Changes in the activities of the protoheme-synthesizing system during the growth of yeast under different conditions

Role of the non-respiratory pathways in the utilization of molecular oxygen by Saccharomyces cerevisiae

Saccharomyces cerevisiae is a facultative anaerobe devoid of mitochondrial alternative oxidase. In this yeast, the structure and biogenesis of the respiratory chain, on the one hand, and the functional interactions of oxidative phosphorylation with the cellular energetic metabolism, on the other, are well documented. However, to our knowledge, the molecular aspects and the physiological roles of the non-respiratory pathways that utilize molecular oxygen have not yet been reviewed. In this paper, we review the various non-respiratory pathways in a global context of utilization of molecular oxygen in S. cerevisiae. The roles of these pathways are examined as a function of environmental conditions, using either physiological, biochemical or molecular data. Special attention is paid to the characterization of the so-called 'cyanide-resistant respiration' that is induced by respiratory deficiency, catabolic repression and oxygen limitation during growth. Finally, several aspects of oxygen sensing are discussed.

Identification of rate-limiting steps in yeast heme biosynthesis

The heme biosynthesis pathway in the yeast Saccharomyces cerevisiae is a highly regulated system, but the mechanisms accounting for this regulation remain unknown. In an attempt to identify rate-limiting steps in heme synthesis, which may constitute potential regulatory points, we constructed yeast strains overproducing two enzymes of the pathway: the porphobilinogen synthase (PBG-S) and deaminase (PBG-D). Biochemical analysis of the enzyme-overproducing strains revealed intracellular porphobilinogen and porphyrin accumulation. These results indicate that both enzymes play a rate-limiting role in yeast heme biosynthesis.

Porphyrin biosynthesis in normal and haem mutants of Saccharomyces cerevisiae. Studies on the inheritance of the HEM R+ phenotype

Heme biosynthesis was studied in the segregants of Saccharomyces cerevisiae (DW10 tetrade 2) from D27 and D27/C6 mating, as a function of the carbon source in the growth medium and the physiological state of the cells. The effects of the HEM R+ gene on the 5-aminolevulinate synthase (ALA-S) and 5-aminolevulinate dehydratase (ALA-D) activities of heme biosynthesis in cells grown on nonfermentable and fermentable carbon sources were compared. Profiles obtained for both strains grown on a fermentable carbon source (glucose) were identical. However, in the presence of a nonfermentable carbon source (ethanol), they behave quite different, as if the mutation could only be expressed under these growth conditions. Moreover, their behavior is similar to that found for the parental strains, indicating that for the mutant its particular behavior might be inheritedly linked to the HEM R+ gene, which in turn affects some regulatory aspects of ALA synthesis explaining its characteristic phenotype.

Delta-aminolevulinic acid transport in Saccharomyces cerevisiae

1. This work represents the first approach to characterize the transport system of haem pathway precursors, such as delta-aminolevulinic acid (ALA), in two strains of Saccharomyces cerevisiae, a wild type, D27, and a HEM R+ mutant. 2. ALA transport occurs unidirectionally by a sole active system with an apparent KM of 0.10 mM, at the optimum pH of 5.0. ALA uptake is influenced by both the carbon and nitrogen source; this suggests a rather complex regulation mechanism. 3. This transport is not mediated by the general amino acid permease (GAP). 4. ALA uptake is strongly inhibited by compounds harboring a methyl-amine terminus suggesting that this group is essential for ALA transport; however, the electric environment of the carboxylic group may be also important for the interaction between ALA and its transporter active site. 5. We have found differences in ALA transport which would indicate a different regulation mechanism for this system in both strain cells.

The role of ALA-S and ALA-D in regulating porphyrin biosynthesis in a normal and a HEM R+ mutant strain of Saccharomyces cerevisiae

Catabolite repression and derepression on delta-aminolevulinate synthase (ALA-S) and delta-aminolevulinate dehydratase (ALA-D) in a normal yeast strain, D27, and its derived D27/C6 (HEM R+) were investigated. ALA-S and ALA-D activities and intracellular ALA (I-ALA) at different physiological states of the cells were measured. In YPD medium, under conditions of repression and when glucose was exhausted, both strains behaved identically as if the mutation was not expressed. In YPEt medium, however, both ALA-S and ALA-D activities were higher than in YPD, but the I-ALA content and the enzymic activity profiles shown by the two strains were quite different. It appears, therefore, that the mutation causes a deregulation of ALA-S, so that its activity is kept at a high level throughout the cell cycle. This would explain the increased levels of cytochromes present in the mutant. This mutation may affect some regulatory aspect of ALA formation and renders an ALA-S of high activity; moreover, this enzyme species seems to be more stable than in the normal strain.

Purification of delta-aminolevulinate dehydratase from genetically engineered yeast

Saccharomyces cerevisiae transformed with a multicopy plasmid carrying the yeast structural gene HEM2, which codes for delta-aminolevulinate dehydratase, was enriched 20-fold in the enzyme. Beginning with cell-free extracts of transformed cells, the dehydratase was purified 193-fold to near-homogeneity. This represents a 3900-fold purification relative to the enzyme activity in normal, untransformed yeast cells. The specific activity of the purified enzyme was 16.2 mumol h-1 per mg protein at pH 9.4 and 37.5 degrees C. In most respects the yeast enzyme resembles mammalian enzymes. It is a homo-octamer with an apparent Mr of 275,000, as determined by centrifugation in glycerol density gradients, and under denaturing conditions behaved as a single subunit of Mr congruent to 37,000. The enzyme requires reduced thiol compounds to maintain full activity, and maximum activity was obtained in the presence of 1.0 mM-Zn2+. It is sensitive to inhibition by the heavy metal ions Pb2+ and Cu2+. The enzyme exhibits Michaelis-Menten kinetics and has an apparent Km of 0.359 mM. Like dehydratases from animal tissues, the yeast enzyme is rather thermostable. During the purification process an enhancement in total delta-aminolevulinate dehydratase activity suggested the possibility that removal of an inhibitor of the enzyme could be occurring.

Isolation and characterization of a new mutant of Saccharomyces cerevisiae with altered synthesis of 5-aminolevulinic acid

A new gene, RHM1, required for normal production of 5-aminolevulinic acid by Saccharomyces cerevisiae, was identified by a novel screening method. Ethyl methanesulfonate treatment of a fluorescent porphyric strain bearing the pop3-1 mutation produced nonfluorescent or weakly fluorescent mutants with defects in early stages of tetrapyrrole biosynthesis. Class I mutants defective in synthesis of 5-aminolevulinate regained fluorescence when grown on medium supplemented with 5-aminolevulinate, whereas class II mutants altered in later biosynthetic steps did not. Among six recessive class I mutants, at least three complementation groups were found. One mutant contained an allele of HEM1, the structural gene for 5-aminolevulinate synthase, and two mutants contained alleles of the regulatory gene CYC4. The remaining mutants contained genes complementary to both hem1 and cyc4. Mutant strain DA3-RS3/68 contained mutant gene rhm1, which segregated independently of hem1 and cyc4 during meiosis. 5-Aminolevulinate synthase activity of the rhm1 mutant was 35 to 40% of that of the parental pop3-1 strain, whereas intracellular 5-aminolevulinate concentration was only 3 to 4% of the parental value. Transformation of an rhm1 strain with a multicopy plasmid containing the cloned HEM1 gene restored normal levels of 5-aminolevulinate synthase activity, but intracellular 5-aminolevulinate was increased to only 9 to 10% of normal. We concluded that RHM1 could control either targeting of 5-aminolevulinate synthase to the mitochondrial matrix or the activity of the enzyme in vivo.

Characteristics of L-alanine:4,5-dioxovaleric acid transaminase: an alternate pathway of heme biosynthesis in yeast

The present study reports for the first time the presence of the enzyme L-alanine:4,5-dioxovaleric acid transaminase (EC 2.6.1.43), which catalyzes the irreversible synthesis of 5-aminolevulinic acid in three strains of yeast: Candida albicans 3100, Saccharomyces cerevisiae 3059, and S. cerevisiae S288C. The enzyme was predominantly present in the cytosol and was purified from C. albicans 3100 by about 200-fold with an overall yield of 23% to apparent homogeneity. The purification procedure involved heat treatment, followed by affinity chromatography on L-alanine-Sepharose CL-4B, ion-exchange chromatography on DEAE-cellulose DE-52, and chromatography on hydroxyapatite. As judged by gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the enzyme appeared to be a monomeric protein of relative molecular mass 59,000. The enzyme activity was stimulated by pyridoxal phosphate and was optimally active at 60 degrees C. The purified enzyme had an isoelectric point of 4.7 and a pH optimum of 7.2 Km values of the enzyme for L-alanine, pyridoxal phosphate, and 4,5-dioxovaleric acid were 2.8 X 10(-7), 5.0 X 10(-7), and 1.05 X 10(-5) M, respectively. The Vmax of the enzymatic reaction was 5 X 10(-6) mol/mg/h. Antibody raised against the purified yeast L-alanine:4,5-dioxovaleric acid transaminase did not cross-react with the same enzyme from rat liver and kidney.

The effects in vivo of mutationally modified uroporphyrinogen decarboxylase in different hem12 mutants of baker's yeast (Saccharomyces cerevisiae)

Nine new hem12 haploid mutants of baker's yeast (Saccharomyces cerevisiae), totally or partially deficient in uroporphyrinogen decarboxylase activity, were subjected to both genetic and biochemical analysis. The mutations sites studied are situated far apart within the HEM12 gene located on chromosome IV. Uroporphyrinogen decarboxylase activity in the cell-free extracts of the mutants was decreased by 50-100%. This correlated well with the decrease of haem formation and the increased accumulation and excretion of porphyrins observed in vivo. The pattern of porphyrins (uroporphyrin and its decarboxylation products) accumulated in the cells of mutants partially deficient in uroporphyrinogen decarboxylase activity did not differ significantly, although differences in vitro were found in the relative activity of the mutant enzyme at the four decarboxylation steps. The excreted porphyrins comprised mainly dehydroisocoproporphyrin or pentacarboxyporphyrin. In heterozygous hem12-1/HEM12 diploid cells, a 50% decrease in decarboxylase activity led to an increased accumulation of porphyrins as compared with the wild-type HEM12/HEM12 diploid, which points to the semi-dominant character of the hem12-1 mutation. The biochemical phenotypes of both the haploid and the heterozygous diploid resembles closely the situation encountered in porphyria cutanea tarda, the most common human form of porphyria.

A simple and rapid assay for heme attachment to apocytochrome c

A method for assaying the covalent attachment of heme to apoprotein of cytochrome c was developed. 125I-labeled apoprotein was chemically prepared from 125I-labeled yeast cytochrome c (iso-1-cytochrome c). After incubation of 125I-apocytochrome c with yeast mitochondria, the product was extracted with Triton X-100, digested with trypsin in the presence or absence of a reducing agent, and then precipitated in trichloroacetic acid. The resulting precipitates were collected on nitrocellulose membranes and counted for radioactivity. The radioactivity correlated well with the appearance of a heme-containing peptide in the trypsin digested peptide fragments of cytochrome c. This procedure is simpler and faster than the previously reported methods.

Purification and properties of coproporphyrinogen oxidase from the yeast Saccharomyces cerevisiae

Coproporphyrinogen oxidase has been located in the cytosol of yeast cells. The enzyme was purified to homogeneity from a heme mutant strain exhibiting a high specific activity (15-20 enzyme units/mg soluble protein compared to 1-2 enzyme units/mg soluble protein of by the wild-type strain). The final preparation was homogeneous as judged by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (Mr = 35,000) and isoelectrofocusing (pI = 6.2). Gel filtration on AcA 44 gave a relative molecular mass of 70,000. N-terminal amino-acid sequence analysis revealed a single polypeptide chain. Thus the enzyme appears to be a dimer with identical subunits. Two iron atoms/molecule of native protein were detected; they could not be removed by exhaustive dialysis or gel filtration on Sephadex G-25. However the involvement of the iron atoms in the oxidative catalytic activity of the enzyme was not demonstrated. The Km value for coproporphyrinogen was 0.05 microM. The enzyme was active only when molecular oxygen was used as electron acceptor; no anaerobic activity could be detected. Thiol-directed reagents partially inhibited the enzyme, indicating that an SH group is required for activity. Yeast coproporphyrinogen oxidase was activated by phospholipids or neutral detergents as described for the bovine liver enzyme.

A model for testing compounds influencing porphyrin synthesis

The yeast Saccharomyces cerevisiae cultivated semi-anaerobically in a synthetic medium was used as a model to establish (a) total porphyrin synthesis, (b) ratio of intracellular to extracellular porphyrin concentrations. The antimalarials used for the therapy of porphyria cutanea tarda, chloroquine and pyrimethamine, reduced the total synthesis of porphyrins, pyrimethamine being more effective than chloroquine, like in porphyric patients. Both drugs exerted an antagonistic influence on the release of porphyrins from cells. Chloroquine reduced the concentration ratio of porphyrins while pyrimethamine increased it, apparently through inhibition of permeation of porphyrins. Combined treatment with the two compounds may hold promise for the therapy of porphyria cutanea tarda.

Fluorometric assays for coproporphyrinogen oxidase and protoporphyrinogen oxidase

We describe fluorometric assays for two enzymes of the heme pathway, coproporphyrinogen oxidase and protoporphyrinogen oxidase. Both assays are based on measurement of protoporphyrin IX fluorescence generated from coproporphyrinogen III by the two consecutive reactions catalyzed by coproporphyrinogen oxidase and protoporphyrinogen oxidase. Both enzymatic activities are measured by recording protoporphyrin IX fluorescence increase in air-saturated buffer in the presence of EDTA (to inhibit ferrochelatase that can further metabolize protoporphyrin IX) and in the presence of dithiothreitol (that prevents nonenzymatic oxidation of porphyrinogens to porphyrins). Coproporphyrinogen oxidase (limiting) activity is measured in the presence of a large excess of protoporphyrinogen oxidase provided by yeast mitochondrial membranes isolated from commercial baker's yeast. These membranes are easy to prepare and are stable for at least 1 year when kept at -80 degrees C. Moreover they ensure maximum fluorescence of the generated protoporphyrin (solubilization effect), avoiding use of a detergent in the incubation medium. The fluorometric protoporphyrinogen oxidase two-step assay is closely related to that already described (J.-M. Camadro, D. Urban-Grimal, and P. Labbe, 1982, Biochem. Biophys. Res. Commun. 106, 724-730). Protoporphyrinogen is enzymatically generated from coproporphyrinogen by partially purified yeast coproporphyrinogen oxidase. The protoporphyrinogen oxidase reaction is then initiated by addition of the membrane fraction to be tested. However, when very low amounts of membrane are used, low amounts of Tween 80 (less than 1 mg/ml) have to be added to the incubation mixture to solubilize protoporphyrin IX in order to ensure optimal fluorescence intensity. This detergent has no effect on the rate of the enzymatic reaction when used at concentrations less than 2 mg/ml. Activities ranging from 0.1 to 4-5 nmol protoporphyrin formed per hour per assay are easily and reproducibly measured in less than 30 min.

Isolation and properties of 5-aminolevulinate synthase from the yeast Saccharomyces cerevisiae

5-Aminolevulinate synthase from yeast mitochondria has been purified to homogeneity for the first time. By using affinity chromatography on agarose-hexane-CoA, gel filtration and DEAE-Sepharose chromatography, the enzyme was purified about 7000-fold with an overall yield of 40%. The specific activity of the final preparation was 39000 nmol of 5-aminolevulinate h-1 mg-1 of protein at 30 degrees C. As judged by gel filtration, polyacrylamide gradient gel and sodium dodecyl sulfate/polyacrylamide gel electrophoresis, the enzyme appeared to be composed of two identical subunits of a relative molecular mass of 53000. Electrophoresis of sodium-dodecyl-sulfate-solubilized yeast homogenate followed by immune replica analysis showed that the value of 53000 is the Mr of a non-degraded form. The purified enzyme had an isoelectric point of 5.3 and a pH optimum of 7.4. Pyridoxal 5'-phosphate has been shown to be an essential cofactor. The enzyme activity was sensitive to thiol blocking reagents. Hemin, but not heme, inhibited the activity of the purified enzyme.

Modified uroporphyrinogen decarboxylase activity in a yeast mutant which mimics porphyria cutanea tarda

The isolation of a new mutant Sm1 strain of yeast, Saccharomyces cerevisiae, is described: this strain was partially defective in haem formation and accumulated large amounts of Zn-porphyrins. Genetic analysis showed that the porphyrin accumulation was under the control of a single nuclear recessive mutation. Biochemical analysis showed that the main porphyrins accumulated in the cells were uroporphyrin and heptacarboxyporphyrin, mostly of the isomer-III type. The excreted porphyrins comprised mainly dehydroisocoproporphyrin. Analysis of uroporphyrinogen decarboxylase activity in the cell-free extract revealed a 70-80% decrease of activity in the mutant and showed that the relative rates of the different decarboxylation steps were modified with the mutant enzyme. A 2-3-fold increase in 5-aminolaevulinate synthase activity was measured in the mutant. The biochemical characteristics of the Sm1 mutant are very similar to those described for porphyria cutanea tarda.

In situ assay for 5-aminolevulinate dehydratase and application to the study of a catabolite repression-resistant Saccharomyces cerevisiae mutant

To facilitate the study of the effects of carbon catabolite repression and mutations on 5-aminolevulinate dehydratase (EC 4.2.1.24) from Saccharomyces cerevisiae, a sensitive in situ assay was developed, using cells permeabilized by five cycles of freezing and thawing. Enzymatic activity was measured by colorimetric determination of porphobilinogen with a modified Ehrlich reagent. For normal strains, porphobilinogen production was linear for 15 min, and the reaction rate was directly proportional to the permeabilized cell concentration up to 20 mg (dry weight) per ml. The reaction exhibited Michaelis-Menten-type kinetics, and an apparent Km of 2.6 mM was obtained for 5-aminolevulinic acid. This value is only slightly higher than the value of 1.8 mM obtained for the enzyme assayed in cell extracts. The in situ assay was used to assess catabolite repression-dependent changes in 5-aminolevulinate dehydratase during batch culture on glucose medium. In normal S. cerevisiae cells, the enzyme is strongly repressed as long as glucose is present in the medium. In contrast, a strain bearing the hex2-3 mutation exhibits derepressed levels of enzyme activity during growth on glucose. Synthesis of cytochromes by this strain is also resistant to catabolite repression. Similar studies employing a strain containing the glc1 mutation, which enhances porphyrin accumulation, did not reveal any significant phenotypic change in catabolite regulation of 5-aminolevulinate dehydratase.

Kinetic studies of ferrochelatase in yeast. Zinc or iron as competing substrates

Ferrochelatase (protoheme ferro-lyase, EC 4.99.1.1) has been studied in yeast mitochondrial membranes with special reference to zinc-chelatase and iron-chelatase activities. Using physiological substrates (protoporphyrin IX, Fe(II) and Zn(II), anaerobic conditions of incubation and direct spectrophotometric assay, apparent Km values smaller than those previously described were found for the membrane-bound enzyme. Fe(II) but not Fe(III) was a strong competitive inhibitor of zinc-chelatase activity, while Zn(II) was a slight competitive inhibitor of iron-chelatase activity. These results could point to modes of control of ferrochelatase activity in yeast. We suggest that reduced supply of Fe(II) may explain the in vivo accumulation of zinc-protoporphyrin in yeast cells incubated under 'resting' conditions.

Mechanism of thiamine-induced respiratory deficiency in Saccharomyces carlsbergensis

Cells of Saccharomyces carlsbergensis 4228 grown aerobically with added thiamine (1 microgram . ml-1) in a vitamin B6-free medium contained no detectable heme precursors, such as delta-aminolevulinate, coproporphyrin III, or protoporphyrin IX. The deficiency in heme precursors in the thiamine-grown cells was accompanied by previously reported phenomena, i.e., growth depression, vitamin B6 deficiency, and respiratory deficiency due to a marked decrease in the activities of heme-containing enzymes and cytochrome level (I. Nakamura et al., FEBS Lett. 62: 354-358, 1976). It has been reported that all of the effects of thiamine are abolished by adding pyridoxine to the medium. delta-Aminolevulinate was found to have quite similar effects to those of pyridoxine, except that growth was partially improved by delta-aminolevulinate, whereas it was fully restored by pyridoxine. Incubation of the thiamine-grown cells with delta-aminolevulinate resulted in the appearance of the heme precursors and the heme-containing enzymes. Consistent with the lowered amount of vitamin B6, the thiamine-grown cells had a lowered activity of delta-aminolevulinate synthase, a pyridoxal phosphate-dependent enzyme. Not only the holoenzyme activity but also the apoenzyme activity was very low in these cells. These results indicate that the thiamine-induced vitamin B6 deficiency brings about the decrease in delta-aminolevulinate synthase activity, which leads to heme deficiency and therefore to respiratory deficiency.

Genetic and biochemical characterization of mutants of Saccharomyces cerevisiae blocked in six different steps of heme biosynthesis

Heme-deficient mutants of Saccharomyces cerevisiae have been isolated from two isogenic strains with the use of an enrichment method based on photodynamic properties of Zn-protoporphyrin. They defined seven non-overlapping complementation groups. A mutant representative of each group was further analysed. Genetic analysis showed that each mutant carried a single nuclear recessive mutations. Biochemical studies showed that the observed accumulation and/or excretion of the different heme synthesis precursors by the mutant cells correlated well with the enzymatic deficiencies measured in acellular extracts. Six of the seven mutants were blocked in a different enzyme activity: 5-aminolevulinate synthase, porphobilinogen synthase, uroporphyrinogen I synthase, uroporphyrinogen decarboxylase, coproporphyrinogen III oxidase and ferrochelatase. The other mutant had the same phenotype as the mutant deficient in ferrochelatase activity. However, it possessed a normal ferrochelatase activity when measured in vitro, so this mutant was assumed to be deficient in protoporphyrinogen oxidase activity or in the transport and/or reduction of iron. The absence of PBG synthesis led to a total lack of uroporphyrinogen I synthase activity. The absence of heme, the end product, led to an important increase of coproporphyrinogen III oxidase activity, while the activity of 5-aminolevulinate synthase, the first enzyme of the pathway, was not changed. These results are discussed in terms of possible modes of regulation of heme synthesis pathway in yeast.

Effect of carbon source on the accumulation of cytochrome P-450 in the yeast Saccharomyces cerevisiae

The appearance of cytochrome P-450 in the yeast Saccharomyces cerevisiae depended on the substrate supporting growth. Cytochrome P-450 was apparent in yeast cells grown on a strongly fermentable sugar such as D-glucose, D-fructose or sucrose. When yeast was grown on D-galactose, D-mannose or maltose, where fermentation and respiration occurred concomitantly, cytochrome P-450 was also formed. The cytochrome P-450 concentration was maximal at the beginning of the stationary phase of the culture. Thereafter the concentration decreased, reaching zero at a late-stationary phase. When the yeast was grown on a medium that contained lactose or pentoses (L-arabinose, L-rhamnose, D-ribose and D-xylose), cytochrome P-450 did not occur. When a non-fermentable energy source (glycerol, lactate or ethanol) was used, no cytochrome P-450 was detectable. Transfer of cells from D-glucose medium to ethanol medium caused a slow disappearance of cytochrome P-450, although the amount of the haemoprotein still continued to increase in the control cultures. Cytochrome P-450 appeared thus to accumulate in conditions where the rate of growth was fast and fermentation occurred. Occurrence of this haemoprotein is not necessarily linked, however, with the repression of mitochondrial haemoprotein synthesis.

An enrichment method for heme-less mutants of Saccharomyces cerevisiae based on photodynamic properties of Zn-protoporphyrin

A new and widely applicable technique has been elaborated for enrichment of mutants of Saccharomyces cerevisiae deficient in porphyrin and heme synthesis. The method is based on selective photooxidative killing of the wild-type cells, sensitited by Zn-protoporphyrin synthesized and accumulated in wild-type cells under defined conditions. Using a single cycle of selection, heme-deficient mutants have been obtained with a frequency of 2-2.3%.

Molecular events during the release of delta-aminolevulinate dehydratase from catabolite repression

Transfer of exponential-phase cells of Saccharomyces cerevisiae, previously grown in 2% glucose, to a derepression medium resulted in a prompt increase in the level of delta-aminolevulinate dehydratase, the rate-limiting enzyme of heme biosynthesis under these conditions. This derepression exhibited a lag of 35 min at 23 degrees C and required the participation of both RNA and protein syntheses. Dissection of the molecular events during this lag period disclosed that RNA synthesis, rnal gene function (messenger RNA transport from nucleus to cytosol), and initiation of protein synthesis were completed within less than 10, 18, and 24 min, respectively. The potential regulation of derepression by mitochondrial gene products and mitochondrial function was probed by means of a series of isogenic, respiration-deficient (rho-, pet-, and mit-) mutants; no such regulation was found.

[Inhibition of the yeast respiratory system by Zn-protoporphyrin and effect of photolysis of this substance]

We have shown earlier that yeast cells grown in synthetic mediums supplemented with Zn++ accumulate large amounts of Zn-protoporphyrin within their mitochondria. This accumulation is accompanied by an inhibition of respiration (3). This study deals with the effect of light on the respiratory inhibition and the release of respiratory control which are observed if Zn-protoporphyrin is added to isolated mitochondria which are initially devoid of this pigment. In addition, we have studied the effect of light on the respiratory inhibition exerted by Zn-protoporphyrin accumulated in vivo. The following results were obtained: 1) The light-induced destruction of Zn-protoporphrin which had been added in vitro to Zn-protoporphyrin-free mitochondria significantly inhibits respiration and phosphorylation. Under these conditions, the extent of the inhibitions increases with the concentration of the added Zn-protoporphyrin and the duration of illumination. 2) Accumulation of Zn-protoporphyrin within the cells causes an inhibition of the respiratory activities and the activities of succinate-cytochrome c reductase and NADH-cytochrome c reductase of the mitochondria. Illumination of the isolated mitochondria from Zn-protoporphyrin-containing cells enhances the inhibition of these activities. No light-induced inhibition of these activities is observed with mitochondria from cells devoid of Zn-protoporphyrin.

Analysis of heme biosynthesis in catalase and cytochrome deficient yeast mutants

Mutants of Saccharomyces cerevisiae, described as catalase and cytochromes deficient (Pachecka et al., 1974), have been analyzed for heme biosynthesis ability. Some enzymatic activities involved in protoheme synthesis were measured in acellular extracts, whereas whole cells were analyzed for cytochrome spectra and for possible accumulation of porphyrin synthesis intermediates. A good correlation was found between these in vitro and in vivo studies. Results show that two mutants were impaired in 5-aminolevulinate synthesis, two mutants were devoid of uroporphyrinogen I synthetase activity and one mutant presented defects in coproporphyrinogen III oxidase activity.