Conversion of ATP into Other Adenine Nucleotides Within Isolated Islet Secretory Vesicles. Effect of Cyclic AMP on Phosphorus Translocation

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Abstract
The secretory vesicle fraction (24,000 .times. g) from rat [pancreatic] islets was incubated with labeled ATP and radioactivity recovered as adenine nucleotides and Pi. The ATP for incubation was labeled in the adenyl moiety with 3H along with ATP labeled with 32P either in the .alpha. (ribosyl) or the .gamma. (terminal) P position. After a 1 h period of incubation, 51% of the [3H]ATP that underwent conversion was recovered as AMP in the soluble fraction of the secretory vesicle. Forty-three per cent of the radioactivity was recovered as ADP and 5% as cyclic[c]AMP. The insoluble fraction of the secretory vesicle contained substantially less 3H radioactivity-AMP (0.2%), ADP (0.05%), cAMP (0.05%) and ATP (0.20%). In the insoluble fraction of the secretory vesicle, P radioactivity was found which could not be accounted for by the presence of adenine nucleotides. The ratio of 32P to 3H (incubation with ATP labeled with tritium and in the .gamma. phosphorus position) increased from 0.25-0.53 .+-. 0.041 (P < 0.02). With the addition of cAMP, the ratio increased further to 0.92 .+-. 0.064 (P < 0.02). The increase of the 32P to 3H ratio occurred as a consequence of increased recoverable P in the insoluble fraction. The secretory vesicle fraction possesses the capability of converting ATP to other adenine nucleotides. As part of the metabolism of ATP, there occurs a translocation of the terminal P to insoluble secretory vesicle constituents. The addition of cAMP (3.5 .times. 10-6 M) increases P radioactivity in the insoluble secretory vesicle material to a level above that accounted for by the presence of adenine nucleotides.