Presence of ε‐adenosine tetraphosphate in chromaffin granules after transport of ε‐ATP

Adenosine 5'-tetraphosphate (Ap4) 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-Ap4) at the intragranular level. The epsilon-Ap4 formation followed a hyperbolic saturation kinetic at low epsilon-ATP concentrations with K(m) value of 0.4 microM epsilon-ATP intragranular (1.15 pmol/mg of granular protein). Intragranular concentrations of epsilon-ATP higher than 500 pmol/mg of protein (approximately to 175 microM intragranular) resulted in a non-saturable production of epsilon-Ap4.