Peroxisomes play an indispensible role in
ether lipid biosynthesis as evidenced by the deficiency of
ether phospholipids in fibroblasts and tissues from patients suffering from a number of
peroxisomal disorders. Alkyl-dihydroxyacetonephosphate synthase, a peroxisomal
enzyme playing a key role in the biosynthesis of
ether phospholipids, contains the
peroxisomal targeting signal type 2 in a N-terminal cleavable presequence. Using a polyclonal antiserum raised against alkyl-dihydroxyacetonephosphate synthase, levels of this
enzyme were examined in fibroblast cell lines from patients affected by
peroxisomal disorders. Strongly reduced levels were found in fibroblasts of
Zellweger syndrome and
rhizomelic chondrodysplasia punctata patients, indicating that the
enzyme is not stable in the cytoplasm as a result of defective import into peroxisomes. In a
neonatal adrenoleukodystrophy patient with an isolated import deficiency of
proteins carrying the
peroxisomal targeting signal type 1, the precursor form of alkyl-dihydroxyacetonephosphate synthase was detected at a level comparable to that of the mature form in control fibroblasts, in line with an intraperoxisomal localization. A patient with an isolated deficiency in alkyl-dihydroxyacetonephosphate (DHAP) synthase activity had normal levels of this
protein. Analysis at the
cDNA level revealed a missense mutation leading to a R419H substitution in the
enzyme of this patient. Expression of a
recombinant protein carrying this mutation in Escherichia coli yielded an inactive
enzyme, whereas a comparable control recombinant
enzyme was active, providing further proof that this substitution is responsible for the inactivity of the
enzyme and the phenotype. In line with this result is the observation that wild-type
alkyl-DHAP synthase activity can be inactivated by the
arginine-modifying agent
phenylglyoxal. The
enzyme is efficiently protected against this inactivation when the substrate palmitoyl-DHAP is present at a saturating concentration. The gene encoding human alkyl-dihydroxyacetonephosphate synthase was mapped on chromosome 2q31.