ABSTRACT.    S-adenosylhomocysteine hydrolase (CpSAHH), the essential enzyme of AdoMet metabolism in the apicomplexan parasite Cryptosporidium parvum has been cloned and characterized. RT-PCR analysis confirmed that this single-copy gene is expressed both in intracellular stages (in C. parvum infected HCT-8 cells 24 hours after infection) and in sporozoites. Our data indicate that CpSAHH is a tetramer. CpSAHH was successfully overexpressed in E. coli TB1 cells using the pMAL-c2x vector and after purification, the recombinant protein showed enzymatic activity comparable with SAHH from other organisms. The kinetic data reveal that the maltose binding protein used for affinity purification of the enzyme influences the kinetic properties of its fused protein. The enzymatic activity of CpSAHH was inhibited by D-eritadenine, (S)-DHPA and Ara-A.