A novel human zinc metalloprotease, ACEH, has been identified, showing significant homology to human angiotensin-converting enzyme (ACE). ACEH contains a single HEXXH zinc-binding domain and shares critical residues with the ACE family. The predicted protein sequence is 805 amino acids long, including a potential 17-amino acid N-terminal signal peptide and a putative C-terminal membrane anchor. Expression of a soluble, truncated form of ACEH in Chinese hamster ovary cells produces a 120 kDa glycoprotein that cleaves angiotensin I and II but not bradykinin or Hip-His-Leu. ACEH functions as a carboxypeptidase, is inhibited by EDTA but not by classical ACE inhibitors like captopril, lisinopril, or enalapril. The ACEH gene contains 18 exons, with significant similarity to the first 17 exons of human ACE. ACEH mRNA is most highly expressed in testis, kidney, and heart. This is the first report of a mammalian homolog of ACE, with implications for cardiovascular and renal function. ACE is a zinc metalloprotease of the M2 family, primarily involved in cardiovascular homeostasis by cleaving angiotensin I to produce angiotensin II. It also inactivates bradykinin. Two forms of ACE have been identified: somatic ACE, a type I integral membrane protein, and a soluble form lacking the transmembrane and cytosolic domains. ACE-like enzymes have been found in non-mammalian species, including insects. In this study, a novel human ACEH cDNA was identified, with significant similarity to mammalian ACE. The ACEH gene maps to chromosome Xp22 and is highly expressed in testis, kidney, and heart. Expression of a soluble, truncated ACEH produced a 120 kDa glycosylated protein that hydrolyzed the C-terminal residue from angiotensin I and II. ACEH activity is inhibited by EDTA but not by classical ACE inhibitors. ACEH is a metalloprotease with distinct substrate and inhibitor specificity from ACE. The high expression of ACEH in heart and kidney suggests a role in blood pressure homeostasis, while its expression in testis may indicate reproductive functions. The genomic sequence of ACEH shows remarkable similarity to the ACE gene, with exons 1–4 and 7–12 showing significant similarity. The ACEH gene is located on the X chromosome, similar to the PEX gene associated with X-linked hypophosphatemic rickets. These findings suggest that ACEH may have a significant role in cardiovascular homeostasis and fertility. Further studies are needed to elucidate its physiological roles.A novel human zinc metalloprotease, ACEH, has been identified, showing significant homology to human angiotensin-converting enzyme (ACE). ACEH contains a single HEXXH zinc-binding domain and shares critical residues with the ACE family. The predicted protein sequence is 805 amino acids long, including a potential 17-amino acid N-terminal signal peptide and a putative C-terminal membrane anchor. Expression of a soluble, truncated form of ACEH in Chinese hamster ovary cells produces a 120 kDa glycoprotein that cleaves angiotensin I and II but not bradykinin or Hip-His-Leu. ACEH functions as a carboxypeptidase, is inhibited by EDTA but not by classical ACE inhibitors like captopril, lisinopril, or enalapril. The ACEH gene contains 18 exons, with significant similarity to the first 17 exons of human ACE. ACEH mRNA is most highly expressed in testis, kidney, and heart. This is the first report of a mammalian homolog of ACE, with implications for cardiovascular and renal function. ACE is a zinc metalloprotease of the M2 family, primarily involved in cardiovascular homeostasis by cleaving angiotensin I to produce angiotensin II. It also inactivates bradykinin. Two forms of ACE have been identified: somatic ACE, a type I integral membrane protein, and a soluble form lacking the transmembrane and cytosolic domains. ACE-like enzymes have been found in non-mammalian species, including insects. In this study, a novel human ACEH cDNA was identified, with significant similarity to mammalian ACE. The ACEH gene maps to chromosome Xp22 and is highly expressed in testis, kidney, and heart. Expression of a soluble, truncated ACEH produced a 120 kDa glycosylated protein that hydrolyzed the C-terminal residue from angiotensin I and II. ACEH activity is inhibited by EDTA but not by classical ACE inhibitors. ACEH is a metalloprotease with distinct substrate and inhibitor specificity from ACE. The high expression of ACEH in heart and kidney suggests a role in blood pressure homeostasis, while its expression in testis may indicate reproductive functions. The genomic sequence of ACEH shows remarkable similarity to the ACE gene, with exons 1–4 and 7–12 showing significant similarity. The ACEH gene is located on the X chromosome, similar to the PEX gene associated with X-linked hypophosphatemic rickets. These findings suggest that ACEH may have a significant role in cardiovascular homeostasis and fertility. Further studies are needed to elucidate its physiological roles.