Arylsulfatase C is a microsomal membrane-bound enzyme previously thought to be the same as steroid sulfatase, the only X-linked enzyme known to escape from X inactivation in man. We had shown that arylsulfatase C actually consists of two biochemically distinct isozymes, s and f. Only the s form has steroid sulfatase activity. The f and s forms were thought to be related through posttranslational or posttranscriptional modification of the same gene product. In part consistent with this hypothesis, we now report that in a panel of 28 rodent-human somatic cell hybrids, expression of both s and f was concordant only with the human X chromosome, thus showing that the f form is also X linked. In three separate somatic hybrids containing human X chromosomes in an inactive state, the f form was still expressed. Thus, similar to the s form, the f form also escapes from X inactivation. However, contrary to expectations, the s and f forms were not related by posttranslational modification of the same gene product. A full-length cDNA for the s form failed to hybridize to transcripts produced from an f-expressing cell line, showing that there is little sequence identity between the two. They are also not related by differential splicing of a common primary transcript, since fibroblasts from some patients with steroid sulfatase deficiency due to gene deletion of the s form continue to express the f form. Therefore, although the f and s isozymes of arylsulfatase C are X linked and escape from X inactivation, they are products from separate genes, thus providing a unique isoenzyme system to study possible gene duplication and regulation in the part of the human X chromosome that escapes inactivation.
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