ACE2 Article COVID-19 Gastrointestinal tract Liver SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 alanine aminotransferase alkaline phosphatase angiotensin converting enzyme 2 aspartate aminotransferase atrophic gastritis bilirubin coronavirus disease 2019 dipeptidyl carboxypeptidase disease severity dysbiosis enzyme activity fecal oral transmission feces analysis follow up gastrectomy gastric hypoacidity gastrointestinal disease gastrointestinal hemorrhage gene mutation homeostasis human infection rate inflammation