Ssh. Wang et al., MEMBRANE TOXICITY OF THE PROTEIN-KINASE-C INHIBITOR CALPHOSTIN-A BY AFREE-RADICAL MECHANISM, Neuroscience letters, 157(1), 1993, pp. 25-28
The effects of calphostin A on cytoplasmic calcium levels, receptor-me
diated calcium release, and membrane input resistance were measured in
neuroblastoma cells. Calphostin A is a lipophilic, light-sensitive pe
rylenequinone that generates singlet oxygen when illuminated. It inhib
its the activity of protein kinase C (IC50 = 250 nM), but only in the
presence of light. Phorbol esters normally attenuate carbachol-evoked
calcium release. This effect was blocked by simultaneous exposure to l
ight and calphostin A (40 nM) for 30 min. At higher doses (0.5-1 muM)
calphostin A also approximately doubled the resting calcium level and
decreased cell input resistance by 51%. These toxic effects did not oc
cur in the dark or after preincubation with the antioxidant alpha-toco
pherol. These data support the hypothesis that the calphostins act by
partitioning into the membrane and producing singlet oxygen and endope
roxides which then irreversibly modify protein kinase C and other memb
rane proteins and lipids.