Nitrobenzylthioinosinate binding sites in HeLa cells: relationship with ecto-5'-nucleotidase

Nitrobenzylthioinosine (NBTI), a substrate for the ecto-5'-nucleotidase of HeLa cells, was used to probe the relationship between ecto-5'-nucleotidase dephosphorylation site and the dephosphorylated NBTI binding site. It may be assumed that the dephosphorylation site of the enzyme is separate and functions independently of the NBTI binding site. Evidences supporting these conclusions were based on the following observations: i. NBTI-P dephosphorylation progressed with similar rates in the presence or absence of NBTI in the incubation medium; ii. adenosine-5'-monophosphate inhibited the dephosphorylation of NBTI-P but did not affect the binding of NBTI. The inhibition was competitive and dependent on concentration; iii. the effect of the NBTI nonisotopic medium on the binding of free G-3H-NBTI and G-3H-NBTI derived from G-3H-NBTI-P was different, and an instantaneous isotopic dilution was observed with G-3H-NBTI, but not with G-3H-NBTI-P, as substrate.