Investigation of glyceraldehyde-3-phosphate dehydrogenase from human sperms

Glyceraldehyde-3-phosphate dehydrogenase (GAPDs) was purified from human sperms and properties of the enzyme were investigated. After sonication of sperms, the most part of GAPDs is associated with the insoluble cell fraction. Trypsin treatment results in the cleavage of part of the N-terminal domain of the enzyme yielding a soluble fragment that was purified by hydrophobic chromatography on Phenyl-Sepharose. The isolated fragment was shown to be a tetramer with molecular weight of approximately 150 kD (according to Blue Native PAGE) and composed of subunits of 40 kD (according to SDS-PAGE). The specific activity of the isolated fragment reached 374 U/mg. It is supposed that GAPDs exists in sperms as the tetrameric molecule bound to the fibrous sheath of the flagellum through the N-terminus of one or two subunits. Comparative study of the amino acid sequences of mammalian GAPDs revealed conservative cysteine residues (C21, C94, and C150) that are specific for the sperm isoenzyme and absent in the somatic isoenzyme. Residue C21 can be involved in the formation of the disulfide bond between the N-terminal domain of GAPDs and fibrous sheath proteins.