Cyclic AMP powerfully inhibits the fMet-Leu-Phe-dependent respiratory burst and exocytosis of azurophilic and specific granules without affecting Ca2+ release from intracellular stores. The elevation of [Ca2+]i induced by fMet-Leu-Phe is short-lived in cyclic AMP-treated cells and similar to that of untreated cells stimulated in the absence of external Ca2+. Nevertheless, in these latter cells fMet-Leu-Phe induces metabolic activation. We therefore suggest that the inhibitory action of cyclic AMP on neutrophil responses is not due to its effects on [Ca2+]i homoeostasis.

The catalytic subunit of cAMP-dependent protein kinase (EC 2.7.1.37) was purified for the first time from human placenta by DEAE-cellulose and HTP chromatography. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis showed a single band of average molecular weight of 42 kDa (SEM = 0.52). Kinetic experiments showed a Km for ATP of 12.6 ± 1.2 μmol/l, for histone II-AS of 1.3 ± 0.05 mg-ml^-1, for kemptide of 11.4 ± 4.4 μmol/l. The synthetic inhibitor IP(20)-amide showed a competitive mechanism of inhibition with a K, of 5.0 nmol/1. The protein kinase inhibitors H7 and H9 showed an apparent K, of 8.3 and 4.9 μmol/1 respectively. Preparative isoelectric focusing revealed the presence of 5 different isoforms with an average pi of 6.17, 6.70, 7.15, 7.67, 8.9.

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