Human placental NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH) is a member of the short-chain dehydrogenase family of enzymes. It has been proposed that a highly conserved serine residue (corresponding to serine 138 of 15-PGDH) may be involved in the catalytic mechanism of many of these enzymes. Site-directed mutagenesis was used to change serine 138 of NAD+-dependent 15-hydroxyprostaglandin dehydrogenase to an alanine. The mutant protein was then expressed in E. coli. Western blot analysis indicated that the S138A mutant protein was expressed at levels similar to the wild type enzyme; however, the mutant protein was found to be inactive. These results support the proposed role of this highly conserved serine in enzyme activity.