Heparin-like substances (HLS) have their structure similar with heparin, the highly sulfated glycosaminoglycan (GAGs). The polysaccharide, a product of mast cells, is isolated from animal tissues (pig intestinal mucosa, bovine lung), and owes its anticoagulant activity, activates antithrombin and proteases inhibitor. Disclosed herein are novel methods of isolation, purification and characterization of HLS from snail mucus (Achatina fulica), and its uses. This HLS can be used to develop a simple, sensitive and specific assay for its biological activities of anti-SARs-CoV-2 (COVID-19) and anti-PD-L1 on the breast cancer cell.