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 puta tive tumor suppressor gene, is a typical dinucleoside 5',5 triple prim e-P-1,P-3-triphosphate (Ap(3)A) hydrolase (EC 3.6.1.29) on the basis o f its enzymatic properties we report here. Ap(3)A is the preferred sub strate among Ap(n)A (n = 3-6), and AMP is always one of the reaction p roducts. Mn2+ and Mg2+ are equally stimulatory, while Zn2+ is inhibito ry 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 const ant, 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 a ll four conserved histidines are required for full activity, and the c entral histidine of the triad is absolutely essential for Ap(3)A hydro lase activity. This putative tumor suppressor is the first evidence fo r a connection between dinucleotide oligophosphate metabolism and tumo rigenesis. Also, Fhit is the first HIT protein in which the histidine residues have been demonstrated by mutagenesis to be critical for func tion.