Human glyceraldehyde 3-phosphate dehydrogenase-2 gene is expressed specifically in spermatogenic cells

Although the process of glycolysis is highly conserved in eukaryotes, sever al glycolytic enzymes have unique structural or functional features in sper matogenic cells. We previously identified and characterized the mouse compl ementary DNA (cDNA) and a gene for 1 of these enzymes, glyceraldehyde 3-pho sphate dehydrogenase-s (Gapds). This gene is expressed only in spermatids. The enzyme appears to have an essential role in energy production required for fertilization, and it is reported to be susceptible to inhibition by ce rtain environmental chemicals. We have now cloned and sequenced the cDNA fo r the human homologue of glyceraldehyde 3-phosphate dehydrogenase (GAPD2) a nd determined the structure of the gene. The messenger RNA (mRNA) was detec ted in testis, but not in 15 other human tissues analyzed by Northern blot technique. The deduced GAPD2 protein contains 408 amino acids and is 68% id entical with somatic cell GAPD. GAPD2 has a 72-amino acid segment at the am ino terminal end that is not present in somatic cell GAPD. This segment is proline-rich but contains smaller stretches of polyproline and is 30 amino acids shorter than the comparable segment of mouse GAPDS. The structure of the human GAPD2 gene was determined by polymerase chain reaction (PGR) to i dentify exon-intron junctions in a genomic clone and in total genomic DNA. The locations of these junctions in the GAPD2 gene corresponded precisely t o those of the 11 exon-intron junctions in the mouse Gapds gene. Immunohist ochemical studies found that GAPD2 is located in the principal piece of the flagellum of human spermatozoa, as are GAPDS in mouse and rat spermatozoa. GAPD2 extracted from human spermatozoa and analyzed by Western blot techni que migrated with an apparent molecular weight of similar to 56 000, althou gh the calculated molecular weight is 44 501. The conserved nature of the m ouse, rat. and human enzymes suggests that they serve similar roles in thes e and other mammalian species.