Arylsulfatase D gene in Xp22.3 encodes two protein isoforms

The human genome contains six arylsulfatase genes (ARSA-ARSF), of which fou r are clustered in a distal region of the short arm of the X chromosome (Xp 22.3), They were probably generated by a series of evolutionary duplication events; their exon-intron boundaries are identical, Nevertheless, differen t transcript lengths and the absence of cross-hybridizations point to a spe cific function of each gene in human cell metabolism, and multiple transcri pts suggest the coding of protein isoforms, We identified a novel protein i soform of the ARSD gene by isolation of a series of cDNA clones from a huma n testis cDNA library. The clones were only partially identical to another series of ARSD clones isolated earlier (now designated ARSD alpha clones). Their specific C-terminal region (1160 nt) encodes a novel ARSD peptide of 48 amino acids and was identified as part of intron 6 of the ARSD gene in X p22.3. We therefore designate them ARSD beta clones. Expression analyses of ARSD alpha and ARSD beta by semiquantitative RT-PCR revealed the presence of both in multiple human tissues, although in different quantities. A phys iologic substrate for arylsulfatase D proteins is not known. We therefore e stimated their sulfatase activities in vitro with the aid of the 4-methylum belliferyl sulfate (4-MUS) assay. Surprisingly, neither ARSD protein isofor m demonstrated any sulfatase activity alone or in combination, although the ir catalytic peptide domain is strongly conserved in comparison with that o f the other X-chromosomal arylsulfatase enzymes (ARSC, ARSE, ARSE), all of which are functionally active in the 4-MUS assay.