The protein kinase C inhibitor, calphostin C, inhibits succinate-dependent mitochondrial reduction of MTT by a mechanism that does not involve protein kinase C.

The light-activated protein kinase C inhibitor, calphostin C, is shown to inhibit the ability of IL-3-dependent 32D cells to reduce the tetrazolium salt, MTT. To determine whether this inhibition was mediated through mitochondria which have been implicated in MTT reduction, isolated mitochondria were treated with calphostin C in the presence of various substrates for mitochondrial electron transport and EDTA (to exclude PKC involvement). Calphostin C extensively inhibited succinate-dependent MTT reduction (IC50 = 110nM) but had little effect on either NADH- or NADPH-dependent MTT reduction. An alternative protein kinase C inhibitor, H7, did not affect succinate-dependent mitochondrial MTT reduction, and the protein kinase A inhibitor, KT5720, had little effect on either cellular or mitochondrial MTT reduction. These results show that in addition to its role as a PKC inhibitor, calphostin C is also a potent inhibitor of succinate-dependent mitochondrial electron transport.