Inhibition of alpha-tocopherol and calcium calmodulin-stimulated phosphodiesterase activity in vitro by anthracyclines.

Abstract

1. The inhibition of alpha-tocopherol and calmodulin-stimulated phosphodiesterase activities was investigated in vitro. 2. Anthracyclines--doxorubicin, daunorubicin and aclacinomycin--inhibited calcium calmodulin-stimulated cyclic 3',5'-AMP (cAMP) nucleotide phosphodiesterase (EC. 3.1.4-17) activity (IC50 = 33.00 +/- 3.50-36.50 +/- 2.75 mumol/l). The stimulation of this enzyme by alpha-tocopherol was also inhibited by doxorubicin (IC50 = 18.50 +/- 4.00 mumol/l). 3. The anthracycline-induced inhibition of the calcium calmodulin and alpha-tocopherol-stimulated phosphodiesterase activity was competitive with calmodulin and alpha-tocopherol respectively. Increasing the concentration of the substrate, cAMP or calcium ions did not attenuate the drug-induced inhibition. The basal activity of the enzyme was not inhibited by concentration of doxorubicin up to 50 mumol/l. 4. In vivo, single dose drug distribution studies of the fluorescence of doxorubicin indicate that in the heart after a cardiotoxic dose (20 mg/kg), myocardial concentrations were achieved which could cause 70-80% inhibition of this phosphodiesterase enzyme. 5. Inhibition of calmodulin function by anthracyclines via direct interaction with calmodulin may contribute significantly to the effects of anthracyclines, such as disturbance in calcium homeostasis as well as acute and chronic deleterious effects on the myocardium. The action of alpha-tocopherol to bind or complex anthracycline may in part contribute to its protection against anthracycline-induced membrane damage and cardiotoxicity.