Peroxisomal disorders are lethal inherited diseases caused by either defects in peroxisome assembly or dysfunction of single or multiple enzymatic function(s). The peroxisomal matrix
proteins are targeted to peroxisomes via the interaction of
peroxisomal targeting signal sequences 1 and 2 (PTS1 or PTS2) with their respective cytosolic receptors. We have studied human skin fibroblast cell lines that have
multiple peroxisomal dysfunctions with normal packaging of PTS1 and PTS2 signal-containing
proteins but lack
catalase in peroxisomes. To understand the defect in targeting of
catalase to peroxisomes and the loss of multiple
enzyme activities, we transfected the mutant cells with normal
catalase modified to contain either PTS1 or PTS2
signal sequence. We demonstrate the integrity of these pathways by targeting
catalase into peroxisomes via PTS1 or PTS2 pathways. Furthermore, restoration of peroxisomal functions by targeting
catalase-SKL
protein (a catalase fused to the PTS1 sequence) to peroxisomes indicates that loss of multiple functions may be due to their inactivation by H2O2 or other
oxygen species in these
catalase-negative peroxisomes. In addition to
enzyme activities, targeting of
catalase-SKL chimera to peroxisomes also corrected the in situ levels of
fatty acids and
plasmalogens in these mutant cell lines. In normal fibroblasts treated with
aminotriazole to inhibit
catalase, we found that peroxisomal functions were inhibited to the level found in mutant cells, an observation that supports the conclusion that multiple peroxisomal
enzyme defects in these patients are caused by H2O2 toxicity in
catalase-negative peroxisomes. Moreover, targeting of
catalase to peroxisomes via PTS1 and PTS2 pathways in these mutant cell lines suggests that there is another pathway for
catalase import into peroxisomes and that an abnormality in this pathway manifests as a peroxisomal disease.