MOLECULAR-CLONING OF THE MATURE NAD(-DEPENDENT SUCCINIC SEMIALDEHYDE DEHYDROGENASE FROM RAT AND HUMAN - CDNA ISOLATION, EVOLUTIONARY HOMOLOGY, AND TISSUE EXPRESSION())

Three rat brain cDNA clones similar to 3500, 1465, and 1135 base pairs in length encoding succinic semialdehyde dehydrogenase (SSADH; EC 1.2 .1.24) were isolated from two cDNA libraries using a polymerase chain reaction derived probe. Restriction mapping and DNA sequencing reveale d that the 3.5-kilobase clone contained an 84-base pair (28 amino acid ) insert in the coding region. Composite clones encoding mature SSADH predicted proteins with 488 amino acids (M(r) = 52,188) when including the insert and 460 amino acids (M(r) = 48,854) without the insert. Th e cDNA clones were confirmed by expression of enzyme activity in bacte ria and protein sequence data obtained from sequencing purified rat br ain SSADH. Two human liver SSADH cDNA clones of 1091 and 899 base pair s were also isolated. Human and rat SSADH share 83 and 91% identity in nucleotide and protein sequence, respectively Northern blot analysis revealed two differentially expressed SSADH transcripts of approximate ly 2.0 and 6.0 kilobases in both rat and human tissues. Human genomic Southern blots indicate that the two SSADH transcripts are encoded by a greater than 20-kilobase single copy gene. Mammalian SSADH contains significant homology to bacterial NADP(+)-succinic semialdehyde dehydr ogenase (EC 1.2.1.16) and conserved regions of general aldehyde dehydr ogenases (EC 1.2.1.3), suggesting it is a member of the aldehyde dehyd rogenase superfamily of proteins.