Long-term safety and efficacy of enzyme replacement therapy for Fabry disease

Elsewhere, we reported the safety and efficacy results of a multicenter phase 3 trial of recombinant human alpha -galactosidase A (rh-alpha GalA) replacement in patients with Fabry disease. All 58 patients who were enrolled in the 20-wk phase 3 double-blind, randomized, and placebo-controlled study received subsequently 1 mg/kg of rh-alpha GalA (agalsidase beta, Fabrazyme, Genzyme Corporation) biweekly in an ongoing open-label extension study. Evidence of long-term efficacy, even in patients who developed IgG antibodies against rh- alpha GalA, included the continuously normal mean plasma globotriaosylceramide (GL-3) levels during 30 mo of the extension study and the sustained capillary endothelial GL-3 clearance in 98% (39/40) of patients who had a skin biopsy taken after treatment for 30 mo (original placebo group) or 36 mo (original enzyme-treated group). The mean serum creatinine level and estimated glomerular filtration rate also remained stable after 30-36 mo of treatment. Infusion-associated reactions decreased over time, as did anti-rh- alpha GalA IgG antibody titers. Among seroconverted patients, after 30-36 mo of treatment, seven patients tolerized (no detectable IgG antibody), and 59% had > or =4-fold reductions in antibody titers. As of 30 mo into the extension trial, three patients were withdrawn from the study because of positive serum IgE or skin tests; however, all have been rechallenged successfully at the time of this report. Thus, enzyme replacement therapy for 30-36 mo with agalsidase beta resulted in continuously decreased plasma GL-3 levels, sustained endothelial GL-3 clearance, stable kidney function, and a favorable safety profile.     

Lysosomal storage diseases: mechanisms of enzyme replacement therapy

Summary Lysosomal diseases result from deficiency of one of the many enzymes involved in the normal, step-wise breakdown of macromolecules. Studies in vitro have shown that cells from enzyme-deficient patients can be corrected by an exogenous supply of the missing enzyme. This occurs by receptor-mediated endocytosis of normal enzyme added to tissue culture medium and also by direct transfer from normal leukocytes during cell-to-cell contact. Immunohistochemical analysis has revealed that these processes have similar pathways of intracellular transport of the acquired enzymes, which ultimately reach mature lysosomes in the recipient cells. Moreover, recent studies suggest that both mechanisms are important in the therapy of lysosomal storage diseases by bone marrow transplantation. Advances in gene technology are likely to improve the successful treatment of these disorders, by facilitating the large scale production of clinically effective proteins and also by enabling the stable and safe introduction of normal lysosomal genes into cells of affected patients.     

Reduction of elevated plasma globotriaosylsphingosine in patients with classic Fabry disease following enzyme replacement therapy

Fabry disease is treated by two-weekly infusions with α-galactosidase A, which is deficient in this X-linked globotriaosylceramide (Gb3) storage disorder. Elevated plasma globotriaosylsphingosine (lysoGb3) is a hallmark of classical Fabry disease. We investigated effects of enzyme replacement therapy (ERT) on plasma levels of lysoGb3 and Gb3 in patients with classical Fabry disease treated with agalsidase alfa at 0.2 mg/kg, agalsidase beta at 0.2 mg/kg or at 1.0 mg/kg bodyweight. Each treatment regimen led to prominent reductions of plasma lysoGb3 in Fabry males within 3 months (P = 0.0313), followed by relative stability later on. Many males developed antibodies against α-galactosidase A, particularly those treated with agalsidase beta. Patients with antibodies tended towards smaller correction in plasma lysoGb3 concentration, whereas treatment with high dose agalsidase beta allowed a reduction comparable to patients without antibodies. Pre-treatment plasma lysoGb3 concentrations of Fabry females were relatively low. In all females and with each treatment regimen, ERT gave reduction or stabilisation of plasma lysoGb3. Our investigation revealed that ERT of Fabry patients reduces plasma lysoGb3, regardless of the recombinant enzyme used. This finding shows that ERT can correct a characteristic biochemical abnormality in Fabry patients.     

Cardiomyopathy and response to enzyme replacement therapy in a male mouse model for Fabry disease

Fabry disease is an X-linked disorder of glycosphingolipid metabolism that results in progressive accumulation of neutral glycosphingolipids, (predominately globotriaosylceramide; GL-3) in lysosomes, as well as other cellular compartments and the extracellular space. Our aim was to characterize the cardiac phenotype of male knock-out mice that are deficient in alpha-galactosidase A activity, as a model for Fabry disease and test the efficacy of Enzyme Replacement Therapy with agalsidase-beta. Male mice (3-4 months of age) were characterized with awake blood pressure and heart rate measurements, cardiac echocardiography and electrocardiography measurements under light anesthesia, histological studies and molecular studies with real-time polymerase chain reaction. The Fabry knock-out mouse has bradycardia and lower blood pressure than control wild type (CB7BL/6J) mice. In Fabry knock-out mice, the cardiomyopathy associated mild hypertrophy at echography with normal systolic LV function and mild diastolic dysfunction. Premature atrial contractions were more frequent in without conduction defect. Heart weight normalized to tibial length was increased in Fabry knock-out mice. Ascending aorta dilatation was observed. Molecular studies were consistent with early stages of cardiac remodeling. A single dose of agalsidase-beta (3 mg/kg) did not affect the LV hypertrophy, function or heart rate, but did improve the mRNA signals of early cardiac remodeling. In conclusion, the alpha-galactosidase A deficient mice at 3 to 4 months of age have cardiac and vascular alterations similar to that described in early clinical stage of Fabry disease in children and adolescents. Enzyme replacement therapy affects cardiac molecular remodeling after a single dose.     

Production in yeast of alpha-galactosidase A,a lysosomal enzyme applicable to enzyme replacement therapy for Fabry disease

A mammalian-like sugar moiety was created in glycoprotein by Saccharomyces cerevisiae in combination with bacterial alpha-mannosidase to produce a more economic enzyme replacement therapy for patients with Fabry disease. We introduced the human alpha-galactosidase A (alpha-GalA) gene into an S. cerevisiae mutant that was deficient in the outer chains of N-linked mannan. The recombinant alpha-GalA contained both neutral (Man(8)GlcNAc(2)) and acidic ([Man-P](1-2)Man(8)GlcNAc(2)) sugar chains. Because an efficient incorporation of alpha-GalA into lysosomes of human cells requires mannose-6-phosphate (Man-6-P) residues that should be recognized by the specific receptor, we trimmed down the sugar chains of the alpha-GalA by a newly isolated bacterial alpha-mannosidase. Treatment of the alpha-GalA with the alpha-mannosidase resulted in the exposure of a Man-6-P residue on a nonreduced end of oligosaccharide chains after the removal of phosphodiester-linked nonreduced-end mannose. The treated alpha-GalA was efficiently incorporated into fibroblasts derived from patients with Fabry disease. The uptake was three to four times higher than that of the nontreated alpha-GalA and was inhibited by the addition of 5 mM Man-6-P. Incorporated alpha-GalA was targeted to the lysosome, and hydrolyzed ceramide trihexoside accumulated in the Fabry fibroblasts after 5 days. This method provides an effective and economic therapy for many lysosomal disorders, including Fabry disease.     

Reduced glucosylceramide in the mouse model of Fabry disease: Correction by successful enzyme replacement therapy

Fabry disease is an X-linked lysosomal storage disease (LSD) caused by deficient activity of α-Galactosidase A (α-Gal A). As a result, glycosphingolipids, mainly globotriaosylceramide (Gb3), progressively accumulate in body fluids and tissues. Studies aiming at the identification of secondary lipid alterations in Fabry disease may be potentially useful for the monitorization of the response to enzyme replacement therapy (ERT) and development of future therapies. The focus of this study was to evaluate if α-Gal A deficiency has an effect on two key groups of molecules of sphingolipids metabolism: glucosylceramides (GlucCers) and ceramides (Cers). Studies performed in a mouse model of Fabry disease showed reduced level of GlucCer and normal level of Cer in plasma, liver, spleen, kidney and heart. Moreover, analysis of GlucCer isoforms in Fabry knockout mice showed that GlucCer isoforms are unequally reduced in different tissues of these animals. ERT had a specific effect on the liver's GlucCer levels of Fabry knockout mice, increasing hepatic GlucCer to the levels observed in wild type mice. In contrast to Fabry knockout mice, plasma of Fabry patients had normal GlucCer and Cer but an increased GlucCer/Cer ratio. This alteration showed a positive correlation with plasma globotriaosylsphingosine (lyso-Gb3) concentration. In conclusion, this work reveals novel secondary lipid imbalances caused by α-Gal A deficiency.     

Tissue and plasma globotriaosylsphingosine could be a biomarker for assessing enzyme replacement therapy for Fabry disease

Fabry disease is a genetic disease caused by a deficiency of α-galactosidase A (GLA), which leads to systemic accumulation of glycolipids, predominantly globotriaosylceramide (Gb3). With the introduction and spread of enzyme replacement therapy (ERT) with recombinant GLAs for this disease, a useful biomarker for assessing the response to ERT is strongly required. We measured the tissue level of lyso-globotriaosylsphingosine (lyso-Gb3) in Fabry mice by means of high performance liquid chromatography, and compared it with the Gb3 level. The results revealed a marked increase in the lyso-Gb3 level in most tissues of Fabry mice, and which decreased after the administration of a recombinant GLA as in the case of Gb3, which is usually used as a biomarker of Fabry disease. The response was more impressive for lyso-Gb3 compared with for Gb3, especially in kidney tissues, in which a defect significantly influences the morbidity and mortality in patients with this disease. The plasma level of lyso-Gb3 also decreased after the injection of the enzyme, and it was well related to the degradation of tissue lyso-Gb3. Thus, lyso-Gb3 is expected to be a useful new biomarker for assessing the response to ERT for Fabry disease.     

Lysosomal enzyme activities among Chinese: leukocyte alpha-galactosidase and beta-galactosidase

alpha-Galactosidase and beta-galactosidase activities have been determined in leukocyte preparations from 100 randomly selected Chinese adults. For alpha-galactosidase, two groups with low activities were identified: group I consisted of 3 females having activities below 40% of normal, and group II consisted of 5 males and 1 female with activities about 60% of normal. Family studies suggested that these low alpha-galactosidase activities are genetically determined. Only 1 individual was found to have about 50% of normal beta-galactosidase activity; presumably he is a carrier for beta-galactosidase deficiency (GM1 gangliosidosis).     

Globotriaosylceramide is correlated with oxidative stress and inflammation in Fabry patients treated with enzyme replacement therapy

Fabry disease is an X-linked inborn error of glycosphingolipid catabolism due to deficient activity of α-galactosidase A that leads to accumulation of the enzyme substrates, mainly globotriaosylceramide (Gb3), in body fluids and lysosomes of many cell types. Some pathophysiology hypotheses are intimately linked to reactive species production and inflammation, but until this moment there is no in vivo study about it. Hence, the aim of this study was to investigate oxidative stress parameters, pro-inflammatory cytokines and Gb3 levels in Fabry patients under treatment with enzyme replacement therapy (ERT) and finally to establish a possible relation between them. We analyzed urine and blood samples of patients under ERT (n = 14) and healthy age-matched controls (n = 14). Patients presented decreased levels of antioxidant defenses, assessed by reduced glutathione (GSH), glutathione peroxidase (GPx) activity and increased superoxide dismutase/catalase (SOD/CAT) ratio in erythrocytes. Concerning to the damage to biomolecules (lipids and proteins), we found that plasma levels of malondialdehyde (MDA) and protein carbonyl groups and di-tyrosine (di-Tyr) in urine were increased in patients. The pro-inflammatory cytokines IL-6 and TNF-α were also increased in patients. Urinary Gb3 levels were positively correlated with the plasma levels of IL-6, carbonyl groups and MDA. IL-6 levels were directly correlated with di-Tyr and inversely correlated with GPx activity. This data suggest that pro-inflammatory and pro-oxidant states occur, are correlated and seem to be induced by Gb3 in Fabry patients.     

Globotriaosylceramide is correlated with oxidative stress and inflammation in Fabry patients treated with enzyme replacement therapy

Fabry disease is an X-linked inborn error of glycosphingolipid catabolism due to deficient activity of α-galactosidase A that leads to accumulation of the enzyme substrates, mainly globotriaosylceramide (Gb3), in body fluids and lysosomes of many cell types. Some pathophysiology hypotheses are intimately linked to reactive species production and inflammation, but until this moment there is no in vivo study about it. Hence, the aim of this study was to investigate oxidative stress parameters, pro-inflammatory cytokines and Gb3 levels in Fabry patients under treatment with enzyme replacement therapy (ERT) and finally to establish a possible relation between them. We analyzed urine and blood samples of patients under ERT (n=14) and healthy age-matched controls (n=14). Patients presented decreased levels of antioxidant defenses, assessed by reduced glutathione (GSH), glutathione peroxidase (GPx) activity and increased superoxide dismutase/catalase (SOD/CAT) ratio in erythrocytes. Concerning to the damage to biomolecules (lipids and proteins), we found that plasma levels of malondialdehyde (MDA) and protein carbonyl groups and di-tyrosine (di-Tyr) in urine were increased in patients. The pro-inflammatory cytokines IL-6 and TNF-α were also increased in patients. Urinary Gb3 levels were positively correlated with the plasma levels of IL-6, carbonyl groups and MDA. IL-6 levels were directly correlated with di-Tyr and inversely correlated with GPx activity. This data suggest that pro-inflammatory and pro-oxidant states occur, are correlated and seem to be induced by Gb3 in Fabry patients.     

Decrease of plasma taurine in Gaucher disease and its sustained correction during enzyme replacement therapy

Summary. Gaucher disease is caused by an autosomal-recessive deficiency of glucocerebrosidase. Cells of monocytic/macrophagic origin accumulate glucosylceramide. This leads to hepatosplenomegaly, bone destruction, thrombocytopenia and anemia. Enzyme replacement therapy (ERT) with macrophage-targeted glucocerebrosidase leads to normalization of these parameters. The way of macrophage activation in Gaucher disease is not known. Recently, the osmolytes taurine, betaine and inositol were identified as important regulators of macrophage function in liver. Therefore, the role of plasma taurine in Gaucher disease as a primarily macrophage-derived disease was studied. Fasting plasma levels were measured from blood samples of healthy control subjects (n = 29, m : f = 11 : 18, mean age 37 ± 3 years), from un-treated Gaucher patients (n = 16, m : f = 7 : 9, mean age 44 ± 4 years) and those treated for 37 ± 2 months (n = 54, m : f = 19 : 35, mean age 47 ± 2 years). Amino acid analysis was carried out in a BioChrom amino acid analyzer. In the untreated patients, plasma taurine was 45 ± 3 μM, as compared to the controls with a plasma taurine of 63 ± 4 μM (p < 0.01). The aver-age increase of plasma taurine during the first year of ERT was 18 ± 8 μM (n = 10). Patients treated for an average of 37 months (range 1–9 years of ERT) had a plasma taurine of 65 ± 4 μM (n = 54), which was not different from the controls. It is concluded that Gaucher patients show decreased plasma taurine levels and that therapy of Gaucher disease might correct this. It has to be established, whether decreased taurine availability is a cofactor of the permanent activation of glucosylceramide-storing monocytes/macrophages in this disease. Received January 25, 2000/Accepted January 31, 2000     

Active-site-specific chaperone therapy for Fabry disease. Yin and Yang of enzyme inhibitors

Protein misfolding is recognized as an important pathophysiological cause of protein deficiency in many genetic disorders. Inherited mutations can disrupt native protein folding, thereby producing proteins with misfolded conformations. These misfolded proteins are consequently retained and degraded by endoplasmic reticulum-associated degradation, although they would otherwise be catalytically fully or partially active. Active-site directed competitive inhibitors are often effective active-site-specific chaperones when they are used at subinhibitory concentrations. Active-site-specific chaperones act as a folding template in the endoplasmic reticulum to facilitate folding of mutant proteins, thereby accelerating their smooth escape from the endoplasmic reticulum-associated degradation to maintain a higher level of residual enzyme activity. In Fabry disease, degradation of mutant lysosomal alpha-galactosidase A caused by a large set of missense mutations was demonstrated to occur within the endoplasmic reticulum-associated degradation as a result of the misfolding of mutant proteins. 1-Deoxygalactonojirimycin is one of the most potent inhibitors of alpha-galactosidase A. It has also been shown to be the most effective active-site-specific chaperone at increasing residual enzyme activity in cultured fibroblasts and lymphoblasts established from Fabry patients with a variety of missense mutations. Oral administration of 1-deoxygalactonojirimycin to transgenic mice expressing human R301Q alpha-galactosidase A yielded higher alpha-galactosidase A activity in major tissues. These results indicate that 1-deoxygalactonojirimycin could be of therapeutic benefit to Fabry patients with a variety of missense mutations, and that the active-site-specific chaperone approach using functional small molecules may be broadly applicable to other lysosomal storage disorders and other protein deficiencies.     

Enzyme replacement reverses abnormal cerebrovascular responses in Fabry disease

Background  

Fabry disease is a lysosomal X-linked enzyme deficiency of α-galactosidase A associated with an increased mortality and morbidity due to renal failure, cardiac disease and early onset stroke.     

A biochemical and pharmacological comparison of enzyme replacement therapies for the glycolipid storage disorder Fabry disease

Fabry disease is a lysosomal storage disease arising from deficiency of the enzyme alpha-galactosidase A. Two recombinant protein therapeutics, Fabrazyme (agalsidase beta) and Replagal (agalsidase alfa), have been approved in Europe as enzyme replacement therapies for Fabry disease. Both contain the same human enzyme, alpha-galactosidase A, but they are produced using different protein expression systems and have been approved for administration at different doses. To determine if there is recognizable biochemical basis for the different doses, we performed a comparison of the two drugs, focusing on factors that are likely to influence biological activity and availability. The two drugs have similar glycosylation, both in the type and location of the oligosaccharide structures present. Differences in glycosylation were mainly limited to the levels of sialic acid and mannose-6-phosphate present, with Fabrazyme having a higher percentage of fully sialylated oligosaccharides and a higher level of phosphorylation. The higher levels of phosphorylated oligomannose residues correlated with increased binding to mannose-6-phosphate receptors and uptake into Fabry fibroblasts in vitro. Biodistribution studies in a mouse model of Fabry disease showed similar organ uptake. Likewise, antigenicity studies using antisera from Fabry patients demonstrated that both drugs were indistinguishable in terms of antibody cross-reactivity. Based on these studies and present knowledge regarding the influence of glycosylation on protein biodistribution and cellular uptake, the two protein preparations appear to be functionally indistinguishable. Therefore, the data from these studies provide no rationale for the use of these proteins at different therapeutic doses.     

Substrate reduction augments the efficacy of enzyme therapy in a mouse model of Fabry disease

Fabry disease is an X-linked glycosphingolipid storage disorder caused by a deficiency in the activity of the lysosomal hydrolase α-galactosidase A (α-gal). This deficiency results in accumulation of the glycosphingolipid globotriaosylceramide (GL-3) in lysosomes. Endothelial cell storage of GL-3 frequently leads to kidney dysfunction, cardiac and cerebrovascular disease. The current treatment for Fabry disease is through infusions of recombinant α-gal (enzyme-replacement therapy; ERT). Although ERT can markedly reduce the lysosomal burden of GL-3 in endothelial cells, variability is seen in the clearance from several other cell types. This suggests that alternative and adjuvant therapies may be desirable. Use of glucosylceramide synthase inhibitors to abate the biosynthesis of glycosphingolipids (substrate reduction therapy, SRT) has been shown to be effective at reducing substrate levels in the related glycosphingolipidosis, Gaucher disease. Here, we show that such an inhibitor (eliglustat tartrate, Genz-112638) was effective at lowering GL-3 accumulation in a mouse model of Fabry disease. Relative efficacy of SRT and ERT at reducing GL-3 levels in Fabry mouse tissues differed with SRT being more effective in the kidney, and ERT more efficacious in the heart and liver. Combination therapy with ERT and SRT provided the most complete clearance of GL-3 from all the tissues. Furthermore, treatment normalized urine volume and uromodulin levels and significantly delayed the loss of a nociceptive response. The differential efficacies of SRT and ERT in the different tissues indicate that the combination approach is both additive and complementary suggesting the possibility of an improved therapeutic paradigm in the management of Fabry disease.     

A distinct urinary biomarker pattern characteristic of female Fabry patients that mirrors response to enzyme replacement therapy

Female patients affected by Fabry disease, an X-linked lysosomal storage disorder, exhibit a wide spectrum of symptoms, which renders diagnosis, and treatment decisions challenging. No diagnostic test, other than sequencing of the alpha-galactosidase A gene, is available and no biomarker has been proven useful to screen for the disease, predict disease course and monitor response to enzyme replacement therapy. Here, we used urine proteomic analysis based on capillary electrophoresis coupled to mass spectrometry and identified a biomarker profile in adult female Fabry patients. Urine samples were taken from 35 treatment-naïve female Fabry patients and were compared to 89 age-matched healthy controls. We found a diagnostic biomarker pattern that exhibited 88.2% sensitivity and 97.8% specificity when tested in an independent validation cohort consisting of 17 treatment-naïve Fabry patients and 45 controls. The model remained highly specific when applied to additional control patients with a variety of other renal, metabolic and cardiovascular diseases. Several of the 64 identified diagnostic biomarkers showed correlations with measures of disease severity. Notably, most biomarkers responded to enzyme replacement therapy, and 8 of 11 treated patients scored negative for Fabry disease in the diagnostic model. In conclusion, we defined a urinary biomarker model that seems to be of diagnostic use for Fabry disease in female patients and may be used to monitor response to enzyme replacement therapy.     

Lysosomal enzyme leakage during the hypoxanthine/xanthine oxidase reaction

Impairment of lysosomal stability due to reactive oxygen species generated during the oxidation of hypoxanthine by xanthine oxidase was studied in rat liver lysosomes isolated in a discontinuous Nycodenz gradient. Production of O2.- and H2O2 during the hypoxanthine/xanthine oxidase reaction occurred for at least 5 min, while lysosomal damage, indicated by the release of N-acetyl-beta-glucosaminidase, occurred within 30 s, there being no further damage to these organelles thereafter. The extent of lysosomal enzyme release increased with increasing xanthine oxidase concentration. Superoxide dismutase and catalase did not prevent lysosomal damage during the hypoxanthine/xanthine oxidase reaction. Lysosomes reduced xanthine oxidase activity, as assessed in terms of O2 consumption, only slightly but substantially inhibited in a competitive manner the O2.- -mediated reduction of cytochrome c. This inhibition was almost completely reversed by potassium cyanide, thus pointing to the presence of a cyanide-sensitive superoxide dismutase in the lysosomal fraction. However, potassium cyanide did not affect the hypoxanthine/xanthine oxidase-mediated lysosomal damage, thus suggesting an inability of the lysosomal superoxide dismutase to protect the organelles. Negligible malondialdehyde formation was observed in the lysosomes either during the hypoxanthine/xanthine oxidase reaction or with different selective experimental approaches known to produce lipid peroxidation in other organelles such as microsomes and mitochondria.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lysosomal enzyme secretion by macrophages during intracellular storage of particles

Cultured mouse peritoneal macrophages containing previously endocytosed zymosan or small-fibre asbestos (but not latex or sucrose) were shown to release selectively into the medium the lysosomal hydrolase β-N-acetylglucosaminidase. Thus macrophage lysosomal enzyme secretion was experimentally dissociated from endocytosis (as the residual external particles were washed away from the cells). The cells remained viable, and total activities of both $\text{N-}\text{acetyl}\text{-β-}\text{D}\text{-}\text{glucosaminidase}$ and of lactate dehydrogenase (a cytosol enzyme) rose with time. The relevance of such secretion by macrophages containing stored materials to chronic inflammatory processes is discussed.     

Lysosomal enzyme leakage during the hypoxanthine/xanthine oxidase reaction

Impairment of lysosomal stability due to reactive oxygen species generated during the oxidation of hypoxanthine by xanthine oxidase was studied in rat liver lysosomes isolated in a discontinuous Nycodenz gradient. Production of O ₂ ^⋅ and H₂O₂ during the hypoxanthine/xanthine oxidase reaction occurred for at least 5 min, while lysosomal damage, indicated by the release of N-acetyl-β-glucosaminidase, occurred within 30 s, there being no further damage to these organelles thereafter. The extent of lysosomal enzyme release increased with increasing xanthine oxidase concentration. Superoxide dismutase and catalase did not prevent lysosomal damage during the hypoxanthine/xanthine oxidase reaction. Lysosomes reduced xanthine oxidase activity, as assessed in terms of O₂ consumption, only slightly but substantially inhibited in a competitive manner the O ₂ ^⋅ -mediated reduction of cytochrome c. This inhibition was almost completely reversed by potassium cyanide, thus pointing to the presence of a cyanide-sensitive Superoxide dismutase in the lysosomal fraction. However, potassium cyanide did not affect the hypoxanthine/xanthine oxidase-mediated lysosomal damage, thus suggesting an inability of the lysosomal superoxide dismutase to protect the organelles. Negligible malondialdehyde formation was observed in the lysosomes either during the hypoxanthine/xanthine oxidase reaction or with different selective experimental approaches known to produce lipid peroxidation in other organelles such as microsomes and mitochondria. These results are interpreted in terms of a possible lysosomal membrane permeability to O ₂ ^⋅ causing organelle impairment by a process that, though leading to enzyme-marker leakage, does not involve lipid peroxidation.