ACE2 (angiotensin-converting enzyme 2) is a metalloproteinase that requires a divalent cation positioned at the active site in order to perform catalysis. It has multiple physiological roles that revolve around its trivalent function: a negative regulator of the renin-angiotensin system, facilitator of amino acid transport, and the severe acute respiratory syndrome-coronavirus (SARS-CoV) and SARS-CoV-2 receptor. ACE2 has recently been identified as the SARS-CoV-2 receptor, the infective agent responsible for the coronavirus disease, providing a critical link between immunity, inflammation, ACE2, and cardiovascular disease. Although sharing a close evolutionary relationship with SARS-CoV, the receptor-binding domain of SARS-CoV-2 differs in several key amino acid residues, allowing for stronger binding affinity with the human ACE2 receptor, which may account for the greater pathogenicity of SARS-CoV-2. The loss of ACE2 function following binding by SARS-CoV-2 is driven by endocytosis and activation of proteolytic cleavage and processing. The ACE2 system is a critical protective pathway against heart failure with reduced and preserved ejection fraction including, myocardial infarction and hypertension, and against lung disease and diabetes mellitus. The control of gut dysbiosis and vascular permeability by ACE2 has emerged as an essential mechanism of pulmonary hypertension and diabetic cardiovascular complications. Mca-APK(Dnp) ACE2 substrate is a FRET peptide for measuring enzymatic activity in cells and tissues. It uses DNP as a quencher molecule to quench the fluorescence of methoxycoumarin (Mca). This interaction is abolished when the enzyme cleaves the proline-lysine residue and restore the fluorescence of Mca. This fluorogenic substrate offers more flexibility and higher throughput than the commonly used HPLC-separation-based methods.