The unique N-terminal sequence of testis angiotensin-converting enzyme is heavily O-glycosylated and unessential for activity or stability.

The testis-specific isozyme of angiotensin-converting enzyme (ACE) is identical, from residue 68 to the C terminus, to the second half or C-terminal domain of somatic ACE. However, the first 67 residues, comprising the signal peptide and a Ser-/Thr-rich 36-residue sequence that constitutes the N terminus of mature testis ACE, are unique. We have expressed a mutant human testis ACE lacking this 36-residue N-terminal sequence and find that compared to the wild-type protein the mutant is 15 kDa smaller due to the loss of greater than 90% of all O-linked sugars, but that it retains full enzymatic activity and is stable in culture. Heavy O-glycosylation is a property of testis ACE that is not shared by the somatic enzyme and is attributable to this unique sequence.