Testis-Specific Expression of Mrnas for a Unique Human Type 1 Hexokinase Lacking the Porin-Binding Domain

Several enzymes in the glycolytic pathway are reported to have spermatogenic cell‐specific isozymes. We reported recently the cloning of cDNAs representing three unique type 1 hexokinase mRNAs (mHk1‐sa, mHk1‐sb, and mHk1‐sc) present only in mouse spermatogenic cells and the patterns of expression of these mRNAs (Mori et al., 1993: Biol Reprod 49:191–203). The mRNAs contain a spermatogenic cell‐specific sequence, but lack the sequence for the porin‐binding domain that somatic cell hexokinases use to bind to a pore‐forming protein in the outer mitochondrial membrane. We now report the cloning of cDNAs representing three unique human type 1 hexokinase mRNAs (hHK1‐ta, hHK1‐tb, and hHK1‐tc) expressed in testis, but not detected by Northern analysis in other human tissues. These mRNAs also contain a testis‐specific sequence not present in somatic cell type 1 hexokinase, but lack the sequence for the porin‐binding domain. The hHK1‐tb and hHK1‐tc mRNAs each contain an additional unique sequence. The testis‐specific sequence of the human mRNAs is similar to the spermatogenic cell‐specific sequence of the mouse mRNAs. Furthermore, Northern analysis of RNA from mouse, hamster, guinea pig, rabbit, ram, human, and rat demonstrated expression of type 1 hexokinase mRNAs lacking the porin‐binding domain in the testes of these mammals. These results suggest that hexokinase may have unique structural or functional features in spermatogenic cells and support a model proposed by others for hexokinase gene evolution in mammals.