Inhibitor of adenosine3′ ,5′-monophosphate binding and protein kinase activity in leucocyte lysosomes

Abstract

In contrast to other tissues (e.g. brain, heart), no cAMP dependent protein kinase activity and little cAMP-binding activity could be detected in crude homogenates of purified human PMN leucocytes. This was due to the presence of an inhibitor of cAMP binding and protein kinase activity in PMN leucocytes. Since the inhibitor was entirely segregated in PMN lysosomes (rich in β-glucuronidase and acid phosphatase), lysosomefree supernatants yielded cAMP-dependent protein kinase (> 5-fold stimulation with 5 μM cAMP) and considerable cAMP binding activity. The inhibitor was not dialyzable, and unlike the usual protein kinase modulators, was heat-labile. Preparations of beef-heart protein kinase, treated with the PMN inhibitor, lost cAMP-binding and protein kinase activities simultaneously. The presence of this lysosomal inhibitor may invalidate studies of cAMP binding and protein kinase activities in crude homogenates prepared from lysosome-rich tissues.