Fhit, a putative tumor suppressor in humans, is a dinucleoside 5',5'''-P-1,P-3-triphosphate hydrolase

Human Fhit (fragile histidine triad) protein, encoded by the FHIT putative tumor suppressor gene, is a typical dinucleoside 5',5 triple prime-P-1,P-3-triphosphate (Ap(3)A) hydrolase (EC 3.6.1.29) on the basis of its enzymatic properties we report here. Ap(3)A is the preferred substrate among Ap(n)A (n = 3-6), and AMP is always one of the reaction products. Mn2+ and Mg2+ are equally stimulatory, while Zn2+ is inhibitory with Ap(3)A as the substrate. Values of the K-m for Ap(3)A and Ap(4)A are 1.3 and 4.6 mu M, respectively. Values of the specificity constant, k(cat)/K-m, for Ap(3)A and Ap(4)A are 2.0 x 10(6) and 6.7 x 10(3) s(-1) M(-1), respectively, for a glutathione S-transferase (GST)-Fhit fusion protein. Site-directed mutagenesis of FHIT demonstrated that all four conserved histidines are required for full activity, and the central histidine of the triad is absolutely essential for Ap(3)A hydrolase activity. This putative tumor suppressor is the first evidence for a connection between dinucleotide oligophosphate metabolism and tumorigenesis. Also, Fhit is the first HIT protein in which the histidine residues have been demonstrated by mutagenesis to be critical for function.