Effects of a microbial pigment violacein on the activities of protein kinases

Violacein, a purple bacterial pigment, has a variety of biological activities, including cytotoxicity against tumour cells. To investigate the physiological properties of this pigment, we studied its inhibitory effect on protein kinases in vitro. We used phosphorylation experiments to assess the inhibitory effect of violacein on protein kinase activity. Violacein inhibited the classical-type protein kinase C (PKC) and some novel and atypical PKC enzymes. The catalytic subunits of protein kinase A (PKA) and classical-type PKC were strongly inhibited by violacein. Kinetic analysis of PKA inhibition displayed mixed-type inhibition with respect to ATP. Slight inhibition of Ca2+/calmodulin-dependent protein kinase was observed while protein tyrosine kinase (Src) was not inhibited by violacein. The inhibition of PKA and PKCs by violacein provided evidence that violacein binds to the catalytic subunit of enzymes. The protein kinase inhibition revealed here may be important to the mechanism of violacein cytotoxicity.

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