1. The present invention provides compositions and methods for reversibly inhibiting S-adenosyl-L-homocysteine (SAH) hydrolase. The compounds of the present invention can be used as an anti-hemorrhagic viral infection agent, an immunosuppressant, a homocysteine lowering agent, or an anti-neoplasm agent. The compositions and methods of the present invention can be used for the prevention and treatment of hemorrhagic virus infection, autoimmune diseases, autograft rejection, neoplasm, hyperhomocysteineuria, cardiovascular disease, stroke, Alzheimer's disease, multiple sclerosis or diabetes. The compound of the present invention and/or used in the present invention has the formula (I): ##STR00001## wherein Z is selected from the group consisting of carbon and nitrogen, R1 and R2 are the same or different, and are selected from the group consisting of hydrogen, hydroxy, alkyl, cycloalkyl, alkenyl, alkoxy, amino, aryl, heteroaryl, and halogen; R3 and R4 are the same or different and are selected from the group consisting of hydrogen, alkyl, acetyl, alkenyl, aryl, and heteroaryl; X is selected from the group consisting of oxygen, nitrogen, and sulfur; and Y is selected from the group consisting of hydrogen, a C.sub.1-10 alkyl group, alkenyl, vinyl, aryl, and heteroaryl, provided that the compound is not (4-adenine-9-yl)-2-hydroxybutanoic acid.