Angiotension Converting Enzyme is a metallopeptidase of the MA(E) subclan of peptidases, also know as the “Glu-zincins” which use the motif HExxH as a catalytic triad. ACE-1 contains two catalytic domains, each domain using the HHEMGH zinc binding catalytic site. Although the catalytic domains (peptidase units 1 and 2) are similar in sequence, and are both catalytically active, they show some substrate specificity differences, and specific inhibitors have been crafted for each of the catalytic domains. In addition to the double peptidase form of ACE-1 (known as somatic ACE, or sACE), there is a testis form that lacks the first catalytic domain, and is referred to as germinal ACE, or gACE. The gACE-1 sequence arises from addition of a second promoter in intron 12 of ACE-1, and the first 72 residues are intron derived. ACE-1 is a type-II transmembrane protein, and the cytoplasmic domain has been shown to be a site of phospho-regulation. A form of ACE-1 lacking the transmembrane region exists, and may be a constitutively soluble form of the enzyme. The stalk region that separates the transmembrane region from the catalytic domain(s) is the cleavage site of a sheddase that releases the catalytic domains of full-length ACE-1 from the cell surface. ACE-1 is best known as a carboxy dipeptidase that cleaves the two carboxyterminal amino acids from Angiotension I, converting it to Angiotension II. ACE-1 is found in the lung, kidney, heart, pancreas, prostate, testis, small intestine and colon, and is found in cell lysates and as shed or processed forms in cell culture media and in biological fluids. The full-length sequence for sACE-1 encodes a 1306 amino acid protein with predicted mass of 149.7 kDa and a pI of 5.93. Glycosylation and other post-translational modifications make the apparent molecular weight on SDS PAGE gels larger, 175-184 kDa by Western blot. The shed forms range in size from 175-130 kDa. Many smaller forms may be seen on Western blot of culture media and biological fluid. The germinal form is 739 amino acids, with predicted mass of 84 kDa and pI of 5.87, and the soluble form lacking the TM region is 732 amino acids, with predicted mass of 83.3 kDa and a pI of 6.13. A recommended starting concentration for Western blots is 1:1000 when using colorimetric substrates such as BCIP/NBT, and 1:5000 for chemiluminescent substrates. Higher concentrations of antibody may be needed for samples from more distantly related species.

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