Ectoenzymatic breakdown of diadenosine polyphosphates by Xenopus laevis oocytes

Xenopus laevis oocytes exhibit ectoenzymatic activity able to hydrolytically cleave extracellular diadenosine polyphosphates (ApnA). The basic properties of this ectoenzyme were investigated using as substrates di‐(1,N6‐ethenoadenosine) 5′,5′′′‐P1,P4‐tetraphospate [ε‐(Ap4A)] and di‐(1,N6‐ethenoadenosine) 5′,5′′′‐P1,P5‐pentaphospate [ε‐(Ap5A)], fluorogenic derivatives of Ap4A and Ap5A, respectively. ε‐(Ap4A) and ε‐(Ap5A) are hydrolysed by folliculated oocytes according to hyperbolic kinetics with Km values of 13.4 and 12.0 µm and Vmax values of 4.8 and 5.5 pmol per oocyte per min, respectively. The ectoenzyme is activated by Ca2+ and Mg2+, reaches maximal activity at pH 8–9 and is inhibited by suramin. Defolliculated oocytes also hydrolyse both substrates with similar Km values but Vmax values are approximately doubled with respect to folliculated controls. Chromatographic analysis indicates that extracellular ε‐(Ap4A) and ε‐(Ap5A) are first cleaved into 1,N6‐ethenoAMP (ε‐AMP) + 1,N6‐ethenoATP (ε‐ATP) and ε‐AMP + 1,N6‐ethenoadenosine tetraphosphate (ε‐Ap4), respectively, which are catabolized to 1,N6‐ethenoadenosine (ε‐Ado) as the end product by folliculated oocytes. Denuded oocytes, however, show a drastically reduced rate of ε‐Ado production, ε‐AMP being the main end‐product of extracellular ε‐(ApnA) catabolism. Results indicate that, whereas the ApnA‐cleaving ectoenzyme appears to be located mainly in the oocyte, ectoenzymes involved in the dephosphorylation of mononucleotide moieties are located mainly in the follicular cell layer.