Angiotensin I Converting Enzyme 2 (ACE2), as?a?transmembrane?protein, serves?as?the?main?entry point into?cells?for?some?coronaviruses. More?specifically, the binding of?the spike S1 protein of?SARS-CoV and?SARS-CoV-2?to the enzymatic?domain?of?ACE2?on?the?surface?of?cells?results?in?endocytosis?and translocation of?both?the?virusand?the?enzyme?into?endosomes located?within?cells. The activity of recombinant rat ACE2 is measured by its ability to cleave a fluorogenic peptide substrate MCA-Tyr-Val-Ala-Asp-Ala-Pro-Lys(DNP)-OH in the assay buffer 50 mM Tris, 1 M NaCl, pH 7.5. The rrACE2 is diluted to 0.5 ug/mL in assay buffer. Loading into a black well plate 50 μL of 0.5 ug/mL rrACE2 and start the reaction by adding 50 μL of 20 μM substrate, with a substrate blank containing 50 μL assay buffer, 50 μL substrate, and no rrACE2. Then read at excitiation and emission wavelengths of 320 nm and 405 nm, respectively, in kinetic mode for 5 minutes. The specific activity of recombinant rat ACE2 is > 2200 pmol/min/μg.