Sensitive assays for the determination of the ganglioside sialidase activity of fibroblast homogenates were established using ganglioside GM3, 3H-labelled in the sphingosine moiety, as a substrate. Ganglioside GM3 sialidase activity was greatly stimulated by the presence of the non-ionic detergent Triton X-100 and was further enhanced by salts such as NaCl; the optimal pH was 4.5. The subcellular localization of this activity was determined by fractionation using free-flow electrophoresis and found to be exclusively associated with the marker for the plasma membrane, but not with that for lysosomes. This Triton-stimulated ganglioside sialidase activity was selectively inhibited by preincubating intact cells in the presence of millimolar concentrations of Cu2+, suggesting that the activity resides on the external surface of the plasma membrane. In normal fibroblasts homogenates, ganglioside GM3 sialidase was also greatly stimulated by sodium cholate. In contrast to the Triton X-100-activated reaction, however, it was not diminished by prior incubation of intact cells in the presence of Cu2+. Only after cell lysis was Cu2+ inhibitory. the cholate-stimulated ganglioside sialidase activity thus paralleled the behaviour of the lysosomal 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (4-MU-NeuAc) sialidase. In fibroblasts from sialidosis patients, the cholate-stimulated ganglioside GM3 sialidase activity, but not that of the Triton-activated enzyme, was profoundly diminished. In fibroblasts from patients with mucolipidosis IV (ML IV), both the Triton X-100- and the cholate-stimulated ganglioside GM3 sialidase activities were in the range of normal controls. The Triton-activated enzyme was associated with the plasma membrane in the same manner as in normal cells. Our findings suggest that, in human fibroblasts, there exist two sialidases that degrade ganglioside GM3: one on the external surface of the plasma membrane, and another that is localized in lysosomes and seems identical with the activity that acts on sialyloligosaccharides and 4-MU-NeuAc. As neither activity was found to be deficient in ML IV fibroblasts, our results argue against the hypothesis of a primary involvement of a ganglioside GM3 sialidase in the pathogenesis of ML IV.

Nuclear RNP complexes, cytoplasmic mRNP particles and free and membrane-bound polysomes were prepared from rat liver and their low-molecular-mass RNA components were analyzed on polyacrylamide/formamide gels. The separated small RNAs transferred to diazophenylthioether paper were hybridized to the nick-translated recombinant plasmid pA6 containing cDNA sequences for the low-M, RNA called 7S(L) RNA.Nuclear RNP particles and free and membrane-bound polysomes were found to contain 7S(L) RNA. In the cytoplasm 7S(L) RNA could be identified as the major small RNA in 20-S cmRNP particles.In previous studies we have described the presence of several RNAs of low molecular mass in free cytoplasmic mRNA-protein complexes (cmRNP particles) from rat liver [I ,2]. Two populations of cmRNP particles, one sedimenting at about 20 S and another at 40 S, have been analyzed with respect to their low-M, RNA components. We designated the most prominent low-M, RNAs in these cmRNP particles as nucleotides have been determined, it was of interest to examine whether our 6 1 RNA is identical with 7S(L)RNA. Moreover, we analyzed various rat liver RNP particles with respect to associated 7S(L)RNA.In the present papcr it will be demonstrated that 7S(L)RNA is found in cmRNP particles sedimenting at about 20 S, whereas 40-S cmRNP particles do not contain 7S(L)RNA. 7S(L)RNA could also be identified in salt extracts obtained from free and membrane-bound polysomes. MATERIALS AND METHODS ChemicalsAmberlite MB-1 and polyvinylpyrolidone were purchased from Serva, Heidelberg; yeast tRNA was from Boehringer, Abbreviations. 7S(L)RNA, a low-M, RNA, scdiinenting at aboul 7S, present in cytoplasmic mRNP and nuclear RNP particles; cmRNP, cytoplasmic messengcr ribonucleoprotcin; pmRNP, polysoinal messenger ribonucleoprotein; nRNP, nuclear ribonucleprokin; scRNA, small cytoplasmic RNA; snRNA. small nuclear RNA; DPT, diazophenyl thioether. ~~ ~-Mannheim; ficoll and o-aminopentyl -agarose were from Sigma. St Louis; octaethylcncglycol mono-n-dodecyl ether (Nikkol) was obtained from Nikko Chemicals. Tokyo. AnimalsMale Wistar rats (generously supplied by Prof. Dr H. Ueberberg, Thomae, Biberach) weighing 180 -300 g had frce access to water and a carbohydrate-rich 2046 protein diet (Altromin, Lage/Lippe). Generally the animals were fasted overnight prior to the experiments. Preparation of 20-S and 40-S cmRNP particlesFor the preparation of 40 S and 20-S cmRNP particles a postpolysomal supernatant from rat liver was added onto a linear sucrose gradient (20-40%) and centrifuged in a Beckman SW27 rotor at 27000 rpm for 23.5 h. The fractions around the sedimentation maxima of 40 S and 20 S were pooled, exposed to 0.5 M KCl, sedimented through a 1.0 M sucrose layer, phenol-extracted and subjected to polyacrylamide gel electrophoresis in the presence of formamidc as described previously [I]. It should be noted that the 20-S cmRNP particles were not always visualized as a wellseparated peak, but rather as a shoulder. Preparation of free and membrane-bound p m R N P partic...

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