and cell types (liver, macrophages and brain) is illustrated by studies with appropriate cellular and animal models. In liver, the high levels of cyoprotective enzymes and antioxidants, means that to observe toxic effects substantial levels of iron loading are required. In reticuloendothelial cells, such as macrophages, relatively small increases in cellular iron (2–3-fold) can affect cellular signalling, as measured by NO production and activation of the nuclear transcription factor NFκB, as well as cellular function, as measured by the capacity of the cells to produce reactive oxygen species when stimulated. The situation in brain, where anti-oxidative defences are relatively low, is highly regionally specific, where iron accumulation in specific brain regions is associated with a number of neurodegenerative diseases. In the brains of animals treated with either trimethylhexanoylferrocene or aluminium gluconate, iron and aluminium accumulate, respectively. With the latter compound, iron also increases, which may reflect an effect of aluminium on the IRP2 protein. Chelation therapy can reduce brain aluminium levels significantly, while iron can also be removed, but with greater difficulty. The prospects for chelation therapy in the treatment and possible prevention of neurodegenerative diseases is reviewed.

Effect of lindane and heptachlor on delta-aminolaevulinate synthase and its regulation.

The effect of lindane and heptachlor on haem metabolism was studied with the aim to elucidate the mechanism of their porphyrinogenic action. The effects of these compounds on delta-aminolaevulinate synthase (ALA-S) and ferrochelatase were evaluated and the mechanism of increase of ALA-S activity was especially studied. The results indicated the following: (1) Lindane and heptachlor produced increases in ALA-S activity; this effect was dependent on the drug dose, the time of treatment, and the development of the animal, the maximum response being obtained prior to hatching. Lindane was observed to have a greater effect on ALA-S than heptachlor. In fact, when effects of lindane and heptachlor were compared we observed that lindane produced: (a) greater increases in ALA-S activity (six fold vs four fold), both with respect to dimethyl sulphoxide (DMSO) controls (3.8+/-0.3 nmol ALA/g liver per h); (b) earlier ALA-S response (1.5 h vs 4 h); (c) responses at lower doses (0.3 mg/egg vs 1 mg/egg). (2) The increase in ALA-S activity produced by lindane or heptachlor is an induction and not an activation process since it depends on protein synthesis and the drugs per se have no effect. Thus, our results obtained from studies in ovo with actinomycin D and cycloheximide suggest that lindane is acting at the translational level while heptachlor interferes at the level of transcription. (3) The study of ALA-S subcellular distribution indicated no accumulation in the cytosol of DMSO controls and in the lindane or heptachlor treated embryos, neither of the chlorinated pesticides alter the normal subcellular distribution of this regulatory enzyme in the liver. (4) Exogenous haem was able to prevent or decrease the induction of ALA-S elicited by both pesticides, thus showing that lindane or heptachlor-induced ALA-S respond to haem regulation. (5) Lindane had no effect on ferrochelatase activity at the doses and times assayed, but heptachlor decreased this enzyme activity. The porphyrinogenic mechanism of lindane and heptachlor is discussed on the basis of the present results.

Haem is necessary for a continued increase in ferrochelatase mRNA in murine erythroleukaemia cells during erythroid differentiation

The level of mRNA encoding ferrochelatase (FeC) was examined in two murine erythroleukaemia (MEL) clones, DS and DR, a DMSO-sensitive, and a DMSO-resistant clone, respectively. DS cells undergo erythroid differentiation by DMSO treatment with a marked increase in haem synthesis, while DR cells fail to do so due to the lack of the erythroid-specific delta-aminolaevulinate synthase (ALAS-E). Both DS and DR cells showed an increase in the level of FeC mRNA within 18 h of DMSO treatment. The level of FeC mRNA in DR cells was then decreased, while that in DS cells continued to increase for 72 h. Treatment with haemin significantly increased FeC mRNA in DR cells. When cells were treated with both DMSO and haemin, the level of FeC mRNA in DR cells increased to a level comparable to that in DS cells. These findings suggest that the failure to maintain increased FeC mRNA DR cells after DMSO treatment may be due to a deficiency of haem in these cells.

Influence of hepatic tumors caused by diethylnitrosamine on hexachlorobenzene-induced porphyria in rats

The response of female BDVI rats bearing diethylnitrosamine (DENA)-induced hepatic tumors to the porphyrinogenic action of hexachlorobenzene (HCB) was studied. (1) The heme pathway operates in these tumors but they were less affected by HCB than the liver. (2) Tumors did not accumulate porphyrins although the surrounding liver accumulated more porphyrins than livers treated with HCB. (3) DENA HCB livers which developed a well defined tumor showed slightly less porphyrinogen carboxylyase inhibition and δ-aminolaevulinate synthase induction than HCB rats. (4) The results of the present work suggest that endogenously formed porphyrins would be unable to be accumulated by DENA-induced tumors when the tumoral development precedes the onset of the porphyria.

On the mechanism of induction of chick embryo hepatic δ-aminolaevulinate synthase by translational blockers

On the mechanism of induction of chick embryo hepatic δ-aminolaevulinate synthase by translational blockers

The effects of acute-phase inducers and dimethyl sulphoxide on δ-aminolaevulinate synthase activity in human HepG2 hepatoma cells

The effects of acute-phase inducers and dimethyl sulphoxide (Me2SO) on delta-aminolaevulinate (ALA) synthase in HepG2 cells were examined. Treatment of cells with Me2SO resulted in a significant increase in ALA synthase activity. Interleukin-6 increased ALA synthase activity only slightly, but it substantially potentiated the induction of ALA synthase by Me2SO. These data suggest that ALA synthase activity in liver is altered during acute-phase reactions.

δ-Aminolaevulinate synthase: mechanism of its response to malathion

δ-Aminolaevulinate synthase: mechanism of its response to malathion

Effect of some antineoplasics on metabolic heme pathway

1. 1. The porphyrinogenic ability of several antineoplasics used in the therapy of the different cancers was evaluated. The action of cyclophosphamide, busulfan and 5-fluorouracil on the amount and nature of the accumulated hepatic porphyrins and on the activity of δ-aminolaevulinate synthase(ALA-S). were estimated at different doses and times of drug treatment in 17-day-old chick embryos. 2. 2. It was observed that cyclophosphamide produces a significant increase in the accumulation of hepatic porphyrins at different doses as well as in the activity of the ALA-S, at all the incubation times. Cyclophosphamide alters the pattern of porphyrins accumulated in the liver, where a coproporphyrin: protoporphyrin ratio higher than in the controls can be observed. 3. 3. Busulfan increased the hepatic porphyrins accumulated in the liver but to a lesser degree than cyclophosphamide. 4. 4. 5-Fluorouracil did not modify the hepatic porphyrin content when it was administered at doses up to 40 mg/embryo. 5. 5. When the embryos were injected with busulfan or 5-fluorouracil no significant differences were observed in the activity of ALA-S up to 11 hr of incubation. 6. 6. These results indicate that cyclophosphamide has a remarkable porphyrinogenic capacity in chick embryo while busulfan. notwithstancling the fact that it alters the haem pathway, it does so to a degree that does not impair the regulation of ALA-S activity. Fluorouracil seems to be non porphyrinogenic in this system, up to 40 mg/embryo.

Effect of anticonvulsant drugs on serum delta-aminolaevulinic acid levels in non-porphyric subjects.

British Journal of Clinical PharmacologyVolume 24, Issue 6 p. 847-848 Free Access Effect of anticonvulsant drugs on serum delta-aminolaevulinic acid levels in non-porphyric subjects. A Gorchein, A Gorchein Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this authorR Webber, R Webber Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this authorD Burnett, D Burnett Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this authorJH Goudie, JH Goudie Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this author A Gorchein, A Gorchein Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this authorR Webber, R Webber Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this authorD Burnett, D Burnett Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this authorJH Goudie, JH Goudie Department of Clinical Pharmacology, St Mary's Hospital Medical School, London.Search for more papers by this author First published: December 1987 https://doi.org/10.1111/j.1365-2125.1987.tb03260.xCitations: 3AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Bonkowsky, H. L. & Pomeroy, J. S. (1977). Human hepatic δ-aminolaevulinate synthase: requirement of an exogenous system for succinyl-coenzyme A generation to demonstrate increased activity in cirrhotic and anticonvulsant-treated subjects. Clin. Sci. mol. Med., 52, 509– 521. Bonkowsky, H. L., Sinclair, P. R., Emery, S. & Sinclair, J. F. (1980). Seizure management in acute hepatic porphyria: risks of valproate and clonazepam. Neurology, 30, 588– 592. Gorchein, A. & Webber, R. (1987). δ-Aminolaevulinic acid in plasma, cerebrospinal fluid, saliva and erythrocytes: studies in normal, uraemic and porphyric subjects. Clin. Sci., 72, 103– 112. McGuire, G. M., Brodie, M. J., Moore, M. R. & Goldberg, A. (1986). The effect of carbamazepine on 5-aminolaevulinate dehydratase in cultured human lymphocytes. Biochem. Soc. Trans., 14, 95– 96. McGuire, G. M., Macphee, G. J. A., Thompson, G. G. & Brodie, M. J. (1987). Effect of sodium valproate on haem biosynthesis in man. Clin. Sci., 72 (Suppl. 16), 23P– 24P. Moore, M. R. & Disler, P. B. (1983). Drug-induction of the acute porphyrias. In Adverse drug reactions and acute poisoning reviews, ed. D. M. Davies, 2, 149– 189. Rapeport, W. G., Connell, J. C., Thompson, G. G., Moore, M. R. & Brodie, M. J. (1984). Effect of carbamazepine on haem biosynthesis in man. Eur. J. clin. Invest., 14, 107– 110. Smith, A. G. & De Matteis, F. (1980). Drugs and the hepatic porphyrias. In Clinics in Haematology, 9, pp. 399– 425. London: Saunders. Citing Literature Volume24, Issue6December 1987Pages 847-848 ReferencesRelatedInformation

Effect of desferrioxamine on the development of hexachlorobenzene-induced porphyria

The present work deals with the effect of desferrioxamine (DP) on hexachlorobenzene (HCB)-induced porphyria in female rats with the purpose of further investigation of the role of iron in the development of this porphyria. The results obtained show that the repeated injection of DF (three times a week: 100 mg/kg each i.m.) delayed and diminished remarkably the urinary excretion of precursors and porphyrins as well as the accumulation of the latter in liver promoted by HCB (1 g/kg daily given by stomach tube). This was probably due to attenuation by DF of the alterations produced by the fungicide in the two key enzymes: porphyrinogen carboxy-lyase (PCL) and δ-aminolaevulinate synthase (ALA-S). In fact, DF by reducing liver iron levels produced a smaller decrease of the target enzyme (PCL) and a concomitant smaller induction of ALA-S. DF alone did not modify any of these variables or the liver to body weight ratio. DF added at 10 −2 and 10 −3 M to the incubation media of ALA-S and PCL did not alter either of the enzymatic activities, whether in normal or HCB-porphyric preparations. The results obtained show that DF improved the biochemical picture during HCB porphyria. They suggest that iron plays an indirect role in the decrease of PCL enzyme, possibly at the HCB metabolization step. A common iron-involving mechanism for the production of porphyria by different chlorinated compounds is suggested.

A mathematical analysis of aging influences on enzyme deactivation/activation kinetics. Examples of the influence of regional brain development and drugs in rats

A series-type enzyme deactivation/activation model involving active enzyme states is utilized to theoretically quantify the influence of regional brain development and drugs on enzyme activity levels in rats. Continuous hexachlorobenzene administration with, or without, phenobarbitone pretreatment has different effects on the deactivation/activation kinetics of porphyrinogen carboxylase, δ-aminolaevulinate synthase and δ-aminolaevulinate dehydratase. The deactivation/activation kinetics exhibited by pyruvate dehydrogenase, citrate synthase, and D-3-hydroxybutyrate dehydrogenase during the development of the medulla oblongata, mid-brain, striatum, and hypothalamus sections exhibit similarities as well as discrepancies. These are identified and made more quantitative.

Biogenesis of mitochondrial proteins. Regulation of production of delta-aminolaevulinate synthase by haemin in embryonic-chick liver.

The effect of haemin on the biogenesis of delta-aminolaevulinate synthase (ALA synthase) was investigated in primary cultures of embryonic-chick liver. The activity of the enzyme and the amount of the enzyme detected by 'immune-blotting' were determined in hepatocytes incubated with the porphyrogenic agent allylisopropylacetamide. The results of these studies indicated that the loss in ALA synthase activity in cells incubated in the presence of haemin (10 microM) was roughly proportional to a loss in the immune-reactive mass of the enzyme. Haemin was as effective as cycloheximide in causing depletion of ALA synthase in hepatocytes. We had previously established that haemin blocked the maturation of the precursor of ALA synthase [Ades (1983) Biochem. Biophys. Res. Commun. 110, 42-47]. From results reported in the present paper on analyses of immune-precipitated ALA synthase after pulse-labelling with [35S]methionine in the presence and in the absence of haemin, we determined that the inhibition of processing of pre-ALA synthase in cells by haemin was concentration-dependent. A concentration of 2 microM in the culture medium blocked the processing of pre-ALA synthase by 50% in hepatocytes. We also determined that, after inhibition of its maturation by haemin, pre-ALA synthase turned-over with a half-time of 30 min; on the other hand, mature ALA synthase turned-over with a half-time of 120 min.

Effect of Thiamin-induced Vitamin B6 Deficiency on NAD- and NADP-linked Glutamate Dehydrogenases in Yeast

Thiamin-grown cells of Saccharomyces carlsbergensis exhibited decreased activity of NAD-linked glutamate dehydrogenase (NAD-GDH) and, in contrast, increased activity of NADP-linked glutamate dehydrogenase (NADP-GDH), as compared with control cells. Normal levels of glutamate dehydrogenase (GDH) activities were restored by adding pyridoxine. δ-Aminolaevulinate also prevented the thiamin-induced decrease in NAD-GDH activity, but had no effect on the increase in NADP-GDH activity. NAD-GDH activity was decreased similarly under anaerobic conditions, but NADP-GDH activity was little affected. High concentrations of glucose brought about a decrease in NAD-GDH activity and an increase in NADP-GDH activity, as observed in the thiamin-grown cells. When glycerol was used as carbon source in place of glucose, the thiamin effect on GDH activities was not marked. These results suggest that GDH enzymes are independently controlled by thiamin. NAD-GDH activity is decreased mainly through the thiamin-induced respiratory deficiency, and the increase in NADP-GDH activity is due to the thiamin-enhanced glucose effect. Amino acid metabolism seemed not to be involved in the thiamin effect on GDH activities since the intracellular pools of NH4 + and glutamate were not altered by thiamin. The activities of glutamate-oxaloacetate transaminase and glutamate-pyruvate transaminase were not influenced greatly by thiamin, unlike the activity of δ-aminolaevulinate synthase.

Purification of rat liver mitochondrial δ-aminolaevulinate synthase

Mitochondrial δ-aminolaevulinate synthase was purified from drug-induced adult rat liver and identified for the first time as a protein of minimum molecular weight 70,000. The enzyme was also identified in mitochondria by pulse-labelling and immunoprecipitation and shown to have a molecular weight of 70,000. The purified enzyme was degraded by papain to smaller forms of molecular weight about 56,000 with no loss of enzyme activity. In vitro translation experiments suggest that the enzyme is synthesized initially as a larger precursor of molecular weight 76,000.

Isolation of the mature subunit of delta-aminolaevulinate synthase from embryonic chick liver.

We presented evidence indicating that the established procedure for purifying delta-aminolaevulinate (ALA) synthase from embryonic-chick liver yielded an enzyme with a partially degraded subunit of molecular weight 51000 [Ades & Harpe (1981) J. Biol. Chem. 256, 9329-9333]. We now report the purification from livers of porphyric embryos of a preparation of ALA synthase which consisted primarily of a 63000-Da polypeptide and a component migrating as a smear of polypeptides with a minimum molecular weight of 52 000. Neither component could be recovered from liver mitochondria of normal embryos, where the amounts of ALA synthase were relatively low. The 52 000-Da component had been established to be the partially degraded subunit of the enzyme. Peptide-mapping analyses indicated that the 63 000- and the 52 000-Da components possessed significant structural homologies, and it was concluded that the 63 000-Da polypeptide represented the mature subunit of ALA synthase.

Alterations in haem biosynthesis during the human menstrual cycle: studies in normal subjects and patients with latent and active acute intermittent porphyria.

1. The timing of onset of attacks of acute intermittent porphyria (AIP) in relation to the menstrual cycle has been studied in three patients experiencing frequent attacks. Nineteen of their 27 admissions in attack for which no exogenous precipitating causes could be identified were during the 7 days before the onset of menstruation. 2. Haem biosynthesis has been monitored throughout a complete menstrual cycle in six normal females and compared with that in male control subjects. In the females, there was marked fluctuation in the activity of the rate-controlling enzyme of haem biosynthesis, δ-aminolaevulinate (ALA) synthase, which was monitored in peripheral leucocytes. The fluctuation did not show any clear association with menstruation and no association was found between the enzyme activity and ovarian or adrenal steroid production as monitored by measurement of plasma concentrations of androstenedione, dehydroepiandrosterone (DHA), dehydroepiandrosterone sulphate (DHAS), testosterone, oestradiol and progesterone. The activity of uroporphyrinogen I (URO) synthase, the enzyme which is deficient in AIP, was monitored in peripheral erythrocytes of four of the normal female subjects and was similar to that found in four male control subjects. The urinary excretion of ALA and porphobilinogen (PBG) was also similar in the male and female subjects and there was no association with the phase of the menstrual cycle or activity of the enzymes studied. 3. Studies of haem biosynthesis have been made throughout a complete menstrual cycle in a 26 year old female with latent AIP. The activity of leucocyte ALA synthase showed more marked fluctuation than in the normal female subjects and was highest at the time of menstruation. No association was apparent between the activity of ALA synthase and plasma concentrations of androstenedione, DHA and DHAS. The fluctuation in activity of erthrocyte URO synthase was similar to that of the normal subjects. The urinary excretion of ALA and PBG was normal throughout and showed no correlation with fluctuations in enzyme activity. 4. The human menstrual cycle modifies haem biosynthesis in normal subjects as well as in subjects with latent and manifested AIP. 5. The human female menstrual cycle modifies haem biosynthesis in peripheral blood cells as well as in the liver.

Changes in indices of rat testicular heme biosynthesis and microsomal cytochrome P-450 during sexual maturation

The levels of microsomal cytochrome P-450 and heme were measured in the rats testes from weaning (22 days) through attainment of sexual maturity (60–70 days) and beyond (70–125 days). Microsomal heme and cytochrome P-450 increased from 22 days of age to attain maximal levels at 40 days, the approximate age when rats enter the pubertal stage of sexual development. Microsomal heme and cytochrome P-450 levels then declined to attain plateau levels at approximately 65–80 days of age. Testicular mitochondrial δ-aminolaevulinate (ALA) synthase (EC 2.3.1.37) activity was shown to decrease from 22 days and to attain a plateau level at approximately 65 days of age. Administration of human chorionic gonadotropin (HCG) increased testicular ALA synthase activity at 49 and 89 days of age, but it failed to alter activity of this enzyme in prepubertal rats of 24 and 37 days of age. Testicular uroporphyrinogen (uro) I synthase (EC 4.3.1.8) activity was also shown to diminish from weaning through sexual maturity. The activity of uro I synthase was not affected by treatment with HCG at any age tested. These data suggest that testicular heme synthesis undergoes a change in regulation during sexual maturation.