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By both histological and biochemical criteria, peroxisomes in patients with Zellweger syndrome appear to be absent or severely deficient. By using 15-30% (wt/vol) Nycodenz/sucrose gradients to study the subcellular localization of extraperoxisomal catalase activity, a commonly used marker for mature peroxisomes, we detected a single peak of activity in Zellweger syndrome fibroblasts at an equilibrium density of 1.13 g/cm3, lower than the expected 1.17 g/cm3 of mature peroxisomes. Upon recentrifugation in either the original gradient or one with a higher salt concentration, essentially all catalase activity was recovered in fractions of the original densities. The activity of the catalase peak was further analyzed by a digitonin titration and filtration assay in combination with Triton X-100 treatment. The catalase activity passed through 0.1-microns and 0.22-microns but was retained on 0.025-microns membrane filters (mean pore size). After treatment with Triton X-100 nearly all catalase activity passed through the filters. The results from fractionations data, digitonin latency measurement, and the detergent effect on the filtration behavior suggest that catalase is not free in the cytosol of Zellweger syndrome fibroblasts as commonly thought but in particles (W-particles). Similar low-density catalase-containing particles, distinct from peroxisomes, are also found in normal fibroblasts. We found that L-alpha-hydroxyacid oxidase, another peroxisomal matrix enzyme, is also present in W-particles derived from normal and Zellweger syndrome fibroblasts. We speculate that the low-density catalase-containing W-particle may represent an immature or incomplete form of peroxisome distinct from previously described "peroxisomal ghosts" in Zellweger syndrome fibroblasts.

Low-density particles (W-particles) containing catalase in Zellweger syndrome and normal fibroblasts.