The ABC transporter proteins Pat1 and Pat2 are required for import of long-chain fatty acids into peroxisomes of Saccharomyces cerevisiae

Peroxisomes of Saccharomyces cerevisiae are the exclusive site of fatty acid beta-oxidation. We have found that fatty acids reach the peroxisomal matrix via two independent pathways. The subcellular site of fatty acid activation varies with chain length of the substrate and dictates the pathway of substrate entry into peroxisomes. Medium-chain fatty acids are activated inside peroxisomes hby the acyl-CoA synthetase Faa2p. On the other hand, long-chain fatty acids are imported from the cytosolic pool of activated long-chain fatty acids via Pat1p and Pat2p, peroxisomal membrane proteins belonging to the ATP binding cassette transporter superfamily. Pat1p and Pat2p are the first examples of membrane proteins involved in metabolite transport across the peroxisomal membrane.

Synergistic effect of 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8-tetrachlorodibenzo-p-dioxin on hepatic porphyrin levels in the rat

We studied the effect of polychlorinated biphenyls (PCBs) on hepatic porphyrin accumulation in female Sprague-Dawley rats by feeding them diets containing 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), 2,3,3',4,4',5-hexachlorobiphenyl (PCB 156), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), or combinations of the single PCB congeners with TCDD for 13 weeks. A dose-dependent increase in hepatic porphyrin accumulation occurred after TCDD, PCB 126, or PCB 156 administration, reaching maximal levels of about twice control values. The lowest dose levels for which a significant increase in hepatic porphyrin accumulation was found were 0.7 microgram TCDD/kg diet, 50 micrograms PCB 126/kg diet, or 6 mg PCB 156/kg diet. These doses are equivalent to 47 ng TCDD/kg/day, 3.2 micrograms PCB 126/kg/day, and 365 micrograms PCB 156/kg/day. Relative potencies for hepatic porphyrin accumulation, using TCDD as a reference, ranged from 0.015 to 0.06 for PCB 126 and from 0.0001 to 0.0003 for PCB 156. CYP1A2 activities significantly correlated with hepatic porphyrin levels, with coefficients of 0.629, 0.483, or 0.808 for TCDD, PCB 126, or PCB 156, respectively. Administration of PCB 153 alone did not result in hepatic porphyrin accumulation. Co-administration of PCB 153 and TCDD revealed a strong synergistic effect on porphyrin accumulation (about 800 times control levels). This synergistic effect was significant in rats fed diets containing any combination of PCB 153 with TCDD. Uroporphyrin III and heptacarboxylic porphyrin were accumulated in porphyrinogenic livers. These results suggest that TCDD induction of CYP1A2 may be involved, leading to oxidation of uroporphyrinogen III to uroporphyrin III, in combination with an increase in delta-aminolevulinic acid synthetase induced by PCB 153. Under porphyrinogenic conditions, an inhibitor of CYP1A2 activity may also be formed. The interactive effects on porphyrin accumulation after co-administration of dioxinlike and non-dioxinlike compounds may have significant implications for the risk assessment of these chemicals.

Synergistic effect of 2,2',4,4',5,5'-hexachlorobiphenyl and 2,3,7,8-tetrachlorodibenzo-p-dioxin on hepatic porphyrin levels in the rat

We studied the effect of polychlorinated biphenyls (PCBs) on hepatic porphyrin accumulation in female Sprague-Dawley rats by feeding them diets containing 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153), 2,3,3',4,4',5-hexachlorobiphenyl (PCB 156), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), or combinations of the single PCB congeners with TCDD for 13 weeks. A dose-dependent increase in hepatic porphyrin accumulation occurred after TCDD, PCB 126, or PCB 156 administration, reaching maximal levels of about twice control values. The lowest dose levels for which a significant increase in hepatic porphyrin accumulation was found were 0.7 microgram TCDD/kg diet, 50 micrograms PCB 126/kg diet, or 6 mg PCB 156/kg diet. These doses are equivalent to 47 ng TCDD/kg/day, 3.2 micrograms PCB 126/kg/day, and 365 micrograms PCB 156/kg/day. Relative potencies for hepatic porphyrin accumulation, using TCDD as a reference, ranged from 0.015 to 0.06 for PCB 126 and from 0.0001 to 0.0003 for PCB 156. CYP1A2 activities significantly correlated with hepatic porphyrin levels, with coefficients of 0.629, 0.483, or 0.808 for TCDD, PCB 126, or PCB 156, respectively. Administration of PCB 153 alone did not result in hepatic porphyrin accumulation. Co-administration of PCB 153 and TCDD revealed a strong synergistic effect on porphyrin accumulation (about 800 times control levels). This synergistic effect was significant in rats fed diets containing any combination of PCB 153 with TCDD. Uroporphyrin III and heptacarboxylic porphyrin were accumulated in porphyrinogenic livers. These results suggest that TCDD induction of CYP1A2 may be involved, leading to oxidation of uroporphyrinogen III to uroporphyrin III, in combination with an increase in delta-aminolevulinic acid synthetase induced by PCB 153. Under porphyrinogenic conditions, an inhibitor of CYP1A2 activity may also be formed. The interactive effects on porphyrin accumulation after co-administration of dioxinlike and non-dioxinlike compounds may have significant implications for the risk assessment of these chemicals.

Airway responsiveness after a single dose of salmeterol and during four months of treatment in children with asthma

BACKGROUND

Inhalation of a single dose of the long-acting beta 2-adrenoceptor agonist salmeterol protects against methacholine-induced airway obstruction and other bronchoconstricting stimuli for at least 12 hours. Hypothetically, twice daily dosing of salmeterol may result in continuous protection.

OBJECTIVE

This study was designed to investigate the protective effect of a single dose of salmeterol and of continuous twice daily treatment on airway responsiveness to methacholine.

METHODS

In a double- blind, parallel study, salmeterol 50 micrograms twice daily was compared with salbutamol 200 micrograms twice daily. Thirty children with mild asthma, who had little or no bronchial obstruction and were hyperresponsive to methacholine (PD20 < or = 150 micro g) were allocated to receive either salmeterol or salbutamol. Airway responsiveness was measured before study entry, 12 hours after a single dose of drug was given, and monthly during 4 months of daily treatment. Measurements were always performed at the same time of the day, 12 hours after the last dose of medication was administered.

RESULTS

No significant differences in FEV 1 were found between treatments at any time point. PD20 significantly increased after the first dose of salmeterol was given (geometric mean, 100 micro g). Geometric mean PD20 values were significantly better during salmeterol treatment than during salbutamol treatment, 52 and 25 micro g, respectively (p = 0.005).

CONCLUSION

The protection provided by salmeterol during maintenance treatment was less than that provided after the first dose (p <0.001). However, protection did not diminish during the 4-month treatment period and remained significant compared with baseline (p = 0.003).

Folic acid treatment of hyperhomocysteinemia in dialysis patients

We measured fasting total plasma homocysteine (Hcy) in 10 chronic hemodialysis (HD) and 10 chronic peritoneal dialysis (PD) patients. Mean (+/- SEM) Hcy was 55.7 +/- 10.1 and 50.5 +/- 14.3 mumol/l, respectively (normal range 6-19 mumol/l). Hemodialysis treatment lowered Hcy by about 30%. Daytime Hcy concentrations were stable in the PD patients. Six weeks of treatment with folic acid (FA) significantly lowered Hcy in HD and PD patients to 24.0 +/- 1.8 and 21.0 +/- 3.6 mumol/l, respectively. After withdrawal, Hcy rose slowly, in parallel with the gradually decreasing plasma FA concentrations, which were greatly elevated during treatment. Chronic treatment with FA of another group of patients showed a similar effect on Hcy. Preliminary results of oral methionine loading in chronic dialysis patients were compatible with delayed homocysteine metabolism via the transsulfuration pathway. Further studies on the optimal treatment of hyperhomocysteinemia in chronic dialysis patients are needed.

Plasma homocysteine and severity of atherosclerosis in young patients with lower-limb atherosclerotic disease

Elevated plasma homocysteine levels are recognized as an independent risk factor for atherosclerotic disease. It is not known (1) whether the severity of atherosclerotic disease is related to hyperhomocyst(e)inemia or (2) whether any such relation differs between fasting and post-methionine loading plasma homocysteine levels. Therefore, in 171 consecutive patients under 55 years of age with first symptoms of lower-limb disease, we examined the relation between severity of atherosclerosis and plasma homocysteine concentration. Severity of atherosclerotic disease was estimated from the prevalence of coronary artery disease and cerebrovascular disease and from the angiographic extent of lower-limb disease. Plasma homocysteine was measured after a period of fasting and in response to methionine loading (0.1 g/kg). In multivariate analysis, the prevalence of coronary artery disease plus cerebrovascular disease was related to both fasting and postmethionine homocysteine levels (odds ratio [OR] for the upper quartile versus the lower three quartiles, 2.8, 95% confidence interval [CI], 1.1 to 7.5; and OR 3.0, 95% CI, 1.1 to 7.8, respectively). The extent of lower-limb disease was weakly related to the fasting homocysteine level (partial correlation coefficient, .12; P = .17) and more strongly related to the postmethionine homocysteine level (partial correlation coefficient, .25; P = .003). These relations tended to be more pronounced in women than in men. They were independent of age, total serum cholesterol, blood pressure, and smoking habit. We concluded that the severity of atherosclerotic disease in young patients with lower-limb atherosclerotic disease is associated with high postmethionine and fasting homocysteine concentrations.

Localization of peroxisomal 3-oxoacyl-CoA thiolase in particles of varied density in rat liver: implications for peroxisome biogenesis

In this paper we report on the subcellular localization of peroxisomal thiolase in rat liver using density-gradient centrifugation and immunoelectron microscopy. The results obtained show that peroxisomes display great biochemical heterogeneity and can not be regarded as one homogeneous population of particles. We conclude that rat liver contains at least three distinct populations of peroxisomes, which are present both in normal-fed rats as well in rats treated with a plasticizer, di-(2-ethylhexyl)phthalate, known to induce peroxisomes. The following types of peroxisomes could be discerned: (1) Low-density peroxisomal particles containing 69-kDa peroxisomal membrane protein (PMP), dihydroxyacetonephosphate acyltransferase (DHAPAT) and the precursor form of peroxisomal thiolase (44-kDa). (2) Intermediate-density peroxisomal particles containing 69-kDa peroxisomal membrane protein, dihydroxyacetonephosphate acyltransferase, both 41-kDa (mature) and 44-kDa (immature) peroxisomal thiolase, catalase and D-aminoacid oxidase. (3) High-density peroxisomes containing 69-kDa peroxisomal membrane protein, dihydroxyacetonephosphate acyltransferase, 41-kDa thiolase, catalase and D-aminoacid oxidase.

Factor IX Zutphen: a Cys18-->Arg mutation results in formation of a heterodimer with alpha 1-microglobulin and the inability to form a calcium-induced conformation

Factor IX Zutphen is a variant factor IX molecule isolated from the blood of a patient with severe haemophilia B. The molecular defect in factor IX Zutphen is a Cys18-->Arg mutation as a result of a T-->C transition at residue 6427 of the factor IX gene of the patient. The mutation disrupts the disulphide bond in the Gla-domain between Cys18 and Cys23. The remaining free cysteine residue results in the formation of a 95 kDa complex with alpha 1-microglobulin through an intermolecular disulphide bond. The same complex circulates at high levels in plasma of carriers of the mutation. The variant molecule has a calcium-binding defect, which is shown not to be caused by incomplete gamma-carboxylation. Factor IX Zutphen can not bind to phospholipids and can not be activated by factor XIa or by factor VIIa-tissue factor complex. Two sequential metal ion-dependent conformational transitions (factor IX-->factor IX'-->factor IX*) have been proposed for human factor IX [Liebman (1987) J. Biol. Chem. 262, 7605-7612], based upon the metal ion requirements for binding to anti-factor IX:Mg(II) antibodies, which are specific for the factor IX' conformation, and anti-factor IX:Ca(II) antibodies, which are specific for the factor IX* conformation. We used these conformation-specific antibodies, and antibodies raised against a synthetic peptide corresponding to residues 35-50 of human factor IX [anti-factor IX(35-50)] to study the metal ion-induced conformation of factor IX Zutphen. The disruption of the disulphide bond in the Gla-domain, maybe in combination with the complex with alpha 1-microglobulin, destabilized the factor IX' conformation. The formation of the factor IX* conformation was prevented independent of the presence of alpha 1-microglobulin. The disulphide bond in the Gla-domain is therefore essential for the calcium-dependent conformation and function of factor IX.

Individual PCBs as predictors for concentrations of non and mono-ortho PCBs in human milk

32 Dutch human milk samples were analyzed for PCBs with either HRGC-ECD or HRGC-LRMS in the NCI mode. Samples were collected from three different locations in The Netherlands: Amsterdam, Rotterdam and Groningen. Quantitatively, no differences could be observed between the three localities, while in addition the congener specific pattern showed a striking similarity for all individual samples. Only principal component analysis revealed slight individual differences. Based on similarities in the PCB profiles, linear relationships were calculated between 2,3'4,4',5-PnCB (#118) or 2,2'4,4'5,5'HxCB (#153) and the most relevantnon andmonoortho PCBs exhibiting dioxinlike activity. These PCBs included 2,3,3',4,4'-PnCB (#105), 3,3',4,4'5-PnCB (#126) 2,3,3',4,4',5-HxCB (#156), 2,3,3',4,4',5'-HxCB (#157), 2,3',4,4',5,5'-HxCB (#167) and 3,3',4,4',5'5-HxCB (#169).Good linear relationships were observed between individual PCBs. Based on the results of this study, PCB #118 can be used to predict concentrations of the PCBs #105 and #126. PCB #153 can be used as a predictor for the PCBs #156, #157, #167 and #169, but also for the total toxic equivalencies (TEQs) ofnon andmonoortho PCBs present in human milk. This method using certain PCBs as predictors for other toxicological relevant congeners, can be useful and cost effective, e.g. for epidemiological studies. However, before applied a number of conditions should be met. These are: 1) A stable composition of the PCB matrix should be established. 2) A possible time dependent change in composition of the matrix should first be excluded when used over different time periods.

Hyperhomocysteinaemia; with reference to its neuroradiological aspects

Severe or even mild hyperhomocysteinaemia can cause a wide range of neurological problems. In recent years its vascular complications, including cerebral stroke, in children and young adults have gained special interest, because hyperhomocysteinaemia is treatable and recurrence of vascular incidents may be preventable. Current knowledge about biochemical mechanisms leading to hyperhomocysteinaemia, the pathogenesis of vascular pathology and neurological disfunction, and the various patterns of cerebral damage are reviewed. The significance of MRI in diagnosis, follow-up and research on hyperhomocysteinaemia is discussed.

Inhibition of ethoxyresorufin-O-deethylase (EROD) activity in mixtures of 2,3,7,8-tetrachlorodibenzo-p-dioxin and polychlorinated biphenyls : EROD acitivity as biomarker in TCDD and PCB risk assessment

Induction of ethoxyresorufin-O-deethylase (EROD) activity and porphyrin accumulation shows different structure-activity relationships for different polychlorinated biphenyls (PCBs) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Interactions between the two responses can strongly influence the induction and activity of EROD. The results support the conclusion that there are nonadditive interactions between nondioxin-like PCBs and dioxin-like compounds. The interaction between EROD activity and the porphyrin biosynthesis makes the prerequisite of additivity in the toxic equivalency factor concept for environmental mixtures highly spurious. Inhibition of EROD activity caused by non-dioxin like compounds could have a significant impact on the value of EROD activity as a biomarker in the present methods of risk assessment for these compounds.

Hyperhomocysteinaemia: a role in the accelerated atherogenesis of chronic renal failure?

Moderate hyperhomocysteinaemia has recently been established as an independent risk factor for atherothrombotic disease. It might be caused by heterozygosity for cystathionine beta-synthase deficiency, an enzyme involved in the conversion of methionine to cysteine through the transsulphuration pathway or by inherited thermolability of the enzyme which remethylates homocysteine into methionine. In chronic renal failure (CRF) homocysteine levels are significantly elevated at a relatively early stage. The normal kidney possibly plays an important role in homocysteine catabolism, which cannot be performed in CRF. Alternatively, decreased extrarenal catabolism can contribute to the hyperhomocysteinaemia in this disease state. Treatment with folic acid, 5 mg daily, significantly lowers homocysteine levels in chronic renal patients.

The Nijmegen modification of the Bethesda assay for factor VIII:C inhibitors: improved specificity and reliability

Antibodies against factor VIII coagulant activity can appear in haemophiliacs who are treated with factor VIII preparations but also spontaneously in non-haemophiliacs. The Bethesda assay is the most commonly used method to detect these antibodies, but it lacks specificity especially in the lower range resulting in unreliable data. Two modifications are proposed and tested to resolve the imperfections: 1. Buffering the normal plasma used in the assay- and control mixture with 0.1 M imidazole to pH 7.4. 2. Replacing the imidazole buffer in the control mixture by immunodepleted factor VIII deficient plasma. These modifications allow better discrimination between positive and negative samples and improve reliability.

Combined vitamin B6 plus folic acid therapy in young patients with arteriosclerosis and hyperhomocysteinemia

PURPOSE

Hyperhomocysteinemia is associated with arteriosclerotic and thromboembolic events. The homocysteine-lowering effect of combined treatment with vitamin B6 plus folic acid has never been explored in a large group of patients with vascular disease. Therefore we studied the effects of at least 6 weeks treatment with these vitamins in 72 patients with cardiovascular disease and mild hyperhomocysteinemia (defined as an increase of the plasma homocysteine level after methionine loading greater than 97.5 percentile of age-matched control subjects but less than 200 mumol/L).

METHODS

The existence of mild hyperhomocysteinemia was investigated in 309 consecutive patients under 50 years of age with peripheral arterial occlusive disease, cerebral arterial occlusive disease, or coronary artery occlusive disease. All patients with an abnormal loading test result were treated with vitamin B6, 250 mg daily, plus folic acid, 5 mg daily. After 6 weeks of treatment a second methionine loading test was performed to assess the homocysteine-lowering effect.

RESULTS

Mild hyperhomocysteinemia was detected in 72 patients (23%), 33 (46%) of whom also had hyperhomocysteinemia when fasting. Treatment with vitamin B6 plus folic acid normalized the postload plasma homocysteine concentration in 66 of the 72 patients (92%), whereas fasting hyperhomocysteinemia was normalized in 30 of 33 (91%) patients. In six patients therapy failed to achieve normalization of the postload homocysteine levels. In three of these patients, the same treatment was continued for an additional 6 weeks, and in the remaining three patients betaine was added to the treatment regimen. After 6 weeks of additional treatment all six patients had normal postload plasma homocysteine concentrations.

CONCLUSION

The prevalence of mild hyperhomocysteinemia in young patients with arterial occlusive disease is high. Simple and inexpensive therapy with vitamin B6 plus folic acid will normalize homocysteine metabolism, as assessed by the homocysteine plasma level after methionine loading, in virtually all these patients.

Developmental regulation of epidermal growth factor receptor kinase in rat intestine

BACKGROUND/AIMS

Intraluminal epidermal growth factor (EGF) may regulate intestinal growth and function. The ontogeny, localization, and phosphorylation of the EGF receptor in rat small intestine were studied.

METHODS

EGF-receptor phosphorylation was assayed by antiphosphotyrosine Western blot after EGF administration in vivo and EGF incubation to everted sacs in vitro. EGF-receptor abundance and localization were assayed by Western blot and immunofluorescence using anti-EGF-receptor antibodies.

RESULTS

In vivo, orogastric EGF enhanced EGF-receptor phosphorylation in newborn rat jejunum and liver. However, intraluminal EGF had no effect on EGF-receptor phosphorylation in adult intestine or liver. In vitro, mucosal EGF stimulated a fourfold increase in EGF-receptor phosphorylation in suckling jejunum but not in weanling or adult jejunum. In Western blot, EGF-receptor abundance was similar in 22-day-old fetal, 8-day-old suckling, and adult jejunum. In both crypt and villus, EGF-receptor was localized to the basolateral membrane but not to the apical membrane in fetal, suckling, and adult jejunum.

CONCLUSIONS

Mucosal EGF stimulates EGF-receptor phosphorylation in immature but not in mature intestine. These differences are not explained by EGF-receptor abundance or localization and suggest that the greater mucosal permeability of the immature gut may allow EGF to bind to the EGF-receptor in the basolateral membrane to mediate its effects in suckling rat intestine.

IgE and IgG cross-reactivity among Lol p I and Lol p II/III. Identification of the C-termini of Lol p I, II, and III as cross-reactive structures

In this study, the homologous C-termini of Lol p I, Lol p II, and Lol p III were shown to contain cross-reactive B-cell epitopes. This was demonstrated by inhibition studies with purified Lol p I, II, and III and synthetic peptides of their C-termini. It was ruled out that the observed cross-reactivity was caused by cross-contamination of the purified allergens. Both human IgE and IgG bound to the C-terminus of Lol p I. These antibodies were cross-reactive with Lol p II and, more specifically, with its C-terminus. Within a small panel of allergic patients, no cross-reactivity with Lol p III was found. A hyperimmune polyclonal rabbit antiserum against Lol p I also recognized the Lol p I C-terminus. As for human antibodies, cross-reactivity with Lol p II and its C-terminus was demonstrated. Cross-reactivity with Lol p III was demonstrated with C-terminal peptides, but not with native Lol p III. A polyclonal rabbit antiserum against Lol p II bound to the C-terminal peptides of both Lol p II and III. This binding was inhibited with Lol p I, confirming that cross-reactive structures exist not only on the C-termini of Lol p II and Lol p I, but also of Lol p III and Lol p I. The existence of cross-reactivity between Lol p I and Lol p II and III possibly contributes to the frequently observed cosensitization for these allergens in grass-pollen-allergic patients.

First high-performance liquid chromatography assay of formoterol concentrations in the low-picogram-per-milliliter range

A method for the assay of plasma concentrations of the long-acting beta 2-adrenoceptor agonist, formoterol is described. This method, in which high-performance liquid chromotography is used with electrochemical detection, enables, for the first time, pharmacokinetic and pharmacodynamic research with this drug. After column extraction of plasma samples, the eluent is injected on the chromatographic system. Retention times of formoterol and the internal standard, bromo-formoterol, were 5.6 and 10 min, respectively. Our results show that this is a sensitive and reproducible method with a very low limit of detection (20 pg/ml), which creates the possibility of measuring concentrations in a range achievable in humans.

IgE epitopes on the cat (Felis domesticus) major allergen Fel d I: a study with overlapping synthetic peptides

BACKGROUND

The major cat allergen Fel d I is composed of two disulfide-linked polypeptide chains, chain 1 (70 amino acid residues) and chain 2 (92 amino acid residues). Reduction and alkylation of Fel d I eliminates almost all antigenic and allergenic activity, and detection of linear epitopes with synthetic peptides is therefore not expected.

METHODS

We synthesized synthetic peptides of both chains of about 14 amino acid residues, overlapping by 7 residues. The peptides were coupled to Sepharose (Pharmacia, Uppsala, Sweden) and tested with sera of patients with cat allergy.

RESULTS

Three peptides showed specific binding of human IgE, residues 25-38 and 46-59 of chain 1 and residue 15-28 of chain 2. IgE binding was inhibited by Fel d I and the corresponding peptide. Of 61 patients with cat allergy tested, 65% showed IgE binding to at least one of the peptides; 46% showed IgE binding to peptide 25-38, 11% to peptide 46-59, and 28% to peptide 15-28. Each peptide was recognized by only one of the 78 patients with negative RAST results. By affinity chromatography with peptide-Sepharose anti-Fel d I antibodies were isolated, also confirming the specificity of IgE binding to the peptides. The percentage of IgE antibodies against Fel d I reactive with the peptides varied with the serum and the peptide-Sepharose used and ranged from 2% to 55%.

CONCLUSIONS

Because the affinity of IgE binding to the peptides was very low and only serum samples with high titers of Fel d I-specific IgE antibodies (RAST 4+/5+) showed significant binding, these peptides are not suitable for diagnostic purposes. However, the peptides are useful tools for comparing IgE and IgG responses and for studying the relationship to the T-cell epitopes on this molecule.

An efficient positive selection procedure for the isolation of peroxisomal import and peroxisome assembly mutants of Saccharomyces cerevisiae

To study peroxisome biogenesis, we developed a procedure to select for Saccharomyces cerevisiae mutants defective in peroxisomal protein import or peroxisome assembly. For this purpose, a chimeric gene was constructed encoding the bleomycin resistance protein linked to the peroxisomal protein luciferase. In wild-type cells this chimeric protein is imported into the peroxisome, which prevents the neutralizing interaction of the chimeric protein with its toxic phleomycin ligand. Peroxisomal import and peroxisome assembly mutants are unable to import this chimeric protein into their peroxisomes. This enables the bleomycin moiety of the chimeric protein to bind phleomycin, thereby preventing its toxicity. The selection is very efficient: upon mutagenesis, 84 (10%) of 800 phleomycin resistant colonies tested were unable to grow on oleic acid. This rate could be increased to 25% using more stringent selection conditions. The selection procedure is very specific; all oleic acid non utilizing (onu) mutants tested were disturbed in peroxisomal import and/or peroxisome assembly. The pas (peroxisome assembly) mutants that have been used for complementation analysis represent 12 complementation groups including three novel ones, designated pas20, pas21 and pas22.