Presence of epsilon-adenosine tetraphosphate in chromaffin granules after transport of epsilon-ATP

Adenosine 5'-tetraphosphate (Ap(4)) is a natural constituent of chromaffin granules with concentration values of 2.2 +/- 0.1 nmol/mg of protein and a ratio 245 +/- 40 times lower with respect to ATP (n = 4). The granular transport of epsilon-ATP resulted in a time- and concentration-dependent production of epsilon-adenosine tetraphosphate (epsilon-Ap(4)) at the intragranular level. The epsilon-Ap(4) formation followed a hyperbolic saturation kinetic at low epsilon-ATP concentrations with K-m value of 0.4 mu M epsilon-ATP intragranular (1.15 pmol/mg of granular protein). Intragranular concentrations of epsilon-ATP higher than 500 pmol/mg of protein (approximate to 175 mu M intragranular) resulted in a non-saturable production of epsilon-Ap(4).