An adjacent pair of human NUDT genes on chromosome X are preferentially expressed in testis and encode two new isoforms of diphosphoinositol polyphosphate phosphohydrolase

Combinatorial expression of the various isoforms of diphosphoinositol synthases and phosphohydrolases determines the rates of phosphorylation/dephosphorylation cycles that have been functionally linked to vesicle trafficking, stress responses, DNA repair, and apoptosis. We now describe two new 19-kDa diphosphoinositol polyphosphate phosphohydrolases (DIPPs), named types 3α and 3β, which possess the canonical Nudix-type catalytic motif flanked on either side by short Gly-rich sequences. The two enzymes differ only in that Pro-89 in the a form is replaced by Arg-89 in the β form, making the latter ∼2-fold more active in vitro. Another Nudix substrate, diadenosine hexaphosphate, was hydrolyzed less efficiently (k_cat/K_m = 0.2 × 10⁵ M^-1 s^-1) compared with diphosphoinositol polyphosphates (k_cat/K_m = 2-40 × 10⁵ M^-1 s^-1). Catalytic activity in vivo was established by individual overexpression of the human (h) DIPP3 isoforms in HEK293 cells, which reduced cellular levels of diphosphoinositol polyphosphates by 40-50%. The hDIPP3 mRNA is preferentially expressed in testis, accompanied by relatively weak expression in the brain, contrasting with hDIPP1 and hDIPP2 which are widely expressed. The hDIPP3 genes (NUDT10 encodes hDIPP3α; NUDT11 encodes hDIPP3β) are only 152 kbp apart at p11.22 on chromosome X and probably arose by duplication. Transcription of both genes is inactivated on one of the X chromosomes of human females to maintain appropriate gene dosage. The hDIPP3 pair add tissue-specific diversity to the molecular mechanisms regulating diphosphoinositol polyphosphate turnover.