Effect of Triton X-100 on the isoelectric focusing profile of fibroblast sphingomyelinase

Treatment of sphingomyelinase deficiency by repeated implantations of amniotic epithelial cells

Five young patients with Niemann-Pick disease type B were treated with repeated implantations of amniotic epithelial cells, as a source of exogenous sphingomyelinase. This treatment abolished the recurrent infections, mainly of the respiratory tract, and led to other improvements of the general conditions of the patients. In particular, we noticed a disappearance of vomiting, a recovery from muscular hypotrophy, and significantly reduced pulmonary distress. In four subjects, who were in a prepuberal state, there was a puberal spurt with a concomitant burst of growth. In two cases, characterized by a greater than normal content of sphingomyelin in urinary sediments, a single implantation caused a sustained normalization of sphingomyelin and total phospholipids in the urine. Finally, sphingomyelinase activity of peripheral leukocytes, when assayed 0.5 to 4 months after some of the implantations, showed a rise to heterozygous values in 30-40% of the assays.

Properties of acid ceramidase from human spleen

We have characterised ceramidase activity in extracts of human spleen from control subjects and from patients with Gaucher disease. In Triton X-100 extracts of control spleens, a broad pH optimum of pH 3.5-5.0 was found; no ceramidase activity was detectable at neutral or alkaline pH. About 45-60% of acid ceramidase could be extracted from spleen without detergents, but for complete extraction, Triton X-100 was required. For the radiolabelled substrate oleoylsphingosine, a Km of 0.22 +/- 0.09 mM and a Vmax of 57 +/- 11 nmol/h per mg protein was calculated in spleen from a control subject. Flat-bed isoelectric focussing in the presence of Triton X-100 revealed a pI of 6.0-7.0 for acid ceramidase; similar values were found for sphingomyelinase and glucerebrosidase. HPLC-gel filtration indicated that in the presence of Triton X-100, acid ceramidase has an Mr of about 100 kDa. In the absence of detergents, the enzyme forms high-molecular-weight aggregates. Similar aggregation behaviour was observed for sphingomyelinase, while the elution of beta-hexosaminidase was not affected by detergents. The elution profile of glucocerebrosidase was only slightly altered by Triton X-100. There was no difference in the properties of acid ceramidase present in spleen from control subjects and from patients with type I Gaucher disease.

Studies on the activation of the enzymatic hydrolysis of sphingomyelin liposomes

Two pH optima were observed for the hydrolysis of sphingomyelin liposomes by brain and fibroblast extracts; one at pH 4.2-4.5, the other at pH 7-8. The proportion of the acidic activity in fibroblasts was affected greatly by the culturing conditions. Both the acidic and neutral enzyme activities were deficient in Niemann-Pick Type A fibroblasts, suggesting that both were genetically related. Partially purified activators from normal as well as Gaucher disease spleen stimulated the hydrolysis of sphingomyelin, at both pH values, by fibroblast and brain extracts. After further purification by DE-52 and Sephacryl 200 column chromatography the Gaucher activator retained its ability to stimulate sphingomyelinase and was active as well towards beta-glucocerebrosidase and beta-galactocerebrosidase.

Sphingomyelinase in cultured skin fibroblasts from normal and Niemann-Pick type C patients

The sphingomyelinase activities of extracts of normal cultured skin fibroblasts were compared with those obtained from Niemann-Pick disease Type A and B patients, and from a number of patients with a provisional clinical diagnosis of Niemann-Pick disease Type C. Even though fibroblasts from Type A and B patients were shown to be clearly deficient (less than 5% residual activity) the activity in Type C fibroblasts varied from approximately 50% of the lowest control value to normal activity. Isoelectric focussing of extracts of normal cultured skin fibroblasts on polyacrylamide gels revealed the presence of sphingomyelinase heterogeneity. However, no characteristic change in the behaviour of the enzyme in corresponding extracts from Niemann-Pick Type C cells was observed, suggesting that, if the defect in this condition is expressed fibroblasts, it does not manifest biochemically in the appearance or disappearance of a specific sphingomyelinase isoenzyme. Our data suggest that the heterogeneity observed in fibroblast extracts may simply reflect an interaction of the enzyme either with itself or with other hydrophobic components present in the cellular extracts.

Studies on sphingomyelinase and beta-glucosidase activities in Niemann-Pick disease variants. Phosphodiesterase activities measured with natural and artificial substrates

Cultured fibroblasts were studied from 12 cases of Niemann-Pick disease group C. In 11, sphingomyelinase and glucocerebrosidase (and beta-glucosidase) activities were reduced to around 50% of those of controls. On isoelectric focusing, all 12 strains lacked sphingomyelinase activity in the major cathodic region (pI 8.0). The defect was also demonstrated with the artificial phosphodiester substrates bis(4-methylumbelliferyl) phosphate and 4-methylumbelliferyl pyrophosphate diester. In control fibroblasts and those heterozygous for types A or B or group C Niemann-Pick disease, the major sphingomyelinase peak electrofocused at pI 8.0. No direct interaction could be demonstrated by mixing experiments between group C Niemann-Pick extracts and those of type A disease or Gaucher disease. Profiles for beta-glucosidase activity appeared normal in Niemann-Pick group C fibroblasts. No reduction of sphingomyelinase or glucocerebrosidase activities was found in Niemann-Pick group C liver, nor any attenuation of cathodic sphingomyelinase activity in the affected tissue. Results suggest that sphingomyelinase expression differs in fibroblasts and liver. Enzyme defects associated with Niemann-Pick disease group C were only observed in cultured cells.

Chromatofocusing of skin fibroblast sphingomyelinase: alterations in Niemann-Pick disease type C shared by GM1-gangliosidosis

Sphingomyelinase activity of cultivated skin fibroblast extracts from normal individuals was resolved by chromatofocusing in the pH range 8-5 into three major components with pI's of 7.3, 6.3 and 5.9, respectively. Chromatofocusing proved a more efficient and reproducible separation technique than preparative flat-bed isoelectric focusing and it gave a constant profile even when detergent concentration varied. In skin fibroblasts from five patients with Niemann-Pick disease type C, a varying degree of reduction in the proportion of the 7.3 peak was observed. In a patient with clinical features of Niemann-Pick disease type C, the finding of such a profile would thus be a good argument for the diagnosis, but it is not pathognomonic as we found similar changes in two cases with GM1-gangliosidosis, while some cases of Niemann-Pick disease type C have borderline normal profiles. These results challenge the concept of a specific sphingomyelinase isoenzyme deficiency as the basic defect in Niemann-Pick disease type C.

Enzymatic hydrolysis of sphingomyelin liposomes by normal tissues and tissues from patients with Niemann-Pick disease

Liposomes of [3H]sphingomyelin are readily hydrolyzed by extracts of human spleen, liver, cultured skin fibroblasts and purified placental sphingomyelinase in the absence of detergents. The pH optimum for hydrolysis by liver and spleen extracts was 6.5-7.0 while the fibroblast activity showed an optimum at pH 4.0-4.3. However, the pH optimum for purified placental sphingomyelinase in the presence of Triton X-100 (pH 5.0) is only slightly different from that displayed with liposomes (pH 5.3). The data clearly show that hydrolysis of liposomal sphingomyelin by sphingomyelinase is affected by the composition and purity of the enzyme source.

The active site of lysosomal sphingomyelinase: evidence for the involvement of hydrophobic and ionic groups

The natural substrate for sphingomyelinase contains hydrophobic and polar moieties. In this study, we have employed pH rate studies and examined hydrophobic compounds and phosphorylated esters for their effect on sphingomyelinase activity in an attempt to determine some of the kinetic properties of this enzyme. Sphingomyelinase, purified from human placentae, undergoes noncompetitive inhibition by octylglucoside and Nonidet P-40, two nonionic detergents containing terminal octyl groups. The effect of these detergents at the hydrophobic binding site is somewhat different from that of Triton X-100, which contains an isooctyl terminal group, and this may serve to identify a structural basis for the effects. Sphingomyelinase activity is also modulated by several nucleotides. Inhibition by 5'-adenosine monophosphate (5'-AMP) is also noncompetitive. Other nucleotide monophosphates (such as 5'-uridine monophosphate (5'-UMP), 5'-cytidine monophosphate (5'-CMP), 2'-adenosine monophosphate (2'-AMP), and 3'-adenosine monophosphate (3'-AMP) and phosphorylated intermediates (such as phosphorylcholine, phosphorylethanolamine and hexose phosphates) have a lower inhibitory effect. The data suggest that the inhibition by 5'-AMP involves the combined effect of the phosphate group and the purine ring, structural requirements which may also be satisfied by bis(4-methylumbelliferyl)phosphate, a synthetic enzyme substrate. Studies of pH rate indicate that the maximal velocity for the hydrolysis of sphingomyelin is independent of pH over the range 3.5-6.2 while the Km value shows a pH dependence. The Km value is lowest from pH 4.0-5.2 and rises at pH values outside this range. The log Vmax/Km and pKm relationships, when plotted as a function of pH, have been used to identify the dissociation constants for the binding of sphingomyelin by the enzyme. These occur at pK values of 4.1 and 5.5. The activity of sphingomyelinase is also reduced when the enzyme is photooxidized in the presence of methylene blue or rose bengal and carbamylated by diethylpyrocarbonate (DEPC). These results are interpreted to show that 1). the enzyme contains a hydrophobic binding site which involves linear aliphatic moieties containing at least eight carbon atoms; 2) two ionic groups are involved in formation of the enzyme substrate complex, one of which is presumed to be the carboxylate group of aspartate or glutamate (represented by pK 4.1) and the second may be the protonated imidazolium group of histidine (represented by pK 5.5); and 3) since the maximal velocity shows no pH dependence, the interactions involving the hydrophobic and ionic groups affect only the binding of the substrate to the enzyme and formation of the enzyme-substrate complex.

Acid sphingomyelinase of human brain: purification to homogeneity

Acid sphingomyelinase (sphingomyelin phosphodiesterase, EC 3.1.4.12) was purified from human brain by extraction with 0.1% Triton X-100, followed by sequential chromatography on Concanavalin A-Sepharose, octyl-Sepharose, hydroxylapatite, DEAE-cellulose, red A-agarose, Sephadex G-200, and DEAE-cellulose with ampholyte elution. Sphingomyelinase activity was purified more than 20,000-fold from the starting homogenate with a 1% yield. Specific activity of up to 800 mumol/h/mg protein could be achieved. Gel electrophoresis with 6% polyacrylamide containing sodium dodecyl sulfate gave a single protein band with a molecular weight of 70,000, in good agreement with the molecular weight previously estimated from sucrose density gradient centrifugation in 0.1% Triton X-100. Triton X-100 could be readily removed from the enzyme by sucrose density gradient centrifugation. The Triton-free enzyme showed the same Km and pH optimum. Heat stability of the enzyme was reversibly affected by Triton X-100, in that removal of the detergent made the enzyme more heat labile. The Km of purified enzyme for sphingomyelin was 36 microM. It was unaffected by sulfhydryl reagents, but was inhibited by dithiothreitol at high concentrations. The preparation was free of all lysosomal hydrolase activities tested, including galactosylceramidase and alpha-mannosidase, which tended to copurify in our previous procedure. The enzyme was inactive toward sphingosylphosphorylcholine. It was active with bis[p-nitrophenyl]- and bis[4-methylumbelliferyl]phosphate and the chromogenic and fluorogenic sphingomyelin analogues.

Effect of sample preparation on analysis of superoxide dismutase activity and isoenzymes

The effect of superoxide dismutase (SOD) activity and isoenzyme pattern of detergents, incubation time, and sonication in the preparation of rat liver samples was investigated. The activity of the manganese form of the enzyme (Mn-SOD) was found to decrease significantly after 4 hr of incubation at room temperature, and activity of the copper, zinc form of the enzyme (Cu, Zn-SOD) was not changed significantly even after 24 hr, although levels were somewhat decreased. Sonication of the sample did not affect Cu, Zn-SOD activity, but total Mn-SOD activity was increased. Addition of detergents did not increase Mn-SOD activity when homogenates were sonicated, indicating that Mn-SOD is not membrane bound. Detergents also had no effect on Cu, Zn-SOD activity. None of the treatments investigated altered the isoenzyme patterns, providing evidence that these isoenzymes are not degradation products.

Isoelectric forms of bacteriorhodopsin from Halobacterium halobium

Isoelectric focusing was performed on bacteiorhodopsin isolated from Halobacterium halobium, R1 strain, solubilized in Nonidet P-40, on sucrose density gradient stabilized columns. When 560 nm absorbance was monitored, four forms of bacteriorhodopsin were observed, having isoelectric points (pI) of: A. 3.93; B, 4.43; C, 5.03; D, 5.49. Instability of some of the isoelectric forms during the very process of electrofocusing was observed. When focused over a period of 6 days, the relative abundance of the forms changed although their pI values remains constant. Refocusing of the isolated forms A or B led to the production f form C. The latter species was stable to refocusing. Form B was unstable to storage either as an aqueous suspension of the purple membrane or as a detergent extract. Each of the forms had the absorption spectrum typical for bacteriorhodopsin and each showed identical patterns after sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Purification of sphingomyelinase to apparent homogeneity by using hydrophobic chromatography

Placental sphingomyelinase has been purified to apparent homogeneity by a procedure that makes extensive use of hydrophobic interaction chromatography on sphingosylphosphocholine-CH-, octyl-, hexyl- and Blue-Sepharoses. Enzyme purification is about 10000- 14000-fold over starting extract with excellent yield (usually greater than 28%). Purification of bis-4-methylumbelliferyl phosphate phosphodiesterase activity generally paralleled that of sphingomyelinase during the final stages of the procedure. The enzyme also hydrolysed bis-p-nitrophenyl phosphate, but at a lower rate compared with bis-4-methylumbelliferyl phosphate. A single major protein was observed under non-denaturing conditions. Sphingomyelinase, denatured by reduction and alkylation, is composed of a major polypeptide chain with an apparent molecular weight of 89 100 on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Two minor lower-molecular-weight components were consistently obtained at 47 500 and 30 700. These results were also obtained after maleoylation of the reduced and alkylated sample. The enzyme contains a blocked-N-terminal amino acid. An extensive search for contaminating enzymes revealed the presence of minor amounts of acid phosphatase, which were removed from the final enzyme sample. The highly purified enzyme is stable for several weeks when stored with Triton X-100 at 4 degrees C. The pure enzyme aggregates under denaturing and electrophoretic conditions and special care must be taken to ensure that hydrophobic bonding of the protein is decreased as much as possible. The reproducibility and large scale of this procedure should facilitate further study on the structure and kinetic properties of the enzyme.

Studies on the hydrophobic properties of sphingomyelinase

Crude liver lysosomal sphingomyelinase (EC 3.1.4.12) displays a heterogeneous electrofocusing profile. The majority of the enzyme resolves into two major components with acidic pI values near pH 4.6 and 4.8. Several additional minor peaks of activity are seen at more basic pH values (up to pH 8.0). In the presence of 0.1% Triton X-100 (or Cutscum), the location of sphingomyelinase is shifted by about 1 pH unit to more basic pH values. Triton X-100 also increases the apparent heterogeneity of sphingomyelinase. Removal of detergent by treatment with Bio Beads SM-2 restores the acidic pI profile. This behaviour appears to be specific, since it was not shared by six glycosidases several of which hydrolyse sphingolipids. The electrofocusing profile of 3H-labelled Triton X-100 was distinct and separate from sphingomyelinase, suggesting that only a small fraction of detergent interacted directly with the enzyme. To study this behaviour in more detail we examined the effect of detergents on elution of sphingomyelinase from sphingosylphosphocholine-Sepharose. Sphingosylphosphocholine is a competitive inhibitor of sphingomyelinase (Ki 0.5 mM). Binding of enzyme was pH-dependent. Triton X-100, Cutscum and Tween 20 eluted significant amounts of enzyme at 0.01-0.02%. Total elution was achieved with up to 0.1% detergent. These data suggest that sphingomyelinase binds to neutral detergent monomers with a high degree of affinity. In excess detergent (5-7 times the critical micellar concentration) the surface charge on the protein is changed, leading to a pI shift. This behaviour probably does not occur at the active site of the enzyme, since there is no appreciable effect on substrate hydrolysis and substrate analogues were ineffective in eluting the enzyme.

Artifacts in isoelectric focusing of the microsomal enzymes cytochrome P-450 and NADPH-cytochrome P-450 reductase

Highly-purified rat liver microsomal cytochrome P-450 and NADPH-cytochrome P-450 reductase (NADPH-ferricytochrome oxidoreductase, EC 1.6.2.4) preparations gave rise to a large number of bands under a variety of isoelectric focusing conditions, as observed after staining for either zymogen or protein. The binding patterns were not independent of sample concentration and position of application, and eluted bands did not refocus as expected. The artifactual heterogeneity is attributed to strong protein-protein interactions and perhaps to complexation of proteins with carrier ampholytes. These findings suggest caution in using isoelectric focusing to resolve mixtures of membrane proteins.

Diagnosis of Niemann-Pick disease using a simple and sensitive fluorimetric assay of sphingomyelinase activity

Phosphodiesterase activity of cultured cells was determined with bis-(4-methylumbelliferyl) phosphate as substrate. In the presence of Triton X-100 an acid component was evident and results indicated that this enzyme was identical with sphingomyelinase. Acid phosphodiesterase activity was specifically inhibited by sphingomyelin. In fibroblasts from patients with Niemann-Pick diseases types A, B and C, acid phosphodiesterase activity was deficient whereas neutral activity was normal. Neutral activity could, however, be removed by acid precipitation or by binding to DEAE-cellulose. Hence a simple and sensitive fluorimetric method is described for the assay of sphingomyelinase activity in the diagnosis of Niemann-Pick disease.

Expression of lactate dehydrogenase isozyme 5 (LDH-5) in cultured mouse blastocysts in the absence of implantation and outgrowth

Extensive extraction studies with Triton X-100 revealed only LDH-1 (B4) but no trace of LDH-5 (A4) in one-cell and two-cell mouse and rat embryos. The LDH isozyme pattern of preimplantation mouse embryos changes from the maternally inherited B subunit isozyme (LDH-1) to a pattern dominated by LDH-5 when mouse blastocysts are cultured under conditions that prevent hatching but allow trophoblast giant cell transformation. During differentiation of mouse blastocysts in vitro, implantation is therefore not essential for the appearance of the A subunit form of LDH (LDH-5) coded for by the embryonic genome. Mechanisms controlling the expression of LDH-5 in mouse blastocysts during in vivo development are discussed.